libpq libpq is the C application programmer's interface to PostgreSQL. libpq is a set of library functions that allow client programs to pass queries to the PostgreSQL backend server and to receive the results of these queries. libpq is also the underlying engine for several other PostgreSQL application interfaces, including libpq++ (C++), libpgtcl (Tcl), Perl, and ECPG. So some aspects of libpq's behavior will be important to you if you use one of those packages. Three short programs are included at the end of this chapter () to show how to write programs that use libpq. There are also several complete examples of libpq applications in the directory src/test/examples in the source code distribution. Client programs that use libpq must include the header file libpq-fe.h and must link with the libpq library. The following functions deal with making a connection to a PostgreSQL backend server. An application program can have several backend connections open at one time. (One reason to do that is to access more than one database.) Each connection is represented by a PGconn object which is obtained from the function PQconnectdb or PQsetdbLogin. Note that these functions will always return a non-null object pointer, unless perhaps there is too little memory even to allocate the PGconn object. The PQstatus function should be called to check whether a connection was successfully made before queries are sent via the connection object. PQconnectdb Makes a new connection to the database server. PGconn *PQconnectdb(const char *conninfo); This function opens a new database connection using the parameters taken from the string conninfo. Unlike PQsetdbLogin below, the parameter set can be extended without changing the function signature, so use either of this function or the nonblocking analogues PQconnectStart and PQconnectPoll is preferred for new application programming. The passed string can be empty to use all default parameters, or it can contain one or more parameter settings separated by whitespace. Each parameter setting is in the form keyword = value. (To write an empty value or a value containing spaces, surround it with single quotes, e.g., keyword = 'a value'. Single quotes and backslashes within the value must be escaped with a backslash, i.e., \' and \\.) Spaces around the equal sign are optional. The currently recognized parameter key words are: host Name of host to connect to. If this begins with a slash, it specifies Unix-domain communication rather than TCP/IP communication; the value is the name of the directory in which the socket file is stored. The default is to connect to a Unix-domain socket in /tmp. hostaddr IP address of host to connect to. This should be in the standard IPv4 address format, e.g., 172.28.40.9. If your machine supports IPv6, you can also use those addresses. If a nonzero-length string is specified, TCP/IP communication is used. Using hostaddr instead of host allows the application to avoid a host name look-up, which may be important in applications with time constraints. However, Kerberos authentication requires the host name. The following therefore applies: If host is specified without hostaddr, a host name lookup is forced. If hostaddr is specified without host, the value for hostaddr gives the remote address; if Kerberos is used, this causes a reverse name query. If both host and hostaddr are specified, the value for hostaddr gives the remote address; the value for host is ignored, unless Kerberos is used, in which case that value is used for Kerberos authentication. Note that authentication is likely to fail if libpq is passed a host name that is not the name of the machine at hostaddr. Without either a host name or host address, libpq will connect using a local Unix domain socket. port Port number to connect to at the server host, or socket file name extension for Unix-domain connections. dbname The database name. user User name to connect as. password Password to be used if the server demands password authentication. connect_timeout Time space in seconds given to connection function. Zero or not set means infinite. options Configuration options to be sent to the server. tty A file or TTY for optional debug output from the server. requiressl If set to 1, an SSL connection to the server is required. libpq will then refuse to connect if the server does not accept an SSL connection. If set to 0 (default), libpq will negotiate the connection type with server. This option is only available if PostgreSQL is compiled with SSL support. service Service name to use for additional parameters. It specifies a service name in pg_service.conf that holds additional connection parameters. This allows applications to specify only a service name so connection parameters can be centrally maintained. See PREFIX/share/pg_service.conf.sample for information on how to set up the file. If any parameter is unspecified, then the corresponding environment variable (see ) is checked. If the environment variable is not set either, then hardwired defaults are used. PQsetdbLogin Makes a new connection to the database server. PGconn *PQsetdbLogin(const char *pghost, const char *pgport, const char *pgoptions, const char *pgtty, const char *dbName, const char *login, const char *pwd); This is the predecessor of PQconnectdb with a fixed number of parameters but the same functionality. PQsetdb Makes a new connection to the database server. PGconn *PQsetdb(char *pghost, char *pgport, char *pgoptions, char *pgtty, char *dbName); This is a macro that calls PQsetdbLogin with null pointers for the login and pwd parameters. It is provided primarily for backward compatibility with old programs. PQconnectStart PQconnectPoll nonblocking connection Make a connection to the database server in a nonblocking manner. PGconn *PQconnectStart(const char *conninfo); PostgresPollingStatusType PQconnectPoll(PGconn *conn); These two functions are used to open a connection to a database server such that your application's thread of execution is not blocked on remote I/O whilst doing so. The database connection is made using the parameters taken from the string conninfo, passed to PQconnectStart. This string is in the same format as described above for PQconnectdb. Neither PQconnectStart nor PQconnectPoll will block, as long as a number of restrictions are met: The hostaddr and host parameters are used appropriately to ensure that name and reverse name queries are not made. See the documentation of these parameters under PQconnectdb above for details. If you call PQtrace, ensure that the stream object into which you trace will not block. You ensure for yourself that the socket is in the appropriate state before calling PQconnectPoll, as described below. To begin a nonblocking connection request, call conn = PQconnectStart("connection_info_string"). If conn is null, then libpq has been unable to allocate a new PGconn structure. Otherwise, a valid PGconn pointer is returned (though not yet representing a valid connection to the database). On return from PQconnectStart, call status = PQstatus(conn). If status equals CONNECTION_BAD, PQconnectStart has failed. If PQconnectStart succeeds, the next stage is to poll libpq so that it may proceed with the connection sequence. Loop thus: If PQconnectPoll(conn) last returned PGRES_POLLING_READING, perform a select() for reading on the socket determined using PQsocket(conn). If it last returned PGRES_POLLING_WRITING, perform a select() for writing on that same socket. If you have yet to call PQconnectPoll, i.e., after the call to PQconnectStart, behave as if it last returned PGRES_POLLING_WRITING. If select() shows that the socket is ready, consider it active. If it has been decided that this connection is active, call PQconnectPoll(conn) again. If this call returns PGRES_POLLING_FAILED, the connection procedure has failed. If this call returns PGRES_POLLING_OK, the connection has been successfully made. Note that the use of select() to ensure that the socket is ready is merely a (likely) example; those with other facilities available, such as a poll() call, may of course use that instead. At any time during connection, the status of the connection may be checked, by calling PQstatus. If this gives CONNECTION_BAD, then the connection procedure has failed; if it gives CONNECTION_OK, then the connection is ready. Both of these states are equally detectable from the return value of PQconnectPoll, described above. Other states may also occur during (and only during) an asynchronous connection procedure. These indicate the current stage of the connection procedure and may be useful to provide feedback to the user for example. These statuses are: CONNECTION_STARTED Waiting for connection to be made. CONNECTION_MADE Connection OK; waiting to send. CONNECTION_AWAITING_RESPONSE Waiting for a response from the server. CONNECTION_AUTH_OK Received authentication; waiting for connection start-up to continue. CONNECTION_SETENV Negotiating environment (part of the connection start-up). Note that, although these constants will remain (in order to maintain compatibility), an application should never rely upon these appearing in a particular order, or at all, or on the status always being one of these documented values. An application may do something like this: switch(PQstatus(conn)) { case CONNECTION_STARTED: feedback = "Connecting..."; break; case CONNECTION_MADE: feedback = "Connected to server..."; break; . . . default: feedback = "Connecting..."; } Note that if PQconnectStart returns a non-null pointer, you must call PQfinish when you are finished with it, in order to dispose of the structure and any associated memory blocks. This must be done even if a call to PQconnectStart or PQconnectPoll failed. PQconnectPoll will currently block if libpq is compiled with SSL support. This restriction may be removed in the future. Finally, these functions leave the socket in a nonblocking state as if PQsetnonblocking had been called. PQconndefaults Returns the default connection options. PQconninfoOption *PQconndefaults(void); typedef struct { char *keyword; /* The keyword of the option */ char *envvar; /* Fallback environment variable name */ char *compiled; /* Fallback compiled in default value */ char *val; /* Option's current value, or NULL */ char *label; /* Label for field in connect dialog */ char *dispchar; /* Character to display for this field in a connect dialog. Values are: "" Display entered value as is "*" Password field - hide value "D" Debug option - don't show by default */ int dispsize; /* Field size in characters for dialog */ } PQconninfoOption; converts an escaped string representation of binary data into binary data --- the reverse of PQescapeBytea. Returns a connection options array. This may be used to determine all possible PQconnectdb options and their current default values. The return value points to an array of PQconninfoOption structures, which ends with an entry having a null key-word pointer. Note that the current default values (val fields) will depend on environment variables and other context. Callers must treat the connection options data as read-only. After processing the options array, free it by passing it to PQconninfoFree. If this is not done, a small amount of memory is leaked for each call to PQconndefaults. In PostgreSQL versions before 7.0, PQconndefaults returned a pointer to a static array, rather than a dynamically allocated array. That was not thread-safe, so the behavior has been changed. PQfinish Closes the connection to the server. Also frees memory used by the PGconn object. void PQfinish(PGconn *conn); Note that even if the server connection attempt fails (as indicated by PQstatus), the application should call PQfinish to free the memory used by the PGconn object. The PGconn pointer should not be used after PQfinish has been called. PQreset Resets the communication channel to the server. void PQreset(PGconn *conn); This function will close the connection to the server and attempt to reestablish a new connection to the same server, using all the same parameters previously used. This may be useful for error recovery if a working connection is lost. PQresetStart PQresetPoll Reset the communication channel to the server, in a nonblocking manner. int PQresetStart(PGconn *conn); PostgresPollingStatusType PQresetPoll(PGconn *conn); These functions will close the connection to the server and attempt to reestablish a new connection to the same server, using all the same parameters previously used. This may be useful for error recovery if a working connection is lost. They differ from PQreset (above) in that they act in a nonblocking manner. These functions suffer from the same restrictions as PQconnectStart and PQconnectPoll. To initiate a connection reset, call PQresetStart. If it returns 0, the reset has failed. If it returns 1, poll the reset using PQresetPoll in exactly the same way as you would create the connection using PQconnectPoll. libpq-fe.h libpq-int.h libpq application programmers should be careful to maintain the PGconn abstraction. Use the accessor functions below to get at the contents of PGconn. Avoid directly referencing the fields of the PGconn structure because they are subject to change in the future. (Beginning in PostgreSQL release 6.4, the definition of the struct behind PGconn is not even provided in libpq-fe.h. If you have old code that accesses PGconn fields directly, you can keep using it by including libpq-int.h too, but you are encouraged to fix the code soon.) PQdb Returns the database name of the connection. char *PQdb(const PGconn *conn); PQdb and the next several functions return the values established at connection. These values are fixed for the life of the PGconn object. PQuser Returns the user name of the connection. char *PQuser(const PGconn *conn); PQpass Returns the password of the connection. char *PQpass(const PGconn *conn); PQhost Returns the server host name of the connection. char *PQhost(const PGconn *conn); PQport Returns the port of the connection. char *PQport(const PGconn *conn); PQtty Returns the debug TTY of the connection. char *PQtty(const PGconn *conn); PQoptions Returns the configuration options passed in the connection request. char *PQoptions(const PGconn *conn); PQstatus Returns the status of the connection. ConnStatusType PQstatus(const PGconn *conn); The status can be one of a number of values. However, only two of these are seen outside of an asynchronous connection procedure: CONNECTION_OK and CONNECTION_BAD. A good connection to the database has the status CONNECTION_OK. A failed connection attempt is signaled by status CONNECTION_BAD. Ordinarily, an OK status will remain so until PQfinish, but a communications failure might result in the status changing to CONNECTION_BAD prematurely. In that case the application could try to recover by calling PQreset. See the entry for PQconnectStart and PQconnectPoll with regards to other status codes that might be seen. PQerrorMessage error message Returns the error message most recently generated by an operation on the connection. char *PQerrorMessage(const PGconn* conn); Nearly all libpq functions will set a message for PQerrorMessage if they fail. Note that by libpq convention, a nonempty PQerrorMessage result will include a trailing newline. PQsocket Obtains the file descriptor number of the connection socket to the server. A valid descriptor will be greater than or equal to 0; a result of -1 indicates that no server connection is currently open. int PQsocket(const PGconn *conn); PQbackendPID Returns the process ID of the backend server process handling this connection. int PQbackendPID(const PGconn *conn); The backend PID is useful for debugging purposes and for comparison to NOTIFY messages (which include the PID of the notifying backend process). Note that the PID belongs to a process executing on the database server host, not the local host! PQgetssl SSL Returns the SSL structure used in the connection, or null if SSL is not in use. SSL *PQgetssl(const PGconn *conn); This structure can be used to verify encryption levels, check server certificates, and more. Refer to the OpenSSL documentation for information about this structure. You must define USE_SSL in order to get the prototype for this function. Doing this will also automatically include ssl.h from OpenSSL. Once a connection to a database server has been successfully established, the functions described here are used to perform SQL queries and commands. PQexec Submits a command to the server and waits for the result. PGresult *PQexec(PGconn *conn, const char *command); Returns a PGresult pointer or possibly a null pointer. A non-null pointer will generally be returned except in out-of-memory conditions or serious errors such as inability to send the command to the server. If a null pointer is returned, it should be treated like a PGRES_FATAL_ERROR result. Use PQerrorMessage to get more information about the error. The PGresult structure encapsulates the result returned by the server. libpq application programmers should be careful to maintain the PGresult abstraction. Use the accessor functions below to get at the contents of PGresult. Avoid directly referencing the fields of the PGresult structure because they are subject to change in the future. If autocommit is on, multiple queries sent in a single function call are processed in a single transaction. PQresultStatus Returns the result status of the command. ExecStatusType PQresultStatus(const PGresult *res); PQresultStatus can return one of the following values: PGRES_EMPTY_QUERY The string sent to the server was empty. PGRES_COMMAND_OK Successful completion of a command returning no data. PGRES_TUPLES_OK The query successfully executed. PGRES_COPY_OUT Copy Out (from server) data transfer started. PGRES_COPY_IN Copy In (to server) data transfer started. PGRES_BAD_RESPONSE The server's response was not understood. PGRES_NONFATAL_ERROR A nonfatal error occurred. PGRES_FATAL_ERROR A fatal error occurred. If the result status is PGRES_TUPLES_OK, then the functions described below can be used to retrieve the rows returned by the query. Note that a SELECT command that happens to retrieve zero rows still shows PGRES_TUPLES_OK. PGRES_COMMAND_OK is for commands that can never return rows (INSERT, UPDATE, etc.). A response of PGRES_EMPTY_QUERY often exposes a bug in the client software. PQresStatus Converts the enumerated type returned by PQresultStatus into a string constant describing the status code. char *PQresStatus(ExecStatusType status); PQresultErrorMessage Returns the error message associated with the command, or an empty string if there was no error. char *PQresultErrorMessage(const PGresult *res); Immediately following a PQexec or PQgetResult call, PQerrorMessage (on the connection) will return the same string as PQresultErrorMessage (on the result). However, a PGresult will retain its error message until destroyed, whereas the connection's error message will change when subsequent operations are done. Use PQresultErrorMessage when you want to know the status associated with a particular PGresult; use PQerrorMessage when you want to know the status from the latest operation on the connection. PQclear Frees the storage associated with a PGresult. Every command result should be freed via PQclear when it is no longer needed. void PQclear(PQresult *res); You can keep a PGresult object around for as long as you need it; it does not go away when you issue a new command, nor even if you close the connection. To get rid of it, you must call PQclear. Failure to do this will result in memory leaks in your client application. PQmakeEmptyPGresult Constructs an empty PGresult object with the given status. PGresult* PQmakeEmptyPGresult(PGconn *conn, ExecStatusType status); This is libpq's internal function to allocate and initialize an empty PGresult object. It is exported because some applications find it useful to generate result objects (particularly objects with error status) themselves. If conn is not null and status indicates an error, the current error message of the specified connection is copied into the PGresult. Note that PQclear should eventually be called on the object, just as with a PGresult returned by libpq itself. escaping strings PQescapeString escapes a string for use within an SQL commmand. size_t PQescapeString (char *to, const char *from, size_t length); If you want to use strings that have been received from a source that is not trustworthy (for example, because a random user entered them), you should not directly include them in SQL commands for security reasons. Instead, you have to escape certain characters that are otherwise interpreted specially by the SQL parser. PQescapeString performs this operation. The parameter from points to the first character of the string that is to be escaped, and the length parameter counts the number of characters in this string. (A terminating zero byte is neither necessary nor counted.) to shall point to a buffer that is able to hold at least one more character than twice the value of length, otherwise the behavior is undefined. A call to PQescapeString writes an escaped version of the from string to the to buffer, replacing special characters so that they cannot cause any harm, and adding a terminating zero byte. The single quotes that must surround PostgreSQL string literals are not part of the result string. PQescapeString returns the number of characters written to to, not including the terminating zero byte. Behavior is undefined when the to and from strings overlap. escaping binary strings PQescapeBytea Escapes binary data for use within an SQL command with the type bytea. unsigned char *PQescapeBytea(const unsigned char *from, size_t from_length, size_t *to_length); Certain byte values must be escaped (but all byte values may be escaped) when used as part of a bytea literal in an SQL statement. In general, to escape a byte, it is converted into the three digit octal number equal to the octet value, and preceded by two backslashes. The single quote (') and backslash (\) characters have special alternative escape sequences. See for more information. PQescapeBytea performs this operation, escaping only the minimally required bytes. The from parameter points to the first byte of the string that is to be escaped, and the from_length parameter reflects the number of bytes in this binary string. (A terminating zero byte is neither necessary nor counted.) The to_length parameter points to a variable that will hold the resultant escaped string length. The result string length includes the terminating zero byte of the result. PQescapeBytea returns an escaped version of the from parameter binary string in memory allocated with malloc(), and must be freed using PQfreemem(). The return string has all special characters replaced so that they can be properly processed by the PostgreSQL string literal parser, and the bytea input function. A terminating zero byte is also added. The single quotes that must surround PostgreSQL string literals are not part of the result string. PQunescapeBytea Converts an escaped string representation of binary data into binary data --- the reverse of PQescapeBytea. unsigned char *PQunescapeBytea(const unsigned char *from, size_t *to_length); The from parameter points to an escaped string such as might be returned by PQgetvalue when applied to a bytea column. PQunescapeBytea converts this string representation into its binary representation. It returns a pointer to a buffer allocated with malloc(), or null on error, and puts the size of the buffer in to_length. The memory must be freed using PQfreemem(). PQfreemem Frees memory allocated by libpq void PQfreemem(void *ptr); Frees memory allocated by libpq, particularly PQescapeBytea, PQunescapeBytea, and PQnotifies. It is needed by Win32, which can not free memory across DLL's, unless multithreaded DLL's (/MD in VC6) are used. PQntuples Returns the number of rows (tuples) in the query result. int PQntuples(const PGresult *res); PQnfields Returns the number of columns (fields) in each row of the query result. int PQnfields(const PGresult *res); PQfname Returns the column name associated with the given column number. Column numbers start at 0. char *PQfname(const PGresult *res, int column_number); PQfnumber Returns the column number associated with the given column name. int PQfnumber(const PGresult *res, const char *column_name); -1 is returned if the given name does not match any column. PQftype Returns the column data type associated with the given column number. The integer returned is the internal OID number of the type. Column numbers start at 0. Oid PQftype(const PGresult *res, int column_number); You can query the system table pg_type to obtain the name and properties of the various data types. The OIDs of the built-in data types are defined in the file src/include/catalog/pg_type.h in the source tree. PQfmod Returns the type-specific modification data of the column associated with the given column number. Column numbers start at 0. int PQfmod(const PGresult *res, int column_number); PQfsize Returns the size in bytes of the column associated with the given column number. Column numbers start at 0. int PQfsize(const PGresult *res, int column_number); PQfsize returns the space allocated for this column in a database row, in other words the size of the server's binary representation of the data type. -1 is returned if the column has a variable size. PQbinaryTuples Returns 1 if the PGresult contains binary row data and 0 if it contains text data. int PQbinaryTuples(const PGresult *res); Currently, binary row data can only be returned by a query that extracts data from a binary cursor. PQgetvalue Returns a single column value of one row of a PGresult. Row and colums indices start at 0. char* PQgetvalue(const PGresult *res, int row_number, int column_number); For most queries, the value returned by PQgetvalue is a null-terminated character string representation of the column value. But if PQbinaryTuples returns 1, the value returned by PQgetvalue is the binary representation of the type in the internal format of the backend server (but not including the size word, if the column is variable-length). It is then the programmer's responsibility to cast and convert the data to the correct C type. The pointer returned by PQgetvalue points to storage that is part of the PGresult structure. One should not modify the data it points to, and one must explicitly copy the data into other storage if it is to be used past the lifetime of the PGresult structure itself. PQgetisnull Tests a column for a null value. Row and column numbers start at 0. int PQgetisnull(const PGresult *res, int row_number, int column_number); This function returns 1 if the column is null and 0 if it contains a non-null value. (Note that PQgetvalue will return an empty string, not a null pointer, for a null column.) PQgetlength Returns the length of a column value in bytes. Row and column numbers start at 0. int PQgetlength(const PGresult *res, int row_number, int column_number); This is the actual data length for the particular data value, that is, the size of the object pointed to by PQgetvalue. Note that for character-represented values, this size has little to do with the binary size reported by PQfsize. PQprint Prints out all the rows and, optionally, the column names to the specified output stream. void PQprint(FILE* fout, /* output stream */ const PGresult *res, const PQprintOpt *po); typedef struct { pqbool header; /* print output field headings and row count */ pqbool align; /* fill align the fields */ pqbool standard; /* old brain dead format */ pqbool html3; /* output HTML tables */ pqbool expanded; /* expand tables */ pqbool pager; /* use pager for output if needed */ char *fieldSep; /* field separator */ char *tableOpt; /* attributes for HTML table element */ char *caption; /* HTML table caption */ char **fieldName; /* null-terminated array of replacement field names */ } PQprintOpt; This function was formerly used by psql to print query results, but this is no longer the case and this function is no longer actively supported. PQcmdStatus Returns the command status string from the SQL command that generated the PGresult. char * PQcmdStatus(PGresult *res); PQcmdTuples Returns the number of rows affected by the SQL command. char * PQcmdTuples(PGresult *res); If the SQL command that generated the PGresult was INSERT, UPDATE, or DELETE, this returns a string containing the number of rows affected. If the command was anything else, it returns the empty string. PQoidValue Returns the OID of the inserted row, if the SQL command was an INSERT that inserted exactly one row into a table that has OIDs. Otherwise, returns InvalidOid. Oid PQoidValue(const PGresult *res); The type Oid and the constant InvalidOid will be defined if you include the libpq header file. They will both be some integer type. PQoidStatus Returns a string with the OID of the inserted row, if the SQL command was an INSERT. (The string will be 0 if the INSERT did not insert exactly one row, or if the target table does not have OIDs.) If the command was not an INSERT, returns an empty string. char * PQoidStatus(const PGresult *res); This function is deprecated in favor of PQoidValue and is not thread-safe. nonblocking connection The PQexec function is adequate for submitting commands in normal, synchronous applications. It has a couple of deficiencies, however, that can be of importance to some users: PQexec waits for the command to be completed. The application may have other work to do (such as maintaining a user interface), in which case it won't want to block waiting for the response. Since the execution of the client application is suspended while it waits for the result, it is hard for the application to decide that it would like to try to cancel the ongoing command. (It can be done from a signal handler, but not otherwise.) PQexec can return only one PGresult structure. If the submitted command string contains multiple SQL commands, all but the last PGresult are discarded by PQexec. Applications that do not like these limitations can instead use the underlying functions that PQexec is built from: PQsendQuery and PQgetResult. Older programs that used this functionality as well as PQputline and PQputnbytes could block waiting to send data to the server. To address that issue, the function PQsetnonblocking was added. Old applications can neglect to use PQsetnonblocking and get the old potentially blocking behavior. Newer programs can use PQsetnonblocking to achieve a completely nonblocking connection to the server. PQsetnonblocking Sets the nonblocking status of the connection. int PQsetnonblocking(PGconn *conn, int arg); Sets the state of the connection to nonblocking if arg is 1 and blocking if arg is 0. Returns 0 if OK, -1 if error. In the nonblocking state, calls to PQputline, PQputnbytes, PQsendQuery, and PQendcopy will not block but instead return an error if they need to be called again. When a database connection has been set to nonblocking mode and PQexec is called, it will temporarily set the state of the connection to blocking until the PQexec call completes. More of libpq is expected to be made safe for the nonblocking mode in the future. PQisnonblocking Returns the blocking status of the database connection. int PQisnonblocking(const PGconn *conn); Returns 1 if the connection is set to nonblocking mode and 0 if blocking. PQsendQuery Submits a command to the server without waiting for the result(s). 1 is returned if the command was successfully dispatched and 0 if not (in which case, use PQerrorMessage to get more information about the failure). int PQsendQuery(PGconn *conn, const char *command); After successfully calling PQsendQuery, call PQgetResult one or more times to obtain the results. PQsendQuery may not be called again (on the same connection) until PQgetResult has returned a null pointer, indicating that the command is done. PQgetResult Waits for the next result from a prior PQsendQuery, and return it. A null pointer is returned when the command is complete and there will be no more results. PGresult *PQgetResult(PGconn *conn); PQgetResult must be called repeatedly until it returns a null pointer, indicating that the command is done. (If called when no command is active, PQgetResult will just return a null pointer at once.) Each non-null result from PQgetResult should be processed using the same PGresult accessor functions previously described. Don't forget to free each result object with PQclear when done with it. Note that PQgetResult will block only if a command is active and the necessary response data has not yet been read by PQconsumeInput. Using PQsendQuery and PQgetResult solves one of PQexec's problems: If a command string contains multiple SQL commands, the results of those commands can be obtained individually. (This allows a simple form of overlapped processing, by the way: the client can be handling the results of one command while the server is still working on later queries in the same command string.) However, calling PQgetResult will still cause the client to block until the server completes the next SQL command. This can be avoided by proper use of three more functions: PQconsumeInput If input is available from the server, consume it. int PQconsumeInput(PGconn *conn); PQconsumeInput normally returns 1 indicating no error, but returns 0 if there was some kind of trouble (in which case PQerrorMessage can be used). Note that the result does not say whether any input data was actually collected. After calling PQconsumeInput, the application may check PQisBusy and/or PQnotifies to see if their state has changed. PQconsumeInput may be called even if the application is not prepared to deal with a result or notification just yet. The function will read available data and save it in a buffer, thereby causing a select() read-ready indication to go away. The application can thus use PQconsumeInput to clear the select() condition immediately, and then examine the results at leisure. PQisBusy Returns 1 if a command is busy, that is, PQgetResult would block waiting for input. A 0 return indicates that PQgetResult can be called with assurance of not blocking. int PQisBusy(PGconn *conn); PQisBusy will not itself attempt to read data from the server; therefore PQconsumeInput must be invoked first, or the busy state will never end. PQflush Attempts to flush any data queued to the server, returns 0 if successful (or if the send queue is empty) or EOF if it failed for some reason. int PQflush(PGconn *conn); PQflush needs to be called on a nonblocking connection before calling select() to determine if a response has arrived. If 0 is returned it ensures that there is no data queued to the server that has not actually been sent. Only applications that have used PQsetnonblocking have a need for this. A typical application using these functions will have a main loop that uses select() to wait for all the conditions that it must respond to. One of the conditions will be input available from the server, which in terms of select() means readable data on the file descriptor identified by PQsocket. When the main loop detects input ready, it should call PQconsumeInput to read the input. It can then call PQisBusy, followed by PQgetResult if PQisBusy returns false (0). It can also call PQnotifies to detect NOTIFY messages (see ). Nonblocking connections (that have used PQsetnonblocking) should not use select() until PQflush has returned 0 indicating that there is no buffered data waiting to be sent to the server. A client that uses PQsendQuery/PQgetResult can also attempt to cancel a command that is still being processed by the server. PQrequestCancel Requests that the server abandon processing of the current command. int PQrequestCancel(PGconn *conn); The return value is 1 if the cancel request was successfully dispatched and 0 if not. (If not, PQerrorMessage tells why not.) Successful dispatch is no guarantee that the request will have any effect, however. Regardless of the return value of PQrequestCancel, the application must continue with the normal result-reading sequence using PQgetResult. If the cancellation is effective, the current command will terminate early and return an error result. If the cancellation fails (say, because the server was already done processing the command), then there will be no visible result at all. Note that if the current command is part of a transaction block, cancellation will abort the whole transaction. PQrequestCancel can safely be invoked from a signal handler. So, it is also possible to use it in conjunction with plain PQexec, if the decision to cancel can be made in a signal handler. For example, psql invokes PQrequestCancel from a SIGINT signal handler, thus allowing interactive cancellation of commands that it issues through PQexec. PostgreSQL provides a fast-path interface to send function calls to the server. This is a trapdoor into system internals and can be a potential security hole. Most users will not need this feature. The function PQfn requests execution of a server function via the fast-path interface: PGresult* PQfn(PGconn* conn, int fnid, int *result_buf, int *result_len, int result_is_int, const PQArgBlock *args, int nargs); typedef struct { int len; int isint; union { int *ptr; int integer; } u; } PQArgBlock; The fnid argument is the OID of the function to be executed. result_buf is the buffer in which to place the return value. The caller must have allocated sufficient space to store the return value. (There is no check!) The actual result length will be returned in the integer pointed to by result_len. If a 4-byte integer result is expected, set result_is_int to 1, otherwise set it to 0. (Setting result_is_int to 1 tells libpq to byte-swap the value if necessary, so that it is delivered as a proper int value for the client machine. When result_is_int is 0, the byte string sent by the server is returned unmodified.) args and nargs specify the arguments to be passed to the function. PQfn always returns a valid PGresult pointer. The result status should be checked before the result is used. The caller is responsible for freeing the PGresult with PQclear when it is no longer needed. NOTIFY PostgreSQL offers asynchronous notification via the LISTEN and NOTIFY commands. A server-side session registers its interest in a particular notification condition with the LISTEN command (and can stop listening with the UNLISTEN command). All sessions listening on a particular condition will be notified asynchronously when a NOTIFY command with that condition name is executed by any session. No additional information is passed from the notifier to the listener. Thus, typically, any actual data that needs to be communicated is transferred through a database table. Commonly, the condition name is the same as the associated table, but it is not necessary for there to be any associated table. libpq applications submit LISTEN and UNLISTEN commands as ordinary SQL command. The arrival of NOTIFY messages can subsequently be detected by calling PQnotifies. The function PQnotifies returns the next notification from a list of unhandled notification messages received from the server. It returns a null pointer if there are no pending notifications. Once a notification is returned from PQnotifies, it is considered handled and will be removed from the list of notifications. PGnotify* PQnotifies(PGconn *conn); typedef struct pgNotify { char *relname; /* notification name */ int be_pid; /* process ID of server process */ } PGnotify; After processing a PGnotify object returned by PQnotifies, be sure to free it with PQfreemem(). In PostgreSQL 6.4 and later, the be_pid is that of the notifying backend process, whereas in earlier versions it was always the PID of your own backend process. gives a sample program that illustrates the use of asynchronous notification. PQnotifies() does not actually read data from the server; it just returns messages previously absorbed by another libpq function. In prior releases of libpq, the only way to ensure timely receipt of NOTIFY messages was to constantly submit commands, even empty ones, and then check PQnotifies() after each PQexec(). While this still works, it is deprecated as a waste of processing power. A better way to check for NOTIFY messages when you have no useful commands to execute is to call PQconsumeInput(), then check PQnotifies(). You can use select() to wait for data to arrive from the server, thereby using no CPU power unless there is something to do. (See PQsocket() to obtain the file descriptor number to use with select().) Note that this will work OK whether you submit commands with PQsendQuery/PQgetResult or simply use PQexec. You should, however, remember to check PQnotifies() after each PQgetResult or PQexec, to see if any notifications came in during the processing of the command. COPY with libpq The COPY command in PostgreSQL has options to read from or write to the network connection used by libpq. Therefore, functions are necessary to access this network connection directly so applications may take advantage of this capability. These functions should be executed only after obtaining a result status of PGRES_COPY_OUT or PGRES_COPY_IN from PQexec or PQgetResult. PQgetline Reads a newline-terminated line of characters (transmitted by the server) into a buffer string of size length. int PQgetline(PGconn *conn, char *buffer, int length); This function copies up to length-1 characters into the buffer and converts the terminating newline into a zero byte. PQgetline returns EOF at the end of input, 0 if the entire line has been read, and 1 if the buffer is full but the terminating newline has not yet been read. Note that the application must check to see if a new line consists of the two characters \., which indicates that the server has finished sending the results of the COPY command. If the application might receive lines that are more than length-1 characters long, care is needed to be sure it recognizes the \. line correctly (and does not, for example, mistake the end of a long data line for a terminator line). The code in the file src/bin/psql/copy.c contains example functions that correctly handle the COPY protocol. PQgetlineAsync Reads a newline-terminated line of characters (transmitted by the server) into a buffer without blocking. int PQgetlineAsync(PGconn *conn, char *buffer, int length); This function is similar to PQgetline, but it can be used by applications that must read COPY data asynchronously, that is, without blocking. Having issued the COPY command and gotten a PGRES_COPY_OUT response, the application should call PQconsumeInput and PQgetlineAsync until the end-of-data signal is detected. Unlike PQgetline, this function takes responsibility for detecting end-of-data. On each call, PQgetlineAsync will return data if a complete newline- terminated data line is available in libpq's input buffer, or if the incoming data line is too long to fit in the buffer offered by the caller. Otherwise, no data is returned until the rest of the line arrives. The function returns -1 if the end-of-copy-data marker has been recognized, or 0 if no data is available, or a positive number giving the number of bytes of data returned. If -1 is returned, the caller must next call PQendcopy, and then return to normal processing. The data returned will not extend beyond a newline character. If possible a whole line will be returned at one time. But if the buffer offered by the caller is too small to hold a line sent by the server, then a partial data line will be returned. This can be detected by testing whether the last returned byte is \n or not. The returned string is not null-terminated. (If you want to add a terminating null, be sure to pass a length one smaller than the room actually available.) PQputline Sends a null-terminated string to the server. Returns 0 if OK and EOF if unable to send the string. int PQputline(PGconn *conn, const char *string); Note the application must explicitly send the two characters \. on a final line to indicate to the server that it has finished sending its data. PQputnbytes Sends a non-null-terminated string to the server. Returns 0 if OK and EOF if unable to send the string. int PQputnbytes(PGconn *conn, const char *buffer, int nbytes); This is exactly like PQputline, except that the data buffer need not be null-terminated since the number of bytes to send is specified directly. PQendcopy Synchronizes with the server. int PQendcopy(PGconn *conn); This function waits until the server has finished the copying. It should either be issued when the last string has been sent to the server using PQputline or when the last string has been received from the server using PGgetline. It must be issued or the server may get out of sync with the client. Upon return from this function, the server is ready to receive the next SQL command. The return value is 0 on successful completion, nonzero otherwise. When using PQgetResult, the application should respond to a PGRES_COPY_OUT result by executing PQgetline repeatedly, followed by PQendcopy after the terminator line is seen. It should then return to the PQgetResult loop until PQgetResult returns a null pointer. Similarly a PGRES_COPY_IN result is processed by a series of PQputline calls followed by PQendcopy, then return to the PQgetResult loop. This arrangement will ensure that a COPY command embedded in a series of SQL commands will be executed correctly. Older applications are likely to submit a COPY via PQexec and assume that the transaction is done after PQendcopy. This will work correctly only if the COPY is the only SQL command in the command string. An example: PQexec(conn, "CREATE TABLE foo (a integer, b varchar(16), d double precision);"); PQexec(conn, "COPY foo FROM STDIN;"); PQputline(conn, "3\thello world\t4.5\n"); PQputline(conn, "4\tgoodbye world\t7.11\n"); ... PQputline(conn, "\\.\n"); PQendcopy(conn); PQtrace Enables tracing of the client/server communication to a debugging file stream. void PQtrace(PGconn *conn FILE *stream); PQuntrace Disables tracing started by PQtrace. void PQuntrace(PGconn *conn); The function PQsetNoticeProcessor notice processor controls the reporting of notice and warning messages generated by the server. typedef void (*PQnoticeProcessor) (void *arg, const char *message); PQnoticeProcessor PQsetNoticeProcessor(PGconn *conn, PQnoticeProcessor proc, void *arg); By default, libpq prints notice messages from the server, as well as a few error messages that it generates by itself, on stderr. This behavior can be overridden by supplying a callback function that does something else with the messages, a so-called notice processor. The callback function is passed the text of the message (which includes a trailing newline), plus a void pointer that is the same one passed to PQsetNoticeProcessor. (This pointer can be used to access application-specific state if needed.) The default notice processor is simply static void defaultNoticeProcessor(void * arg, const char * message) { fprintf(stderr, "%s", message); } To use a special notice processor, call PQsetNoticeProcessor just after creation of a new PGconn object. The return value is the pointer to the previous notice processor. If you supply a null callback function pointer, no action is taken, but the current pointer is returned. Once you have set a notice processor, you should expect that that function could be called as long as either the PGconn object or PGresult objects made from it exist. At creation of a PGresult, the PGconn's current notice processor pointer is copied into the PGresult for possible use by functions like PQgetvalue. environment variables The following environment variables can be used to select default connection parameter values, which will be used by PQconnectdb, PQsetdbLogin and PQsetdb if no value is directly specified by the calling code. These are useful to avoid hard-coding database connection information into simple client applications, for example. PGHOST PGHOST sets the default server name. If this begins with a slash, it specifies Unix-domain communication rather than TCP/IP communication; the value is the name of the directory in which the socket file is stored (default /tmp). PGPORT PGPORT sets the default TCP port number or Unix-domain socket file extension for communicating with the PostgreSQL server. PGDATABASE PGDATABASE sets the default PostgreSQL database name. PGUSER PGUSER sets the user name used to connect to the database. PGPASSWORD PGPASSWORD sets the password used if the server demands password authentication. This environment variable is deprecated for security reasons; consider migrating to use the $HOME/.pgpass file (see ). PGREALM sets the Kerberos realm to use with PostgreSQL, if it is different from the local realm. If PGREALM is set, libpq applications will attempt authentication with servers for this realm and use separate ticket files to avoid conflicts with local ticket files. This environment variable is only used if Kerberos authentication is selected by the server. PGOPTIONS sets additional run-time options for the PostgreSQL server. PGTTY sets the file or TTY on which debugging messages from the server are displayed. PGREQUIRESSL sets whether or not the connection must be made over SSL. If set to 1, libpq will refuse to connect if the server does not accept an SSL connection. This option is only available if PostgreSQL is compiled with SSL support. PGCONNECT_TIMEOUT sets the maximum number of seconds that libpq will wait when attempting to connect to the PostgreSQL server. This option should be set to at least 2 seconds. The following environment variables can be used to specify default behavior for every PostgreSQL session. PGDATESTYLE sets the default style of date/time representation. (Equivalent to SET datestyle TO ....) PGTZ sets the default time zone. (Equivalent to SET timezone TO ....) PGCLIENTENCODING sets the default client character set encoding. (Equivalent to SET client_encoding TO ....) PGGEQO sets the default mode for the genetic query optimizer. (Equivalent to SET geqo TO ....) Refer to the SQL command SET for information on correct values for these environment variables. password file .pgpass The file .pgpass in a user's home directory is a file that can contain passwords to be used if the connection requires a password (and no password has been specified otherwise). This file should have lines of the following format: hostname:port:database:username:password Each of these fields may be a literal name or *, which matches anything. The first matching entry will be used, so put more-specific entries first. When an entry contain : or \, it must be escaped with \. The permissions on .pgpass must disallow any access to world or group; achieve this by the command chmod 0600 ~/.pgpass. If the permissions are less strict than this, the file will be ignored. threads with libpq libpq is thread-safe as of PostgreSQL 7.0, so long as no two threads attempt to manipulate the same PGconn object at the same time. In particular, you cannot issue concurrent commands from different threads through the same connection object. (If you need to run concurrent commands, start up multiple connections.) PGresult objects are read-only after creation, and so can be passed around freely between threads. The deprecated functions PQoidStatus and fe_setauthsvc are not thread-safe and should not be used in multithread programs. PQoidStatus can be replaced by PQoidValue. There is no good reason to call fe_setauthsvc at all. libpq applications that use the crypt authentication method rely on the crypt() operating system function, which is often not thread-safe. It is better to use the md5 method, which is thread-safe on all platforms. To build (i.e., compile and link) your libpq programs you need to do all of the following things: Include the libpq-fe.h header file: #include <libpq-fe.h> If you failed to do that then you will normally get error messages from your compiler similar to foo.c: In function `main': foo.c:34: `PGconn' undeclared (first use in this function) foo.c:35: `PGresult' undeclared (first use in this function) foo.c:54: `CONNECTION_BAD' undeclared (first use in this function) foo.c:68: `PGRES_COMMAND_OK' undeclared (first use in this function) foo.c:95: `PGRES_TUPLES_OK' undeclared (first use in this function) Point your compiler to the directory where the PostgreSQL header files were installed, by supplying the -Idirectory option to your compiler. (In some cases the compiler will look into the directory in question by default, so you can omit this option.) For instance, your compile command line could look like: cc -c -I/usr/local/pgsql/include testprog.c If you are using makefiles then add the option to the CPPFLAGS variable: CPPFLAGS += -I/usr/local/pgsql/include pg_config If there is any chance that your program might be compiled by other users then you should not hardcode the directory location like that. Instead, you can run the utility pg_config to find out where the header files are on the local system: $ pg_config --includedir /usr/local/include Failure to specify the correct option to the compiler will result in an error message such as testlibpq.c:8:22: libpq-fe.h: No such file or directory When linking the final program, specify the option -lpq so that the libpq library gets pulled in, as well as the option -Ldirectory to point the compiler to the directory where the libpq library resides. (Again, the compiler will search some directories by default.) For maximum portability, put the -L option before the -lpq option. For example: cc -o testprog testprog1.o testprog2.o -L/usr/local/pgsql/lib -lpq You can find out the library directory using pg_config as well: $ pg_config --libdir /usr/local/pgsql/lib Error messages that point to problems in this area could look like the following. testlibpq.o: In function `main': testlibpq.o(.text+0x60): undefined reference to `PQsetdbLogin' testlibpq.o(.text+0x71): undefined reference to `PQstatus' testlibpq.o(.text+0xa4): undefined reference to `PQerrorMessage' This means you forgot -lpq. /usr/bin/ld: cannot find -lpq This means you forgot the -L option or did not specify the right directory. libpq-int.h If your codes references the header file libpq-int.h and you refuse to fix your code to not use it, starting in PostgreSQL 7.2, this file will be found in includedir/postgresql/internal/libpq-int.h, so you need to add the appropriate -I option to your compiler command line. /* * testlibpq.c * * Test the C version of libpq, the PostgreSQL frontend * library. */ #include <stdio.h> #include <libpq-fe.h> void exit_nicely(PGconn *conn) { PQfinish(conn); exit(1); } main() { char *pghost, *pgport, *pgoptions, *pgtty; char *dbName; int nFields; int i, j; /* FILE *debug; */ PGconn *conn; PGresult *res; /* * begin, by setting the parameters for a backend connection if the * parameters are null, then the system will try to use reasonable * defaults by looking up environment variables or, failing that, * using hardwired constants */ pghost = NULL; /* host name of the backend server */ pgport = NULL; /* port of the backend server */ pgoptions = NULL; /* special options to start up the backend * server */ pgtty = NULL; /* debugging tty for the backend server */ dbName = "template1"; /* make a connection to the database */ conn = PQsetdb(pghost, pgport, pgoptions, pgtty, dbName); /* * check to see that the backend connection was successfully made */ if (PQstatus(conn) == CONNECTION_BAD) { fprintf(stderr, "Connection to database '%s' failed.\n", dbName); fprintf(stderr, "%s", PQerrorMessage(conn)); exit_nicely(conn); } /* debug = fopen("/tmp/trace.out","w"); */ /* PQtrace(conn, debug); */ /* start a transaction block */ res = PQexec(conn, "BEGIN"); if (!res || PQresultStatus(res) != PGRES_COMMAND_OK) { fprintf(stderr, "BEGIN command failed\n"); PQclear(res); exit_nicely(conn); } /* * should PQclear PGresult whenever it is no longer needed to avoid * memory leaks */ PQclear(res); /* * fetch rows from the pg_database, the system catalog of * databases */ res = PQexec(conn, "DECLARE mycursor CURSOR FOR SELECT * FROM pg_database"); if (!res || PQresultStatus(res) != PGRES_COMMAND_OK) { fprintf(stderr, "DECLARE CURSOR command failed\n"); PQclear(res); exit_nicely(conn); } PQclear(res); res = PQexec(conn, "FETCH ALL in mycursor"); if (!res || PQresultStatus(res) != PGRES_TUPLES_OK) { fprintf(stderr, "FETCH ALL command didn't return tuples properly\n"); PQclear(res); exit_nicely(conn); } /* first, print out the attribute names */ nFields = PQnfields(res); for (i = 0; i < nFields; i++) printf("%-15s", PQfname(res, i)); printf("\n\n"); /* next, print out the rows */ for (i = 0; i < PQntuples(res); i++) { for (j = 0; j < nFields; j++) printf("%-15s", PQgetvalue(res, i, j)); printf("\n"); } PQclear(res); /* close the cursor */ res = PQexec(conn, "CLOSE mycursor"); PQclear(res); /* commit the transaction */ res = PQexec(conn, "COMMIT"); PQclear(res); /* close the connection to the database and cleanup */ PQfinish(conn); /* fclose(debug); */ return 0; } /* * testlibpq2.c * Test of the asynchronous notification interface * * Start this program, then from psql in another window do * NOTIFY TBL2; * * Or, if you want to get fancy, try this: * Populate a database with the following: * * CREATE TABLE TBL1 (i int4); * * CREATE TABLE TBL2 (i int4); * * CREATE RULE r1 AS ON INSERT TO TBL1 DO * (INSERT INTO TBL2 values (new.i); NOTIFY TBL2); * * and do * * INSERT INTO TBL1 values (10); * */ #include <stdio.h> #include "libpq-fe.h" void exit_nicely(PGconn *conn) { PQfinish(conn); exit(1); } main() { char *pghost, *pgport, *pgoptions, *pgtty; char *dbName; int nFields; int i, j; PGconn *conn; PGresult *res; PGnotify *notify; /* * begin, by setting the parameters for a backend connection if the * parameters are null, then the system will try to use reasonable * defaults by looking up environment variables or, failing that, * using hardwired constants */ pghost = NULL; /* host name of the backend server */ pgport = NULL; /* port of the backend server */ pgoptions = NULL; /* special options to start up the backend * server */ pgtty = NULL; /* debugging tty for the backend server */ dbName = getenv("USER"); /* change this to the name of your test * database */ /* make a connection to the database */ conn = PQsetdb(pghost, pgport, pgoptions, pgtty, dbName); /* * check to see that the backend connection was successfully made */ if (PQstatus(conn) == CONNECTION_BAD) { fprintf(stderr, "Connection to database '%s' failed.\n", dbName); fprintf(stderr, "%s", PQerrorMessage(conn)); exit_nicely(conn); } res = PQexec(conn, "LISTEN TBL2"); if (!res || PQresultStatus(res) != PGRES_COMMAND_OK) { fprintf(stderr, "LISTEN command failed\n"); PQclear(res); exit_nicely(conn); } /* * should PQclear PGresult whenever it is no longer needed to avoid * memory leaks */ PQclear(res); while (1) { /* * wait a little bit between checks; waiting with select() * would be more efficient. */ sleep(1); /* collect any asynchronous backend messages */ PQconsumeInput(conn); /* check for asynchronous notify messages */ while ((notify = PQnotifies(conn)) != NULL) { fprintf(stderr, "ASYNC NOTIFY of '%s' from backend pid '%d' received\n", notify->relname, notify->be_pid); PQfreemem(notify); } } /* close the connection to the database and cleanup */ PQfinish(conn); return 0; }