libpq libpq is the C application programmer's interface to Postgres. libpq is a set of library routines that allow client programs to pass queries to the Postgres backend server and to receive the results of these queries. libpq is also the underlying engine for several other Postgres 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 section to show how to write programs that use libpq. There are several complete examples of libpq applications in the following directories: ../src/test/regress ../src/test/examples ../src/bin/psql Frontend programs that use libpq must include the header file libpq-fe.h and must link with the libpq library. The following routines deal with making a connection to a Postgres backend server. The 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 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 routine 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 routine or the non-blocking analogues PQconnectStart / PQconnectPoll is prefered for 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 a null value or a value containing spaces, surround it with single quotes, e.g., keyword = 'a value'. Single quotes within the value must be written as \'. Spaces around the equal sign are optional.) The currently recognized parameter keywords 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 standard numbers-and-dots form, as used by the BSD functions inet_aton et al. If a non-zero-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 hostname look-up 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 filename 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. options Trace/debug options to be sent to the server. tty A file or tty for optional debug output from the backend. requiressl Set to '1' to require SSL connection to the backend. Libpq will then refuse to connect if the server does not support SSL. Set to '0' (default) to negotiate with server. If any parameter is unspecified, then the corresponding environment variable (see "Environment Variables" section) is checked. If the environment variable is not set either, then hardwired defaults are used. The return value is a pointer to an abstract struct representing the connection to the backend. 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 Make a connection to the database server in a non-blocking manner. PGconn *PQconnectStart(const char *conninfo) PostgresPollingStatusType PQconnectPoll(PGconn *conn) These two routines 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, 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: Consider a connection 'inactive' by default. If PQconnectPoll last returned PGRES_POLLING_ACTIVE, consider it 'active' instead. If PQconnectPoll(conn) last returned PGRES_POLLING_READING, perform a select for reading on PQsocket(conn). If it last returned PGRES_POLLING_WRITING, perform a select for writing on PQsocket(conn). If you have yet to call PQconnectPoll, i.e. after the call to PQconnectStart, behave as if it last returned PGRES_POLLING_WRITING. If the 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 is CONNECTION_BAD, then the connection procedure has failed; if this is CONNECTION_OK, then the connection is ready. Either of these states should be equally detectable from the return value of PQconnectPoll, as above. Other states may be shown 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 may include: CONNECTION_STARTED: Waiting for connection to be made. CONNECTION_MADE: Connection OK; waiting to send. CONNECTION_AWAITING_RESPONSE: Waiting for a response from the postmaster. CONNECTION_AUTH_OK: Received authentication; waiting for backend start-up. CONNECTION_SETENV: Negotiating environment. 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 USE_SSL defined. This restriction may be removed in the future. These functions leave the socket in a non-blocking state as if PQsetnonblocking had been called. PQconndefaults Returns the default connection options. PQconninfoOption *PQconndefaults(void) struct PQconninfoOption { 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 */ } 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 structs, which ends with an entry having a NULL keyword pointer. Note that the 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 Postgres versions before 7.0, PQconndefaults() returned a pointer to a static array, rather than a dynamically allocated array. That wasn't thread-safe, so the behavior has been changed. PQfinish Close the connection to the backend. Also frees memory used by the PGconn object. void PQfinish(PGconn *conn) Note that even if the backend 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 Reset the communication port with the backend. void PQreset(PGconn *conn) This function will close the connection to the backend and attempt to reestablish a new connection to the same postmaster, 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 port with the backend, in a non-blocking manner. int PQresetStart(PGconn *conn); PostgresPollingStatusType PQresetPoll(PGconn *conn); These functions will close the connection to the backend and attempt to reestablish a new connection to the same postmaster, 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 non-blocking manner. These functions suffer from the same restrictions as PQconnectStart and PQconnectPoll. 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 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 Postgres release 6.4, the definition of struct 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 backend options used in the connection. 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 or 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 Returns the error message most recently generated by an operation on the connection. char *PQerrorMessage(const PGconn* conn); Nearly all libpq functions will set PQerrorMessage if they fail. Note that by libpq convention, a non-empty PQerrorMessage will include a trailing newline. PQbackendPID Returns the process ID of the backend server 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). Note that the PID belongs to a process executing on the database server host, not the local host! PQgetssl 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 certificate 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 Submit a query to Postgres and wait for the result. PGresult *PQexec(PGconn *conn, const char *query); 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 query to the backend. If a NULL 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 query result returned by the backend. 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. (Beginning in Postgres release 6.4, the definition of struct PGresult is not even provided in libpq-fe.h. If you have old code that accesses PGresult fields directly, you can keep using it by including libpq-int.h too, but you are encouraged to fix the code soon.) PQresultStatus Returns the result status of the query. ExecStatusType PQresultStatus(const PGresult *res) PQresultStatus can return one of the following values: PGRES_EMPTY_QUERY -- The string sent to the backend 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 PGRES_FATAL_ERROR If the result status is PGRES_TUPLES_OK, then the routines described below can be used to retrieve the tuples returned by the query. Note that a SELECT that happens to retrieve zero tuples still shows PGRES_TUPLES_OK. PGRES_COMMAND_OK is for commands that can never return tuples (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 query, 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 the PGresult. Every query 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 query, 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 the frontend application. PQmakeEmptyPGresult Constructs an empty PGresult object with the given status. PGresult* PQmakeEmptyPGresult(PGconn *conn, ExecStatusType status); This is libpq's internal routine 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 connection's current errorMessage is copied into the PGresult. Note that PQclear should eventually be called on the object, just as with a PGresult returned by libpq itself. PQntuples Returns the number of tuples (rows) in the query result. int PQntuples(const PGresult *res); PQnfields Returns the number of fields (attributes) in each tuple of the query result. int PQnfields(const PGresult *res); PQfname Returns the field (attribute) name associated with the given field index. Field indices start at 0. char *PQfname(const PGresult *res, int field_index); PQfnumber Returns the field (attribute) index associated with the given field name. int PQfnumber(const PGresult *res, const char *field_name); -1 is returned if the given name does not match any field. PQftype Returns the field type associated with the given field index. The integer returned is an internal coding of the type. Field indices start at 0. Oid PQftype(const PGresult *res, int field_index); You can query the system table pg_type to obtain the name and properties of the various datatypes. The OIDs of the built-in datatypes are defined in src/include/catalog/pg_type.h in the source tree. PQfmod Returns the type-specific modification data of the field associated with the given field index. Field indices start at 0. int PQfmod(const PGresult *res, int field_index); PQfsize Returns the size in bytes of the field associated with the given field index. Field indices start at 0. int PQfsize(const PGresult *res, int field_index); PQfsize returns the space allocated for this field in a database tuple, in other words the size of the server's binary representation of the data type. -1 is returned if the field is variable size. PQbinaryTuples Returns 1 if the PGresult contains binary tuple data, 0 if it contains ASCII data. int PQbinaryTuples(const PGresult *res); Currently, binary tuple data can only be returned by a query that extracts data from a BINARY cursor. PQgetvalue Returns a single field (attribute) value of one tuple of a PGresult. Tuple and field indices start at 0. char* PQgetvalue(const PGresult *res, int tup_num, int field_num); For most queries, the value returned by PQgetvalue is a null-terminated ASCII string representation of the attribute value. But if PQbinaryTuples() is 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 field 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 it, and one must explicitly copy the value into other storage if it is to be used past the lifetime of the PGresult structure itself. PQgetisnull Tests a field for a NULL entry. Tuple and field indices start at 0. int PQgetisnull(const PGresult *res, int tup_num, int field_num); This function returns 1 if the field contains a NULL, 0 if it contains a non-null value. (Note that PQgetvalue will return an empty string, not a null pointer, for a NULL field.) PQgetlength Returns the length of a field (attribute) value in bytes. Tuple and field indices start at 0. int PQgetlength(const PGresult *res, int tup_num, int field_num); 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 ASCII-represented values, this size has little to do with the binary size reported by PQfsize. PQprint Prints out all the tuples and, optionally, the attribute names to the specified output stream. void PQprint(FILE* fout, /* output stream */ const PGresult *res, const PQprintOpt *po); 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; /* insert to HTML table ... */ char *caption; /* HTML 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(const PGresult *res); PQcmdTuples Returns the number of rows affected by the SQL command. char * PQcmdTuples(const 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 object id of the tuple inserted, 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 object id of the tuple inserted, 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. The PQexec function is adequate for submitting queries in simple synchronous applications. It has a couple of major deficiencies however: PQexec waits for the query 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 control is buried inside PQexec, it is hard for the frontend to decide it would like to try to cancel the ongoing query. (It can be done from a signal handler, but not otherwise.) PQexec can return only one PGresult structure. If the submitted query 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 backend, to address that issue, the function PQsetnonblocking was added. Old applications can neglect to use PQsetnonblocking and get the older potentially blocking behavior. Newer programs can use PQsetnonblocking to achieve a completely non-blocking connection to the backend. 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 TRUE, blocking if arg is FALSE. 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 non-blocking mode and PQexec is called, it will temporarily set the state of the connection to blocking until the PQexec completes. More of libpq is expected to be made safe for PQsetnonblocking functionality in the near future. PQisnonblocking Returns the blocking status of the database connection. int PQisnonblocking(const PGconn *conn) Returns TRUE if the connection is set to non-blocking mode, FALSE if blocking. PQsendQuery Submit a query to Postgres without waiting for the result(s). TRUE is returned if the query was successfully dispatched, FALSE if not (in which case, use PQerrorMessage to get more information about the failure). int PQsendQuery(PGconn *conn, const char *query); After successfully calling PQsendQuery, call PQgetResult one or more times to obtain the query results. PQsendQuery may not be called again (on the same connection) until PQgetResult has returned NULL, indicating that the query is done. PQgetResult Wait for the next result from a prior PQsendQuery, and return it. NULL is returned when the query is complete and there will be no more results. PGresult *PQgetResult(PGconn *conn); PQgetResult must be called repeatedly until it returns NULL, indicating that the query is done. (If called when no query is active, PQgetResult will just return NULL 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 query 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 query 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 frontend can be handling the results of one query while the backend is still working on later queries in the same query string.) However, calling PQgetResult will still cause the frontend to block until the backend completes the next SQL command. This can be avoided by proper use of three more functions: PQconsumeInput If input is available from the backend, 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 is set). 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 routine will read available data and save it in a buffer, thereby causing a select(2) 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 query 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 backend; therefore PQconsumeInput must be invoked first, or the busy state will never end. PQflush Attempt to flush any data queued to the backend, 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 non-blocking 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 backend that has not actually been sent. Only applications that have used PQsetnonblocking have a need for this. PQsocket Obtain the file descriptor number for the backend connection socket. A valid descriptor will be >= 0; a result of -1 indicates that no backend connection is currently open. int PQsocket(const PGconn *conn); PQsocket should be used to obtain the backend socket descriptor in preparation for executing select(2). This allows an application using a blocking connection to wait for either backend responses or other conditions. If the result of select(2) indicates that data can be read from the backend socket, then PQconsumeInput should be called to read the data; after which, PQisBusy, PQgetResult, and/or PQnotifies can be used to process the response. Non-blocking 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 backend. A typical frontend using these functions will have a main loop that uses select(2) to wait for all the conditions that it must respond to. One of the conditions will be input available from the backend, which in select's terms is 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 "Asynchronous Notification", below). A frontend that uses PQsendQuery/PQgetResult can also attempt to cancel a query that is still being processed by the backend. PQrequestCancel Request that Postgres abandon processing of the current query. int PQrequestCancel(PGconn *conn); The return value is 1 if the cancel request was successfully dispatched, 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 query will terminate early and return an error result. If the cancellation fails (say, because the backend was already done processing the query), then there will be no visible result at all. Note that if the current query is part of a transaction, 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 queries that it issues through PQexec. Note that PQrequestCancel will have no effect if the connection is not currently open or the backend is not currently processing a query. Postgres provides a fast path interface to send function calls to the backend. This is a trapdoor into system internals and can be a potential security hole. Most users will not need this feature. PQfn Request execution of a backend 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); The fnid argument is the object identifier 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 backend is returned unmodified.) args and nargs specify the arguments to be passed to the function. typedef struct { int len; int isint; union { int *ptr; int integer; } u; } PQArgBlock; PQfn always returns a valid PGresult*. The resultStatus should be checked before the result is used. The caller is responsible for freeing the PGresult with PQclear when it is no longer needed. Postgres supports asynchronous notification via the LISTEN and NOTIFY commands. A backend registers its interest in a particular notification condition with the LISTEN command (and can stop listening with the UNLISTEN command). All backends listening on a particular condition will be notified asynchronously when a NOTIFY of that condition name is executed by any backend. 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 relation. Commonly the condition name is the same as the associated relation, but it is not necessary for there to be any associated relation. libpq applications submit LISTEN and UNLISTEN commands as ordinary SQL queries. Subsequently, arrival of NOTIFY messages can be detected by calling PQnotifies(). PQnotifies Returns the next notification from a list of unhandled notification messages received from the backend. Returns NULL 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[NAMEDATALEN]; /* name of relation * containing data */ int be_pid; /* process id of backend */ } PGnotify; After processing a PGnotify object returned by PQnotifies, be sure to free it with free() to avoid a memory leak. In Postgres 6.4 and later, the be_pid is the notifying backend's, whereas in earlier versions it was always your own backend's PID. The second sample program gives an example of the use of asynchronous notification. PQnotifies() does not actually read backend data; 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 queries, 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 queries to make is to call PQconsumeInput(), then check PQnotifies(). You can use select(2) to wait for backend data to arrive, 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 queries 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 query. COPY with libpq The COPY command in Postgres 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 PGRES_COPY_OUT or PGRES_COPY_IN result object from PQexec or PQgetResult. PQgetline Reads a newline-terminated line of characters (transmitted by the backend server) into a buffer string of size length. int PQgetline(PGconn *conn, char *string, int length) Like fgets(3), this routine copies up to length-1 characters into string. It is like gets(3), however, in that it converts the terminating newline into a null character. PQgetline returns EOF at EOF, 0 if the entire line has been read, and 1 if the buffer is full but the terminating newline has not yet been read. Notice that the application must check to see if a new line consists of the two characters "\.", which indicates that the backend 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 one recognizes the "\." line correctly (and does not, for example, mistake the end of a long data line for a terminator line). The code in src/bin/psql/copy.c contains example routines that correctly handle the copy protocol. PQgetlineAsync Reads a newline-terminated line of characters (transmitted by the backend server) into a buffer without blocking. int PQgetlineAsync(PGconn *conn, char *buffer, int bufsize) This routine 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 routine 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 routine 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 backend, 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 bufsize one smaller than the room actually available.) PQputline Sends a null-terminated string to the backend server. Returns 0 if OK, 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 backend that it has finished sending its data. PQputnbytes Sends a non-null-terminated string to the backend server. Returns 0 if OK, 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 Syncs with the backend. This function waits until the backend has finished the copy. It should either be issued when the last string has been sent to the backend using PQputline or when the last string has been received from the backend using PGgetline. It must be issued or the backend may get "out of sync" with the frontend. Upon return from this function, the backend is ready to receive the next query. The return value is 0 on successful completion, nonzero otherwise. int PQendcopy(PGconn *conn); As an example: PQexec(conn, "create table foo (a int4, b char(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); 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 NULL. 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 in or copy out command embedded in a series of SQL commands will be executed correctly. Older applications are likely to submit a copy in or copy out via PQexec and assume that the transaction is done after PQendcopy. This will work correctly only if the copy in/out is the only SQL command in the query string. PQtrace Enable tracing of the frontend/backend communication to a debugging file stream. void PQtrace(PGconn *conn FILE *debug_port) PQuntrace Disable tracing started by PQtrace void PQuntrace(PGconn *conn) PQsetNoticeProcessor Control reporting of notice and warning messages generated by libpq. typedef void (*PQnoticeProcessor) (void *arg, const char *message); PQnoticeProcessor PQsetNoticeProcessor(PGconn *conn, PQnoticeProcessor proc, void *arg); By default, libpq prints "notice" messages from the backend on stderr, as well as a few error messages that it generates by itself. This behavior can be overridden by supplying a callback function that does something else with the messages. The callback function is passed the text of the error 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 callback function pointer of NULL, 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 routines like PQgetvalue. environment variables The following environment variables can be used to select default connection parameter values, which will be used by PQconnectdb or PQsetdbLogin if no value is directly specified by the calling code. These are useful to avoid hard-coding database names into simple application programs. 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 Postgres backend. PGDATABASE PGDATABASE sets the default Postgres database name. PGUSER PGUSER sets the username used to connect to the database and for authentication. PGPASSWORD PGPASSWORD sets the password used if the backend demands password authentication. PGREALM sets the Kerberos realm to use with Postgres, if it is different from the local realm. If PGREALM is set, Postgres 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 backend. PGOPTIONS sets additional runtime options for the Postgres backend. PGTTY sets the file or tty on which debugging messages from the backend server are displayed. The following environment variables can be used to specify user-level default behavior for every Postgres session: PGDATESTYLE sets the default style of date/time representation. PGTZ sets the default time zone. PGCLIENTENCODING sets the default client encoding (if MULTIBYTE support was selected when configuring Postgres). The following environment variables can be used to specify default internal behavior for every Postgres session: PGGEQO sets the default mode for the genetic optimizer. Refer to the SET SQL command for information on correct values for these environment variables. threads with libpq libpq is thread-safe as of Postgres 7.0, so long as no two threads attempt to manipulate the same PGconn object at the same time. In particular, you can't issue concurrent queries from different threads through the same connection object. (If you need to run concurrent queries, 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 multi-thread programs. PQoidStatus can be replaced by PQoidValue. There is no good reason to call fe_setauthsvc at all. 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 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 it to the directory where libpq 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 or did not specify the right path. 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); free(notify); } } /* close the connection to the database and cleanup */ PQfinish(conn); return 0; }