34.1. Database Connection Control Functions

Makes a new connection to the database server.

PGconn *PQconnectdbParams(const char * const *keywords,
                          const char * const *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.

The currently recognized parameter key words are listed in Section 34.1.2 .

The passed arrays can be empty to use all default parameters, or can contain one or more parameter settings. They must be matched in length. Processing will stop at the first NULL entry in the keywords array. Also, if the values entry associated with a non- NULL keywords entry is NULL or an empty string, that entry is ignored and processing continues with the next pair of array entries.

When expand_dbname is non-zero, the value for the first dbname key word is checked to see if it is a connection string . If so, it is " expanded " into the individual connection parameters extracted from the string. The value is considered to be a connection string, rather than just a database name, if it contains an equal sign ( = ) or it begins with a URI scheme designator. (More details on connection string formats appear in Section 34.1.1 .) Only the first occurrence of dbname is treated in this way; any subsequent dbname parameter is processed as a plain database name.

In general the parameter arrays are processed from start to end. If any key word is repeated, the last value (that is not NULL or empty) is used. This rule applies in particular when a key word found in a connection string conflicts with one appearing in the keywords array. Thus, the programmer may determine whether array entries can override or be overridden by values taken from a connection string. Array entries appearing before an expanded dbname entry can be overridden by fields of the connection string, and in turn those fields are overridden by array entries appearing after dbname (but, again, only if those entries supply non-empty values).

After processing all the array entries and any expanded connection string, any connection parameters that remain unset are filled with default values. If an unset parameter's corresponding environment variable (see Section 34.14 ) is set, its value is used. If the environment variable is not set either, then the parameter's built-in default value is used.

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, or it can contain a URI . See Section 34.1.1 for details.

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 or has a valid connection URI prefix, 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 specified for PQconnectdbParams .

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 * const *keywords,
                             const char * const *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:

To begin a nonblocking connection request, call PQconnectStart or PQconnectStartParams . If the result 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). Next call PQstatus(conn) . If the result is CONNECTION_BAD , the connection attempt has already failed, typically because of invalid connection parameters.

If PQconnectStart or PQconnectStartParams 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. (Caution: do not assume that the socket remains the same across PQconnectPoll calls.) 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. On the first iteration, i.e., if you have yet to call PQconnectPoll , 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 call returns CONNECTION_BAD , then the connection procedure has failed; if the call returns 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 when PQconnectStart or PQconnectStartParams 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. A missing or invalid service file will be silently ignored. 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 the connection options used by a live connection.

PQconninfoOption *PQconninfo(PGconn *conn);

Returns a connection options array. This can be used to determine all possible PQconnectdb options and the values that were used to connect to the server. The return value points to an array of PQconninfoOption structures, which ends with an entry having a null keyword pointer. All notes above for PQconndefaults also apply to the result of PQconninfo .

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 function can be used to extract 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.

All legal options will be present in the result array, but the PQconninfoOption for any option not present in the connection string will have val set to NULL ; default values are not 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 and the function to return NULL ; this indicates an out-of-memory condition.)

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 establish a new connection, 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 establish a new connection, 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 .

PQpingParams reports the status of the server. It accepts connection parameters identical to those of PQconnectdbParams , described above. It is not necessary to supply correct user name, password, or database name values to obtain the server status; however, if incorrect values are provided, the server will log a failed connection attempt.

PGPing PQpingParams(const char * const *keywords,
                    const char * const *values,
                    int expand_dbname);

The function returns one of the following values:

PQping reports the status of the server. It accepts connection parameters identical to those of PQconnectdb , described above. It is not necessary to supply correct user name, password, or database name values to obtain the server status; however, if incorrect values are provided, the server will log a failed connection attempt.

PGPing PQping(const char *conninfo);

The return values are the same as for PQpingParams .