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1.2. Database Connection Functions

The following routines deal with making a connection to a PostgreSQL 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 nonblocking analogues PQconnectStart and PQconnectPoll is preferred 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 an empty value or a value containing spaces, surround it with single quotes, e.g., keyword = 'a value'. Single quotes and backslashes within the value must be escaped with a backslash, e.g., \' or \\.) Spaces around the equal sign are optional. The currently recognized parameter keywords are:


    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.


    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 nonzero-length string is specified, TCP/IP communication is used.

    Using hostaddr instead of host allows the application to avoid a host name look-up, which may be important in applications with time constraints. However, Kerberos authentication requires the host name. The following therefore applies. If host is specified without hostaddr, a host name lookup is forced. If hostaddr is specified without host, the value for hostaddr gives the remote address; if Kerberos is used, this causes a reverse name query. If both host and hostaddr are specified, the value for hostaddr gives the remote address; the value for host is ignored, unless Kerberos is used, in which case that value is used for Kerberos authentication. Note that authentication is likely to fail if libpq is passed a host name that is not the name of the machine at hostaddr.

    Without either a host name or host address, libpq will connect using a local Unix domain socket.


    Port number to connect to at the server host, or socket file name extension for Unix-domain connections.


    The database name.


    User name to connect as.


    Password to be used if the server demands password authentication.


    Trace/debug options to be sent to the server.


    A file or tty for optional debug output from the backend.


    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 Section 1.10) 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 nonblocking 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:


    Waiting for connection to be made.


    Connection OK; waiting to send.


    Waiting for a response from the server.


    Received authentication; waiting for connection start-up to continue.


    Negotiating environment (part of the connection start-up).

    Note that, although these constants will remain (in order to maintain compatibility), an application should never rely upon these appearing in a particular order, or at all, or on the status always being one of these documented values. An application may do something like this:

            feedback = "Connecting...";
        case CONNECTION_MADE:
            feedback = "Connected to server...";
            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 nonblocking 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 PostgreSQL versions before 7.0, PQconndefaults returned a pointer to a static array, rather than a dynamically allocated array. That was not thread-safe, so the behavior has been changed.

  • PQfinish 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 server, using all the same parameters previously used. This may be useful for error recovery if a working connection is lost.

  • PQresetStart PQresetPoll Reset the communication port with the backend, in a nonblocking 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 server, using all the same parameters previously used. This may be useful for error recovery if a working connection is lost. They differ from PQreset (above) in that they act in a nonblocking manner. These functions suffer from the same restrictions as PQconnectStart and PQconnectPoll.

    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 PostgreSQL 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 SSL 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.