The following functions deal with making a connection to a
PostgreSQL backend server. An
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 the function PQconnectdb,
PQconnectdbParams, 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
the return value for a successful connection before queries are sent
via the connection object.
| Warning | On Unix, forking a process with open libpq connections can lead to
unpredictable results because the parent and child processes share
the same sockets and operating system resources. For this reason,
such usage is not recommended, though doing an exec from
the child process to load a new executable is safe.
|
Note: On Windows, there is a way to improve performance if a single
database connection is repeatedly started and shutdown. Internally,
libpq calls WSAStartup() and WSACleanup() for connection startup
and shutdown, respectively. WSAStartup() increments an internal
Windows library reference count which is decremented by WSACleanup().
When the reference count is just one, calling WSACleanup() frees
all resources and all DLLs are unloaded. This is an expensive
operation. To avoid this, an application can manually call
WSAStartup() so resources will not be freed when the last database
connection is closed.
PQconnectdbParams 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 31.1.1.
When expand_dbname is non-zero, the
dbname key word value is allowed to be recognized
as a connection string. More details on the possible formats appear in
Section 31.1.2.
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.
If any parameter is unspecified, then the corresponding
environment variable (see Section 31.14)
is checked. If the environment variable is not set either,
then the indicated built-in defaults are used.
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.
PQconnectdb 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 31.1.2 for details.
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
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 in Section 31.1.2.
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
for backward compatibility with very old programs.
PQconnectStartParams
PQconnectStart
PQconnectPoll
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:
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 in Section 31.1.1 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 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:
- CONNECTION_STARTED
Waiting for connection to be made.
- CONNECTION_MADE
Connection OK; waiting to send.
- CONNECTION_AWAITING_RESPONSE
Waiting for a response from the server.
- CONNECTION_AUTH_OK
Received authentication; waiting for backend start-up to finish.
- CONNECTION_SSL_STARTUP
Negotiating SSL encryption.
- CONNECTION_SETENV
Negotiating environment-driven parameter settings.
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.
PQconndefaults 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.
PQconninfoParse 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.
PQfinish 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.
PQreset 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.
PQresetStart
PQresetPoll 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.
PQpingParams PQpingParams reports the status of the
server. It accepts connection parameters identical to those of
PQconnectdbParams, described above. It is not, however,
necessary to supply correct user name, password, or database name
values to obtain the server status.
PGPing PQpingParams(const char * const *keywords,
const char * const *values,
int expand_dbname);
The function returns one of the following values:
- PQPING_OK
The server is running and appears to be accepting connections.
- PQPING_REJECT
The server is running but is in a state that disallows connections
(startup, shutdown, or crash recovery).
- PQPING_NO_RESPONSE
The server could not be contacted. This might indicate that the
server is not running, or that there is something wrong with the
given connection parameters (for example, wrong port number), or
that there is a network connectivity problem (for example, a
firewall blocking the connection request).
- PQPING_NO_ATTEMPT
No attempt was made to contact the server, because the supplied
parameters were obviously incorrect or there was some client-side
problem (for example, out of memory).
PQping PQping reports the status of the
server. It accepts connection parameters identical to those of
PQconnectdb, described above. It is not, however,
necessary to supply correct user name, password, or database name
values to obtain the server status.
PGPing PQping(const char *conninfo);
The return values are the same as for PQpingParams.
The currently recognized parameter key words 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 behavior when host is not specified
is to connect to a Unix-domain
socket in
/tmp (or whatever socket directory was specified
when PostgreSQL was built). On machines without
Unix-domain sockets, the default is to connect to localhost.
- hostaddr
Numeric IP address of host to connect to. This should be in the
standard IPv4 address format, e.g., 172.28.40.9. If
your machine supports IPv6, you can also use those addresses.
TCP/IP communication is
always used when a nonempty string is specified for this parameter.
Using hostaddr instead of host allows the
application to avoid a host name look-up, which might be important
in applications with time constraints. However, a host name is
required for Kerberos, GSSAPI, or SSPI authentication
methods, as well as for verify-full SSL
certificate verification. The following rules are used:
If host is specified without hostaddr,
a host name lookup occurs.
If hostaddr is specified without host,
the value for hostaddr gives the server network address.
The connection attempt will fail if the authentication
method requires a host name.
If both host and hostaddr are specified,
the value for hostaddr gives the server network address.
The value for host is ignored unless the
authentication method requires it, in which case it will be
used as the host name.
Note that authentication is likely to fail if host
is not the name of the server at network address hostaddr.
Also, note that host rather than hostaddr
is used to identify the connection in ~/.pgpass (see
Section 31.15).
Without either a host name or host address,
libpq will connect using a
local Unix-domain socket; or on machines without Unix-domain
sockets, it will attempt to connect to localhost.
- port
Port number to connect to at the server host, or socket file
name extension for Unix-domain
connections.
- dbname
The database name. Defaults to be the same as the user name.
In certain, very specific contexts, the value is checked for extended
formats; see Section 31.1.2 for more details on
those.
- user
PostgreSQL user name to connect as.
Defaults to be the same as the operating system name of the user
running the application.
- password
Password to be used if the server demands password authentication.
- connect_timeout
Maximum wait for connection, in seconds (write as a decimal integer
string). Zero or not specified means wait indefinitely. It is not
recommended to use a timeout of less than 2 seconds.
- client_encoding
This sets the client_encoding
configuration parameter for this connection. In addition to
the values accepted by the corresponding server option, you
can use auto to determine the right
encoding from the current locale in the client
(LC_CTYPE environment variable on Unix
systems).
- options
Adds command-line options to send to the server at run-time.
For example, setting this to -c geqo=off sets the
session's value of the geqo parameter to
off. For a detailed discussion of the available
options, consult Chapter 18.
- application_name
Specifies a value for the application_name
configuration parameter.
- fallback_application_name
Specifies a fallback value for the application_name configuration parameter.
This value will be used if no value has been given for
application_name via a connection parameter or the
PGAPPNAME environment variable. Specifying
a fallback name is useful in generic utility programs that
wish to set a default application name but allow it to be
overridden by the user.
- keepalives
Controls whether client-side TCP keepalives are used. The default
value is 1, meaning on, but you can change this to 0, meaning off,
if keepalives are not wanted. This parameter is ignored for
connections made via a Unix-domain socket.
- keepalives_idle
Controls the number of seconds of inactivity after which TCP should
send a keepalive message to the server. A value of zero uses the
system default. This parameter is ignored for connections made via a
Unix-domain socket, or if keepalives are disabled. It is only supported
on systems where the TCP_KEEPIDLE or TCP_KEEPALIVE
socket option is available, and on Windows; on other systems, it has no
effect.
- keepalives_interval
Controls the number of seconds after which a TCP keepalive message
that is not acknowledged by the server should be retransmitted. A
value of zero uses the system default. This parameter is ignored for
connections made via a Unix-domain socket, or if keepalives are disabled.
It is only supported on systems where the TCP_KEEPINTVL
socket option is available, and on Windows; on other systems, it has no
effect.
- keepalives_count
Controls the number of TCP keepalives that can be lost before the
client's connection to the server is considered dead. A value of
zero uses the system default. This parameter is ignored for
connections made via a Unix-domain socket, or if keepalives are disabled.
It is only supported on systems where the TCP_KEEPCNT
socket option is available; on other systems, it has no effect.
- tty
Ignored (formerly, this specified where to send server debug output).
- sslmode
This option determines whether or with what priority a secure
SSL TCP/IP connection will be negotiated with the
server. There are six modes:
- disable
only try a non-SSL connection
- allow
first try a non-SSL connection; if that
fails, try an SSL connection
- prefer (default)
first try an SSL connection; if that fails,
try a non-SSL connection
- require
only try an SSL connection. If a root CA
file is present, verify the certificate in the same way as
if verify-ca was specified
- verify-ca
only try an SSL connection, and verify that
the server certificate is issued by a trusted
certificate authority (CA)
- verify-full
only try an SSL connection, verify that the
server certificate is issued by a
trusted CA and that the server host name
matches that in the certificate
See Section 31.18 for a detailed description of how
these options work.
sslmode is ignored for Unix domain socket
communication.
If PostgreSQL is compiled without SSL support,
using options require, verify-ca, or
verify-full will cause an error, while
options allow and prefer will be
accepted but libpq will not actually attempt
an SSL
connection.
- requiressl
This option is deprecated in favor of the sslmode
setting.
If set to 1, an SSL connection to the server
is required (this is equivalent to sslmode
require). libpq will then refuse
to connect if the server does not accept an
SSL connection. If set to 0 (default),
libpq will negotiate the connection type with
the server (equivalent to sslmode
prefer). This option is only available if
PostgreSQL is compiled with SSL support.
- sslcompression
If set to 1 (default), data sent over SSL connections will be
compressed (this requires OpenSSL version
0.9.8 or later).
If set to 0, compression will be disabled (this requires
OpenSSL 1.0.0 or later).
This parameter is ignored if a connection without SSL is made,
or if the version of OpenSSL used does not support
it.
Compression uses CPU time, but can improve throughput if
the network is the bottleneck.
Disabling compression can improve response time and throughput
if CPU performance is the limiting factor.
- sslcert
This parameter specifies the file name of the client SSL
certificate, replacing the default
~/.postgresql/postgresql.crt.
This parameter is ignored if an SSL connection is not made.
- sslkey
This parameter specifies the location for the secret key used for
the client certificate. It can either specify a file name that will
be used instead of the default
~/.postgresql/postgresql.key, or it can specify a key
obtained from an external "engine" (engines are
OpenSSL loadable modules). An external engine
specification should consist of a colon-separated engine name and
an engine-specific key identifier. This parameter is ignored if an
SSL connection is not made.
- sslrootcert
This parameter specifies the name of a file containing SSL
certificate authority (CA) certificate(s).
If the file exists, the server's certificate will be verified
to be signed by one of these authorities. The default is
~/.postgresql/root.crt.
- sslcrl
This parameter specifies the file name of the SSL certificate
revocation list (CRL). Certificates listed in this file, if it
exists, will be rejected while attempting to authenticate the
server's certificate. The default is
~/.postgresql/root.crl.
- requirepeer
This parameter specifies the operating-system user name of the
server, for example requirepeer=postgres.
When making a Unix-domain socket connection, if this
parameter is set, the client checks at the beginning of the
connection that the server process is running under the specified
user name; if it is not, the connection is aborted with an error.
This parameter can be used to provide server authentication similar
to that available with SSL certificates on TCP/IP connections.
(Note that if the Unix-domain socket is in
/tmp or another publicly writable location,
any user could start a server listening there. Use this parameter
to ensure that you are connected to a server run by a trusted user.)
This option is only supported on platforms for which the
peer authentication method is implemented; see
Section 19.3.7.
- krbsrvname
Kerberos service name to use when authenticating with Kerberos 5
or GSSAPI.
This must match the service name specified in the server
configuration for Kerberos authentication to succeed. (See also
Section 19.3.5 and Section 19.3.3.)
- gsslib
GSS library to use for GSSAPI authentication. Only used on Windows.
Set to gssapi to force libpq to use the GSSAPI
library for authentication instead of the default SSPI.
- service
Service name to use for additional parameters. It specifies a service
name in pg_service.conf that holds additional connection parameters.
This allows applications to specify only a service name so connection parameters
can be centrally maintained. See Section 31.16.
Several libpq functions parse a user-specified string to obtain
connection parameters. There are two accepted formats for these strings:
plain keyword = value strings, and URIs.
In the first format, 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 listed in
Section 31.1.1.
The general form for connection URI is the
following:
postgresql://username:password@hostname:port/database?param1=value1¶m2=value2&...
postgresql:///path/to/pgsql/socket/dir?param1=value1
The URI designator can be either
postgresql:// or postgres:// and
each of the URI parts is optional. The following
examples illustrate valid URI syntax uses:
postgresql://
postgresql://localhost
postgresql://localhost:5433
postgresql://localhost/mydb
postgresql://user@localhost
postgresql://user:secret@localhost
postgresql://other@localhost/otherdb
Additional connection parameters may optionally follow the base URI.
Any connection parameters not corresponding to key words listed
below are ignored and a warning message about them is sent to
stderr.
For improved compatibility with JDBC connection URI
syntax, instances of parameter ssl=true are translated
into sslmode=require (see above.)
Percent-encoding may be used to include a symbol with special
meaning in any of the URI parts.
The host part may be either hostname or an IP address. To specify an
IPv6 host address, enclose it in square brackets:
postgresql://[::1]/database
As a special case, a host part which starts with / is
treated as a local Unix socket directory to look for the connection
socket special file:
postgresql:///path/to/pgsql/socket/dir
The whole connection string up to the extra parameters designator
(?) is treated as the absolute path to the socket
directory (/path/to/pgsql/socket/dir in this
example.) To specify a non-default database name in this case use the
following syntax:
postgresql:///path/to/pgsql/socket/dir?dbname=other
|
