31.1. Database Connection Control Functions

Makes a new connection to the database server.

PGconn *PQconnectdbParams(const char **keywords, const char **values, int expand_dbname);

This function opens a new database connection using the parameters taken from two NULL-terminated arrays. The first, keywords, is defined as an array of strings, each one being a key word. The second, values, gives the value for each key word. Unlike PQsetdbLogin below, the parameter set can be extended without changing the function signature, so use of this function (or its nonblocking analogs PQconnectStartParams and PQconnectPoll) is preferred for new application programming.

When expand_dbname is non-zero, the dbname key word value is allowed to be recognized as a conninfo string. See below for details.

The passed arrays can be empty to use all default parameters, or can contain one or more parameter settings. They should be matched in length. Processing will stop with the last non-NULL element of the keywords array.

The currently recognized parameter key words are:

If any parameter is unspecified, then the corresponding environment variable (see Section 31.13) is checked. If the environment variable is not set either, then the indicated built-in defaults are used.

If expand_dbname is non-zero and dbname contains an = sign, it is taken as a conninfo string in exactly the same way as if it had been passed to PQconnectdb(see below). Previously processed key words will be overridden by key words in the conninfo string.

In general key words are processed from the beginning of these arrays in index order. The effect of this is that when key words are repeated, the last processed value is retained. Therefore, through careful placement of the dbname key word, it is possible to determine what may be overridden by a conninfo string, and what may not.

Makes a new connection to the database server.

PGconn *PQconnectdb(const char *conninfo);

This function opens a new database connection using the parameters taken from the string conninfo.

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. Spaces around the equal sign are optional. To write an empty value, or a value containing spaces, surround it with single quotes, e.g., keyword = 'a value'. Single quotes and backslashes within the value must be escaped with a backslash, i.e., \' and \\.

The currently recognized parameter key words are the same as above.

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 set of parameters. It has the same functionality except that the missing parameters will always take on default values. Write NULL or an empty string for any one of the fixed parameters that is to be defaulted.

If the dbName contains an = sign, it is taken as a conninfo string in exactly the same way as if it had been passed to PQconnectdb, and the remaining parameters are then applied as above.

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 for backward compatibility with very old programs.

Make a connection to the database server in a nonblocking manner.

PGconn *PQconnectStartParams(const char **keywords, const char **values, int expand_dbname);

PGconn *PQconnectStart(const char *conninfo);

PostgresPollingStatusType PQconnectPoll(PGconn *conn);

These three functions are used to open a connection to a database server such that your application's thread of execution is not blocked on remote I/O whilst doing so. The point of this approach is that the waits for I/O to complete can occur in the application's main loop, rather than down inside PQconnectdbParams or PQconnectdb, and so the application can manage this operation in parallel with other activities.

With PQconnectStartParams, the database connection is made using the parameters taken from the keywords and values arrays, and controlled by expand_dbname, as described above for PQconnectdbParams.

With PQconnectStart, the database connection is made using the parameters taken from the string conninfo as described above for PQconnectdb.

Neither PQconnectStartParams nor PQconnectStart nor PQconnectPoll will block, so 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 PQconnectdbParams above for details.

• If you call PQtrace, ensure that the stream object into which you trace will not block.

• You ensure that the socket is in the appropriate state before calling PQconnectPoll, as described below.

Note: use of PQconnectStartParams is analogous to PQconnectStart shown below.

To begin a nonblocking connection request, call conn = PQconnectStart("connection_info_string"). If conn is null, then libpq has been unable to allocate a new PGconn structure. Otherwise, a valid PGconn pointer is returned (though not yet representing a valid connection to the database). On return from PQconnectStart, call status = PQstatus(conn). If status equals CONNECTION_BAD, PQconnectStart has failed.

If PQconnectStart succeeds, the next stage is to poll libpq so that it can proceed with the connection sequence. Use PQsocket(conn) to obtain the descriptor of the socket underlying the database connection. Loop thus: If PQconnectPoll(conn) last returned PGRES_POLLING_READING, wait until the socket is ready to read (as indicated by select(), poll(), or similar system function). Then call PQconnectPoll(conn) again. Conversely, if PQconnectPoll(conn) last returned PGRES_POLLING_WRITING, wait until the socket is ready to write, then call PQconnectPoll(conn) again. If you have yet to call PQconnectPoll, i.e., just after the call to PQconnectStart, behave as if it last returned PGRES_POLLING_WRITING. Continue this loop until PQconnectPoll(conn) returns PGRES_POLLING_FAILED, indicating the connection procedure has failed, or PGRES_POLLING_OK, indicating the connection has been successfully made.

At any time during connection, the status of the connection can be checked by calling PQstatus. If this gives CONNECTION_BAD, then the connection procedure has failed; if it gives CONNECTION_OK, then the connection is ready. Both of these states are equally detectable from the return value of PQconnectPoll, described above. Other states might also occur during (and only during) an asynchronous connection procedure. These indicate the current stage of the connection procedure and might be useful to provide feedback to the user for example. These statuses are:

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

switch(PQstatus(conn))
{
        case CONNECTION_STARTED:
            feedback = "Connecting...";
            break;

        case CONNECTION_MADE:
            feedback = "Connected to server...";
            break;
.
.
.
        default:
            feedback = "Connecting...";
}

The connect_timeout connection parameter is ignored when using PQconnectPoll; it is the application's responsibility to decide whether an excessive amount of time has elapsed. Otherwise, PQconnectStart followed by a PQconnectPoll loop is equivalent to PQconnectdb.

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 the connection attempt fails or is abandoned.

Returns the default connection options.

PQconninfoOption *PQconndefaults(void);

typedef struct
{
    char   *keyword;   /* The keyword of the option */
    char   *envvar;    /* Fallback environment variable name */
    char   *compiled;  /* Fallback compiled in default value */
    char   *val;       /* Option's current value, or NULL */
    char   *label;     /* Label for field in connect dialog */
    char   *dispchar;  /* Indicates how to display this field
                          in a connect dialog. Values are:
                          ""        Display entered value as is
                          "*"       Password field - hide value
                          "D"       Debug option - don't show by default */
    int     dispsize;  /* Field size in characters for dialog */
} PQconninfoOption;

Returns a connection options array. This can be used to determine all possible PQconnectdb options and their current default values. The return value points to an array of PQconninfoOption structures, which ends with an entry having a null keyword pointer. The null pointer is returned if memory could not be allocated. Note that the current default values (val fields) will depend on environment variables and other context. Callers must treat the connection options data as read-only.

After processing the options array, free it by passing it to PQconninfoFree. If this is not done, a small amount of memory is leaked for each call to PQconndefaults.

Returns parsed connection options from the provided connection string.

PQconninfoOption *PQconninfoParse(const char *conninfo, char **errmsg);

Parses a connection string and returns the resulting options as an array; or returns NULL if there is a problem with the connection string. This can be used to determine the PQconnectdb options in the provided connection string. The return value points to an array of PQconninfoOption structures, which ends with an entry having a null keyword pointer.

Note that only options explicitly specified in the string will have values set in the result array; no defaults are inserted.

If errmsg is not NULL, then *errmsg is set to NULL on success, else to a malloc'd error string explaining the problem. (It is also possible for *errmsg to be set to NULL even when NULL is returned; this indicates an out-of-memory situation.)

After processing the options array, free it by passing it to PQconninfoFree. If this is not done, some memory is leaked for each call to PQconninfoParse. Conversely, if an error occurs and errmsg is not NULL, be sure to free the error string using PQfreemem.

Closes the connection to the server. Also frees memory used by the PGconn object.

void PQfinish(PGconn *conn);

Note that even if the server connection attempt fails (as indicated by PQstatus), the application should call PQfinish to free the memory used by the PGconn object. The PGconn pointer must not be used again after PQfinish has been called.

Resets the communication channel to the server.

void PQreset(PGconn *conn);

This function will close the connection to the server and attempt to reestablish a new connection to the same server, using all the same parameters previously used. This might be useful for error recovery if a working connection is lost.

Reset the communication channel to the server, in a nonblocking manner.

int PQresetStart(PGconn *conn);

PostgresPollingStatusType PQresetPoll(PGconn *conn);

These functions will close the connection to the server and attempt to reestablish a new connection to the same server, using all the same parameters previously used. This can 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 PQconnectStartParams, PQconnectStart and PQconnectPoll.

To initiate a connection reset, call PQresetStart. If it returns 0, the reset has failed. If it returns 1, poll the reset using PQresetPoll in exactly the same way as you would create the connection using PQconnectPoll.