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), perl5, 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 which 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. PQsetdbLogin Makes a new connection to a backend. PGconn *PQsetdbLogin(const char *pghost, const char *pgport, const char *pgoptions, const char *pgtty, const char *dbName, const char *login, const char *pwd) If any argument is NULL, then the corresponding environment variable (see "Environment Variables" section) is checked. If the environment variable is also not set, then hardwired defaults are used. The return value is a pointer to an abstract struct representing the connection to the backend. PQsetdb Makes a new connection to a backend. 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. PQconnectdb Makes a new connection to a backend. PGconn *PQconnectdb(const char *conninfo) This routine opens a new database connection using parameters taken from a string. Unlike PQsetdbLogin(), the parameter set can be extended without changing the function signature, so use of this routine is encouraged 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 a null value or a value containing spaces, surround it with single quotes, eg, 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 -- host to connect to. If a non-zero-length string is specified, TCP/IP communication is used. Without a host name, 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 -- database name. user -- user name for authentication. password -- password used if the backend demands password authentication. authtype -- authorization type. (No longer used, since the backend now chooses how to authenticate users. libpq still accepts and ignores this keyword for backward compatibility.) options -- trace/debug options to send to backend. tty -- file or tty for optional debug output from backend. Like PQsetdbLogin, PQconnectdb uses environment variables or built-in default values for unspecified options. 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 value */ 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 options - don't create a field by default */ int dispsize; /* Field size in characters for dialog */ }; Returns the address of the connection options structure. 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. 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. 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(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(PGconn *conn) PQpass Returns the password of the connection. char *PQpass(PGconn *conn) PQhost Returns the server host name of the connection. char *PQhost(PGconn *conn) PQport Returns the port of the connection. char *PQport(PGconn *conn) PQtty Returns the debug tty of the connection. char *PQtty(PGconn *conn) PQoptions Returns the backend options used in the connection. char *PQoptions(PGconn *conn) PQstatus Returns the status of the connection. The status can be CONNECTION_OK or CONNECTION_BAD. ConnStatusType *PQstatus(PGconn *conn) 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. PQerrorMessage Returns the error message most recently generated by an operation on the connection. char *PQerrorMessage(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(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! 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. PQresultStatus can return one of the following values: PGRES_EMPTY_QUERY, PGRES_COMMAND_OK, /* the query was a command returning no data */ PGRES_TUPLES_OK, /* the query successfully returned tuples */ PGRES_COPY_OUT, /* Copy Out (from server) data transfer started */ PGRES_COPY_IN, /* Copy In (to server) data transfer started */ PGRES_BAD_RESPONSE, /* an unexpected response was received */ 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. PQresultErrorMessage returns the error message associated with the query, or an empty string if there was no error. const char *PQresultErrorMessage(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. PQntuples Returns the number of tuples (instances) in the query result. int PQntuples(PGresult *res); PQnfields Returns the number of fields (attributes) in each tuple of the query result. int PQnfields(PGresult *res); PQbinaryTuples Returns 1 if the PGresult contains binary tuple data, 0 if it contains ASCII data. int PQbinaryTuples(PGresult *res); Currently, binary tuple data can only be returned by a query that extracts data from a BINARY cursor. PQfname Returns the field (attribute) name associated with the given field index. Field indices start at 0. char *PQfname(PGresult *res, int field_index); PQfnumber Returns the field (attribute) index associated with the given field name. int PQfnumber(PGresult *res, 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(PGresult *res, int field_num); PQfsize Returns the size in bytes of the field associated with the given field index. Field indices start at 0. int PQfsize(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. PQfmod Returns the type-specific modification data of the field associated with the given field index. Field indices start at 0. int PQfmod(PGresult *res, int field_index); PQgetvalue Returns a single field (attribute) value of one tuple of a PGresult. Tuple and field indices start at 0. char* PQgetvalue(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 TRUE, 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. PQgetlength Returns the length of a field (attribute) in bytes. Tuple and field indices start at 0. int PQgetlength(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. PQgetisnull Tests a field for a NULL entry. Tuple and field indices start at 0. int PQgetisnull(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.) 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. const 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. PQoidStatus Returns a string with the object id of the tuple inserted, if the SQL command was an INSERT. Otherwise, returns an empty string. char* PQoidStatus(PGresult *res); PQprint Prints out all the tuples and, optionally, the attribute names to the specified output stream. void PQprint(FILE* fout, /* output stream */ PGresult* res, PQprintOpt* po); struct _PQprintOpt { 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 */ }; This function is intended to replace PQprintTuples(), which is now obsolete. The psql program uses PQprint() to display query results. PQprintTuples Prints out all the tuples and, optionally, the attribute names to the specified output stream. void PQprintTuples(PGresult* res, FILE* fout, /* output stream */ int printAttName,/* print attribute names or not*/ int terseOutput, /* delimiter bars or not?*/ int width); /* width of column, variable width if 0*/ PQdisplayTuples Prints out all the tuples and, optionally, the attribute names to the specified output stream. void PQdisplayTuples(PGresult* res, FILE* fout, /* output stream */ int fillAlign, /* space fill to align columns */ const char *fieldSep, /* field separator */ int printHeader, /* display headers? */ int quiet); /* suppress print of row count at end */ PQdisplayTuples() was intended to supersede PQprintTuples(), and is in turn superseded by PQprint(). 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. 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. 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 TRUE if a query is busy, that is, PQgetResult would block waiting for input. A FALSE 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. 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(PGconn *conn); PQsocket should be used to obtain the backend socket descriptor in preparation for executing select(2). This allows an application 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. 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. It can also call PQnotifies to detect NOTIFY messages (see "Asynchronous Notification", below). An example is given in the sample programs section. 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 TRUE if the cancel request was successfully dispatched, FALSE 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, 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. NOTE: 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. Note that this will work OK whether you use PQsendQuery/PQgetResult or plain old PQexec for queries. 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. 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/psql.c contains 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, 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 char16, d float8)"); PQexec(conn, "copy foo from stdin"); PQputline(conn, "3<TAB>hello world<TAB>4.5\n"); PQputline(conn,"4<TAB>goodbye world<TAB>7.11\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. void PQsetNoticeProcessor (PGconn * conn, void (*noticeProcessor) (void * arg, const char * message), 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 frontend/backend authentication process is handled by PQconnectdb without any further intervention. The authentication method is now determined entirely by the DBA (see pga_hba.conf(5)). The following routines no longer have any effect and should not be used. fe_getauthname Returns a pointer to static space containing whatever name the user has authenticated. Use of this routine in place of calls to getenv(3) or getpwuid(3) by applications is highly recommended, as it is entirely possible that the authenticated user name is not the same as value of the USER environment variable or the user's entry in /etc/passwd. char *fe_getauthname(char* errorMessage) fe_setauthsvc Specifies that libpq should use authentication service name rather than its compiled-in default. This value is typically taken from a command-line switch. void fe_setauthsvc(char *name, char* errorMessage) Any error messages from the authentication attempts are returned in the errorMessage argument. 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 sets the default server name. If a non-zero-length string is specified, TCP/IP communication is used. Without a host name, libpq will connect using a local Unix domain socket. PGPORT sets the default port or local Unix domain socket file extension for communicating with the Postgres backend. PGDATABASE sets the default Postgres database name. PGUSER sets the username used to connect to the database and for authentication. 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. 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. PGRPLANS sets the default mode to allow or disable right-sided plans in the optimizer. PGCOSTHEAP sets the default cost for heap searches for the optimizer. PGCOSTINDEX sets the default cost for indexed searches for the optimizer. PGQUERY_LIMIT sets the maximum number of rows returned by a query. Refer to the SET SQL command for information on correct values for these environment variables. The query buffer is 8192 bytes long, and queries over that length will be rejected. /* * testlibpq.c Test the C version of Libpq, the Postgres 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 (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 instances from the pg_database, the system catalog of * databases */ res = PQexec(conn, "DECLARE mycursor CURSOR FOR select * from pg_database"); if (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 (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 instances */ 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); */ } /* * testlibpq2.c Test of the asynchronous notification interface * * 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]; * * Then start up this program After the program has begun, 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 (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); } /* * testlibpq3.c Test the C version of Libpq, the Postgres frontend * library. tests the binary cursor interface * * * * populate a database by doing the following: * * CREATE TABLE test1 (i int4, d float4, p polygon); * * INSERT INTO test1 values (1, 3.567, '(3.0, 4.0, 1.0, * 2.0)'::polygon); * * INSERT INTO test1 values (2, 89.05, '(4.0, 3.0, 2.0, * 1.0)'::polygon); * * the expected output is: * * tuple 0: got i = (4 bytes) 1, d = (4 bytes) 3.567000, p = (4 * bytes) 2 points boundbox = (hi=3.000000/4.000000, lo = * 1.000000,2.000000) tuple 1: got i = (4 bytes) 2, d = (4 bytes) * 89.050003, p = (4 bytes) 2 points boundbox = * (hi=4.000000/3.000000, lo = 2.000000,1.000000) * * */ #include <stdio.h> #include "libpq-fe.h" #include "utils/geo-decls.h" /* for the POLYGON type */ void exit_nicely(PGconn *conn) { PQfinish(conn); exit(1); } main() { char *pghost, *pgport, *pgoptions, *pgtty; char *dbName; int nFields; int i, j; int i_fnum, d_fnum, p_fnum; 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 = 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); } /* start a transaction block */ res = PQexec(conn, "BEGIN"); if (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 instances from the pg_database, the system catalog of * databases */ res = PQexec(conn, "DECLARE mycursor BINARY CURSOR FOR select * from test1"); if (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 (PQresultStatus(res) != PGRES_TUPLES_OK) { fprintf(stderr, "FETCH ALL command didn't return tuples properly\n"); PQclear(res); exit_nicely(conn); } i_fnum = PQfnumber(res, "i"); d_fnum = PQfnumber(res, "d"); p_fnum = PQfnumber(res, "p"); for (i = 0; i < 3; i++) { printf("type[%d] = %d, size[%d] = %d\n", i, PQftype(res, i), i, PQfsize(res, i)); } for (i = 0; i < PQntuples(res); i++) { int *ival; float *dval; int plen; POLYGON *pval; /* we hard-wire this to the 3 fields we know about */ ival = (int *) PQgetvalue(res, i, i_fnum); dval = (float *) PQgetvalue(res, i, d_fnum); plen = PQgetlength(res, i, p_fnum); /* * plen doesn't include the length field so need to * increment by VARHDSZ */ pval = (POLYGON *) malloc(plen + VARHDRSZ); pval->size = plen; memmove((char *) &pval->npts, PQgetvalue(res, i, p_fnum), plen); printf("tuple %d: got\n", i); printf(" i = (%d bytes) %d,\n", PQgetlength(res, i, i_fnum), *ival); printf(" d = (%d bytes) %f,\n", PQgetlength(res, i, d_fnum), *dval); printf(" p = (%d bytes) %d points \tboundbox = (hi=%f/%f, lo = %f,%f)\n", PQgetlength(res, i, d_fnum), pval->npts, pval->boundbox.xh, pval->boundbox.yh, pval->boundbox.xl, pval->boundbox.yl); } 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); }