path: root/doc/src/sgml/libpq.sgml

<Chapter>
<Title><FileName>libpq</FileName></Title>

<Para>
     <FileName>libpq</FileName> is the application programming interface to <ProductName>Postgres</ProductName>.  
     <FileName>libpq</FileName> is a set of library routines which  allows
     client programs to pass queries to the <ProductName>Postgres</ProductName> backend
     server and to receive the results of these queries.
     This version  of  the  documentation  describes  the  <Acronym>C</Acronym>
     interface  library.   Three short programs are included
     at the end of this section to show how  to  write  programs that use <FileName>libpq</FileName>.
     There are several examples of <FileName>libpq</FileName> applications in the
     following directories:

<ProgramListing>
    ../src/test/regress
    ../src/test/examples
    ../src/bin/psql
</ProgramListing>

<Para>
     Frontend programs which  use  <FileName>libpq</FileName>  must  include  the
     header  file  <FileName>libpq-fe.h</FileName>  and  must link with the <FileName>libpq</FileName>
     library.
</Para>

<Sect1>
<Title>Control and Initialization</Title>

<Para>
     The following environment variables can be used to  set
     up  default  environment  values  to  avoid hard-coding
     database names into an application program:

<ItemizedList>
<ListItem>
<Para>
<Acronym>PGHOST</Acronym> sets the default server name.
If it is set to a non-zero-length string, it causes TCP/IP
communication to be used, rather than the default local Unix domain sockets.
</Para>
</ListItem>
<ListItem>
<Para>
<Acronym>PGOPTIONS</Acronym> sets additional runtime  options  for  the <ProductName>Postgres</ProductName> backend.
</Para>
</ListItem>
<ListItem>
<Para>
<Acronym>PGPORT</Acronym>  sets the default port or local Unix domain socket
file extension for communicating with the <ProductName>Postgres</ProductName>
backend.
</Para>
</ListItem>
<ListItem>
<Para>
<Acronym>PGTTY</Acronym> sets the file or tty on which  debugging  messages from the backend server are displayed.
</Para>
</ListItem>
<ListItem>
<Para>
<Acronym>PGDATABASE</Acronym>  sets the default <ProductName>Postgres</ProductName> database name.
</Para>
</ListItem>
<ListItem>
<Para>
<Acronym>PGREALM</Acronym> sets the Kerberos realm to  use  with  <ProductName>Postgres</ProductName>,
  if  it is different from the local realm.  If
<Acronym>PGREALM</Acronym> is set, <ProductName>Postgres</ProductName> 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 enabled.
</Para>
</ListItem>
</ItemizedList>
</Para>

<Para>
The following environment variables can be used to specify user-level default
behavior for every Postgres session:

<ItemizedList>
<ListItem>
<Para>
<Acronym>PGDATESTYLE</Acronym>
sets the default style of date/time representation.
</Para>
</ListItem>
<ListItem>
<Para>
<Acronym>PGTZ</Acronym>
sets the default time zone.
</Para>
</ListItem>
</ItemizedList>
</Para>

<Para>
The following environment variables can be used to specify default internal
behavior for every Postgres session:

<ItemizedList>
<ListItem>
<Para>
<Acronym>PGGEQO</Acronym>
sets the default mode for the genetic optimizer.
</Para>
</ListItem>
<ListItem>
<Para>
<Acronym>PGRPLANS</Acronym>
sets the default mode to allow or disable right-sided plans in the optimizer.
</Para>
</ListItem>
<ListItem>
<Para>
<Acronym>PGCOSTHEAP</Acronym>
sets the default cost for heap searches for the optimizer.
</Para>
</ListItem>
<ListItem>
<Para>
<Acronym>PGCOSTINDEX</Acronym>
sets the default cost for indexed searches for the optimizer.
</Para>
</ListItem>
</ItemizedList>
</Para>

<Para>
See the set(l)
man page for information on the arguments for these environment variables.

</Sect1>

<Sect1>
<Title>Database Connection Functions</Title>

<Para>
     The following routines deal with making a connection to
     a backend from a <Acronym>C</Acronym> program.
<ItemizedList>
<ListItem>
<Para>
<Function>PQsetdbLogin</Function> 
          Makes a new connection to a backend.
<ProgramListing>
PGconn *PQsetdbLogin(const char *pghost,
                const char *pgport,
                const char *pgoptions,
                const char *pgtty,
                const char *dbName,
                const char *login,
                const char *pwd);
</ProgramListing>
          If  any  argument  is NULL, then the corresponding
          environment variable is checked. If the  environment  variable  is  also  not  set, then hardwired
          defaults are used.
          PQsetdbLogin always returns  a  valid  PGconn  pointer.
          The  PQstatus (see below) command should be called
          to ensure that  a  connection  was  properly  made
          before queries are sent via the connection.  <FileName>libpq</FileName>
          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 as they are subject  to  change  in  the
          future.
</Para>
</ListItem>

<ListItem>
<Para>
<Function>PQsetdb</Function> 
          Makes a new connection to a backend.
<ProgramListing>
PGconn *PQsetdb(char *pghost,
                char *pgport,
                char *pgoptions,
                char *pgtty,
                char *dbName);
</ProgramListing>
          This is a macro that calls PQsetdbLogin() with null pointers
          for the login and pwd parameters.
</Para>
</ListItem>

<ListItem>
<Para>
<Function>PQconndefaults</Function>  
         Returns the default connection options.
<ProgramListing>
PQconninfoOption *PQconndefaults(void)

struct PQconninfoOption
	{
		char   *keyword;   /* The keyword of the option	*/
		char   *environ;   /* Fallback environment variable name */
		char   *compiled;  /* Fallback compiled in default value */
		char   *val;	   /* Options 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 */
	};

</ProgramListing>
	Returns the address of the connection options structure.  This may
	be used to determine all possible options and their current values.
</Para>
</ListItem>

<ListItem>
<Para>
<Function>PQdb</Function>  
         Returns the database name of the connection.
<ProgramListing>
char *PQdb(PGconn *conn)
</ProgramListing>
</Para>
</ListItem>

<ListItem>
<Para>
<Function>PQhost</Function>
         Returns the host name of the connection.
<ProgramListing>
char *PQhost(PGconn *conn)
</ProgramListing>
</Para>
</ListItem>

<ListItem>
<Para>
<Function>PQoptions</Function>
       Returns the pgoptions used in  the  connection.
<ProgramListing>
char *PQoptions(PGconn *conn)
</ProgramListing>
</Para>
</ListItem>

<ListItem>
<Para>
<Function>PQport</Function>
         Returns the pgport of the connection.
<ProgramListing>
char *PQport(PGconn *conn)
</ProgramListing>
</Para>
</ListItem>

<ListItem>
<Para>
<Function>PQtty</Function>
         Returns the pgtty of the connection.
<ProgramListing>
char *PQtty(PGconn *conn)
</ProgramListing>
</Para>
</ListItem>

<ListItem>
<Para>
<Function>PQstatus</Function>
         Returns the status of the connection. 
         The status can be CONNECTION_OK or CONNECTION_BAD.
<ProgramListing>
ConnStatusType *PQstatus(PGconn *conn)
</ProgramListing>
</Para>
</ListItem>

<ListItem>
<Para>
<Function>PQerrorMessage</Function>
         Returns the error  message  associated with the connection
<ProgramListing>
char *PQerrorMessage(PGconn* conn);
</ProgramListing>
</Para>
</ListItem>

<ListItem>
<Para>
<Function>PQfinish</Function>
          Close  the  connection to the backend.  Also frees
          memory used by the PGconn structure.   The  PGconn
          pointer should not be used after PQfinish has been
          called.
<ProgramListing>
void PQfinish(PGconn *conn)
</ProgramListing>
</Para>
</ListItem>

<ListItem>
<Para>
<Function>PQreset</Function>
          Reset the communication  port  with  the  backend.
          This function will close the IPC socket connection
          to the backend and attempt to  reestablish  a  new
          connection to the same postmaster.
<ProgramListing>
void PQreset(PGconn *conn)
</ProgramListing>
</Para>
</ListItem>
</ItemizedList>
</Para>
</Sect1>

<Sect1>
<Title>Query Execution Functions</Title>

<Para>
<ItemizedList>
<ListItem>
<Para>
<Function>PQexec</Function>
          Submit a query to <ProductName>Postgres</ProductName>.   Returns  a  PGresult
          pointer or possibly a NULL pointer.  If a NULL is returned, it
	  should be treated like a PGRES_FATAL_ERROR result: use
	  PQerrorMessage to get more information about the error.
<ProgramListing>
PGresult *PQexec(PGconn *conn,
                 const char *query);
</ProgramListing>
          The  <Function>PGresult</Function>  structure  encapsulates  the  query
          result returned by the backend.  <Function>libpq</Function> programmers
          should   be   careful  to  maintain  the  PGresult
          abstraction. Use the accessor functions  described
          below to retrieve the results of the query.  Avoid
          directly referencing the fields  of  the  PGresult
          structure  as  they  are  subject to change in the
          future.
</Para>
</ListItem>

<ListItem>
<Para>
<Function>PQresultStatus</Function>
          Returns the result status of the query.  PQresultStatus can return one of the following values:
<ProgramListing>
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,
PGRES_COPY_IN,
PGRES_BAD_RESPONSE, /* an unexpected response was received */
PGRES_NONFATAL_ERROR,
PGRES_FATAL_ERROR
</ProgramListing>
          If  the result status is PGRES_TUPLES_OK, then the
          following routines can be  used  to  retrieve  the
          tuples returned by the query.
</Para>
</ListItem>

<ListItem>
<Para>
<Function>PQntuples</Function> returns the number of tuples (instances)
          in the query result.
<ProgramListing>
int PQntuples(PGresult *res);
</ProgramListing>
</Para>
</ListItem>

<ListItem>
<Para>
<Function>PQnfields</Function>
          Returns   the   number    of    fields
          (attributes) in the query result.
<ProgramListing>
int PQnfields(PGresult *res);
</ProgramListing>
</Para>
</ListItem>

<ListItem>
<Para>
<Function>PQfname</Function>
            Returns the field (attribute) name associated with the given field  index.   Field  indices
          start at 0.
<ProgramListing>
char *PQfname(PGresult *res,
              int field_index);
</ProgramListing>
</Para>
</ListItem>

<ListItem>
<Para>
<Function>PQfnumber</Function>
            Returns  the  field  (attribute)  index
          associated with the given field name.
<ProgramListing>
int PQfnumber(PGresult *res,
              char* field_name);
</ProgramListing>
</Para>
</ListItem>

<ListItem>
<Para>
<Function>PQftype</Function>
            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.
<ProgramListing>
Oid PQftype(PGresult *res,
            int field_num);
</ProgramListing>
</Para>
</ListItem>

<ListItem>
<Para>
<Function>PQfsize</Function>
            Returns  the  size  in bytes of the field
          associated with the given field index. If the size
          returned  is  -1,  the  field is a variable length
          field.  Field indices start at 0.
<ProgramListing>
short PQfsize(PGresult *res,
              int field_index);
</ProgramListing>
</Para>
</ListItem>

<ListItem>
<Para>
<Function>PQfmod</Function>
          Returns  the type-specific modification data of the field
          associated with the given field index.
          Field indices start at 0.
<ProgramListing>
int PQfmod(PGresult *res,
           int field_index);
</ProgramListing>
</Para>
</ListItem>

<ListItem>
<Para>
<Function>PQgetvalue</Function>
            Returns the  field  (attribute)  value.
          For most queries, the value returned by PQgetvalue
          is a null-terminated ASCII  string  representation
          of the attribute value.  If the query was a result
          of a <Acronym>BINARY</Acronym> cursor, then  the  value  returned  by
          PQgetvalue  is  the  binary  representation of the
          type in the internal format of the backend server.
          It  is the programmer's responsibility to cast and
          convert the data to the correct C type.  The value
          returned  by  PQgetvalue points to storage that is
          part of the PGresult structure.  One must  explicitly 
          copy the value into other storage if it is to
          be used past the lifetime of the  PGresult  structure itself.
<ProgramListing>
char* PQgetvalue(PGresult *res,
                 int tup_num,
                 int field_num);
</ProgramListing>
</Para>
</ListItem>

<ListItem>
<Para>
<Function>PQgetisnull</Function>
           Tests a field for a NULL entry.
<ProgramListing>
int PQgetisnull(PGresult *res,
                int tup_num,
                int field_num);
</ProgramListing>
            This function returns  1 if the field contains a NULL, 0 if
            it contains a known value.
</Para>
</ListItem>

<ListItem>
<Para>
<Function>PQgetlength</Function>
            Returns   the   length  of  a  field
          (attribute) in bytes.  If the field  is  a  struct
          varlena, the length returned here does not include
          the size field of the varlena, i.e., it is 4 bytes
          less.
<ProgramListing>
int PQgetlength(PGresult *res,
                int tup_num,
                int field_num);
</ProgramListing>
</Para>
</ListItem>

<ListItem>
<Para>
<Function>PQcmdStatus</Function>
          Returns  the  command  status  associated with the
          last query command.
<ProgramListing>
char *PQcmdStatus(PGresult *res);
</ProgramListing>
</Para>
</ListItem>

<ListItem>
<Para>
<Function>PQcmdTuples</Function>
	  Returns the number of rows affected by the last command.
<ProgramListing>
const char *PQcmdTuples(PGresult *res);
</ProgramListing>
          If the last command was INSERT, UPDATE or DELETE, this returns
          a string containing the number of rows affected.  If the last
          command was anything else, it returns the empty string.
</Para>
</ListItem>

<ListItem>
<Para>
<Function>PQoidStatus</Function>
          Returns a string with the object id of  the  tuple
          inserted  if  the last query is an INSERT command.
          Otherwise, returns an empty string.
<ProgramListing>
char* PQoidStatus(PGresult *res);
</ProgramListing>
</Para>
</ListItem>

<ListItem>
<Para>
<Function>PQprint</Function>
          Prints out all the  tuples  and,  optionally,  the
          attribute  names  to  the specified output stream.
<ProgramListing>
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 &lt;table ...&gt; */
		char	*caption;    /* HTML &lt;caption&gt; */
		char	**fieldName; /* null terminated array of replacement field names */
	};
</ProgramListing>
	This funtion is intended to replace PQprintTuples(), which is
	now obsolete.
</Para>
</ListItem>

<ListItem>
<Para>
<Function>PQprintTuples</Function>
          Prints out all the  tuples  and,  optionally,  the
          attribute  names  to  the specified output stream.
<ProgramListing>
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*/
</ProgramListing>
</Para>
</ListItem>

<ListItem>
<Para>
<Function>PQdisplayTuples</Function>
          Prints out all the  tuples  and,  optionally,  the
          attribute  names  to  the specified output stream.
<ProgramListing>
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 */
</ProgramListing>
          PQdisplayTuples() was intended to supersede PQprintTuples(), and
          is in turn superseded by PQprint().
</Para>
</ListItem>
<ListItem>
<Para>
<Function>PQclear</Function>
          Frees  the  storage  associated with the PGresult.
          Every query result should be properly  freed  when
          it  is  no  longer  used.  Failure to do this will
          result in memory leaks in  the  frontend  application.
<ProgramListing>
void PQclear(PQresult *res);
</ProgramListing>
</Para>
</ListItem>
</ItemizedList>
</Para>
</Sect1>

<Sect1>
<Title>Asynchronous Query Processing</Title>

<Para>
The PQexec function is adequate for submitting queries in simple synchronous
applications.  It has a couple of major deficiencies however:

<Para>
<ItemizedList>
<ListItem>
<Para>
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.
</Para>
</ListItem>
<ListItem>
<Para>
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.)
</Para>
</ListItem>
<ListItem>
<Para>
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.
</Para>
</ListItem>
</ItemizedList>
</Para>

<Para>
Applications that do not like these limitations can instead use the
underlying functions that PQexec is built from: PQsendQuery and
PQgetResult.

<Para>
<ItemizedList>
<ListItem>
<Para>
<Function>PQsendQuery</Function>
          Submit a query to <ProductName>Postgres</ProductName> 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).
<ProgramListing>
int PQsendQuery(PGconn *conn,
                const char *query);
</ProgramListing>
	  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.
</Para>
</ListItem>

<ListItem>
<Para>
<Function>PQgetResult</Function>
          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.
<ProgramListing>
PGresult *PQgetResult(PGconn *conn);
</ProgramListing>
	  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.
</Para>
</ListItem>

</ItemizedList>
</Para>

<Para>
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:

<Para>
<ItemizedList>
<ListItem>
<Para>
<Function>PQconsumeInput</Function>
	  If input is available from the backend, consume it.
<ProgramListing>
void PQconsumeInput(PGconn *conn);
</ProgramListing>
	  No direct return value is available from PQconsumeInput, but
	  after calling it, 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.
	  It 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.
</Para>
</ListItem>

<ListItem>
<Para>
<Function>PQisBusy</Function>
	  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.
<ProgramListing>
int PQisBusy(PGconn *conn);
</ProgramListing>
	  PQisBusy will not itself attempt to read data from the backend;
	  therefore PQconsumeInput must be invoked first, or the busy
	  state will never end.
</Para>
</ListItem>

<ListItem>
<Para>
<Function>PQsocket</Function>
	  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.
<ProgramListing>
int PQsocket(PGconn *conn);
</ProgramListing>
	  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.
</Para>
</ListItem>

</ItemizedList>
</Para>

<Para>
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.

<Para>
A frontend that uses PQsendQuery/PQgetResult can also attempt to cancel
a query that is still being processed by the backend.

<Para>
<ItemizedList>
<ListItem>
<Para>
<Function>PQrequestCancel</Function>
	  Request that <ProductName>Postgres</ProductName> abandon
	  processing of the current query.
<ProgramListing>
int PQrequestCancel(PGconn *conn);
</ProgramListing>
	  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.
</Para>
</ListItem>
</ItemizedList>
</Para>

<Para>
Note that if the current query is part of a transaction, cancellation
will abort the whole transaction.

<Para>
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.

</Sect1>

<Sect1>
<Title>Fast Path</Title>

<Para>
<ItemizedList>
<ListItem>
<Para>
<ProductName>Postgres</ProductName>  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.
<ProgramListing>
PGresult* PQfn(PGconn* conn,
               int fnid,
               int *result_buf,
               int *result_len,
               int result_is_int,
               PQArgBlock *args,
               int nargs);
</ProgramListing>
     The fnid argument is the object identifier of the function to be executed.  result_buf is the buffer in which
     to load the return value.  The caller must  have  allocated  sufficient space to store the return value.  The
     result length will be returned in the  storage  pointed
     to  by  result_len.   If the result is to be an integer
     value, than result_is_int should be set to 1; otherwise
     it  should  be  set  to  0.  args and nargs specify the
     arguments to the function.
<ProgramListing>
typedef struct {
             int len;
             int isint;
             union {
                 int *ptr;
                 int integer;
             } u;
         } PQArgBlock;
</ProgramListing>
     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.
</Para>
</ListItem>
</ItemizedList>
</Para>

</Sect1>

<Sect1>
<Title>Asynchronous Notification</Title>

<Para>
<ProductName>Postgres</ProductName> supports asynchronous notification via the
LISTEN and NOTIFY commands.  A backend registers its interest in a particular
notification condition with the LISTEN 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.

<Para>
<FileName>libpq</FileName> applications submit LISTEN commands as ordinary
SQL queries.  Subsequently, arrival of NOTIFY messages can be detected by
calling PQnotifies().

<Para>
<ItemizedList>
<ListItem>
<Para>
<Function>PQnotifies</Function>
          Returns  the next notification from a list of unhandled
          notification messages received from the backend.  Returns NULL if
          there are no pending notifications.  PQnotifies behaves like the
	  popping of a stack.  Once a notification is returned from
	  PQnotifies, it is considered handled and will be removed from the
	  list of notifications.
<ProgramListing>
PGnotify* PQnotifies(PGconn *conn);
</ProgramListing>
	  After processing a PGnotify object returned by PQnotifies,
	  be sure to free it with free() to avoid a memory leak.
	  The  second  sample program gives an example of the use
	  of asynchronous notification.
</Para>
</ListItem>
</ItemizedList>
</Para>

<Para>
PQnotifies() does not actually read backend data; it just returns messages
previously absorbed by another <FileName>libpq</FileName> function.  In prior
releases of <FileName>libpq</FileName>, 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.
</Para>

</Sect1>

<Sect1>
<Title>Functions Associated with the COPY Command</Title>

<Para>
     The copy command in <ProductName>Postgres</ProductName> has options to  read  from
     or  write  to  the  network  connection  used by <FileName>libpq</FileName>.
     Therefore, functions are necessary to access this  network  connection directly so applications may take full
     advantage of this capability.
</Para>

<Para>
     These functions should be executed only after obtaining a PGRES_COPY_OUT
     or PGRES_COPY_IN result object from PQexec or PQgetResult.
</Para>

<Para>
<ItemizedList>
<ListItem>
<Para>
<Function>PQgetline</Function>
          Reads  a  newline-terminated  line  of  characters
          (transmitted  by the backend server) into a buffer
          string of size 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.
          Therefore,  if  the  application  ever  expects to
          receive lines that are more than length-1  characters  long,  the application must be sure to check
          the return value of PQgetline very carefully.
          The code in
<FileName>
../src/bin/psql/psql.c
</FileName>
          contains routines that correctly handle  the  copy
          protocol.
<ProgramListing>
int PQgetline(PGconn *conn,
              char *string,
              int length)
</ProgramListing>
</Para>
</ListItem>
<ListItem>
<Para>
<Function>PQputline</Function>
          Sends  a  null-terminated  string  to  the backend
          server.
          The application must explicitly  send  the  two
          characters  "\." on a final line  to indicate to the backend that it
          has finished sending its data.
<ProgramListing>
void PQputline(PGconn *conn,
               char *string);
</ProgramListing>
</Para>
</ListItem>

<ListItem>
<Para>
<Function>PQendcopy</Function>
          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.
<ProgramListing>
int PQendcopy(PGconn *conn);
</ProgramListing>

<Para>
As an example:

<ProgramListing>
PQexec(conn, "create table foo (a int4, b char16, d float8)");
PQexec(conn, "copy foo from stdin");
PQputline(conn, "3&lt;TAB&gt;hello world&lt;TAB&gt;4.5\n");
PQputline(conn,"4&lt;TAB&gt;goodbye world&lt;TAB&gt;7.11\n");
...
PQputline(conn,"\\.\n");
PQendcopy(conn);
</ProgramListing>
</Para>
</ListItem>
</ItemizedList>
</Para>

<Para>
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.
</Para>

</Sect1>

<Sect1>
<Title><FileName>libpq</FileName> Tracing Functions</Title>

<Para>
<ItemizedList>
<ListItem>
<Para>
<Function>PQtrace</Function>
          Enable  tracing of the frontend/backend communication to a debugging file stream.
<ProgramListing>
void PQtrace(PGconn *conn
             FILE *debug_port)
</ProgramListing>
</Para>
</ListItem>

<ListItem>
<Para>
<Function>PQuntrace</Function>
          Disable tracing started by PQtrace
<ProgramListing>
void PQuntrace(PGconn *conn)
</ProgramListing>
</Para>
</ListItem>
</ItemizedList>
</Para>

</Sect1>

<Sect1>
<Title>User Authentication Functions</Title>

<Para>
     If the user has generated the  appropriate  authentication  credentials
  (e.g.,  obtaining <Acronym>Kerberos</Acronym> tickets),
     the frontend/backend authentication process is  handled
     by  <Function>PQexec</Function>  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.
</Para>

<Para>
<ItemizedList>
<ListItem>
<Para>
<Function>fe_getauthname</Function>
          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  <Acronym>USER</Acronym>
          environment   variable  or  the  user's  entry  in
          <FileName>/etc/passwd</FileName>.
<ProgramListing>
char *fe_getauthname(char* errorMessage)
</ProgramListing>
</Para>
</ListItem>

<ListItem>
<Para>
<Function>fe_setauthsvc</Function>
          Specifies that  <FileName>libpq</FileName>  should  use  authentication
          service  name rather than its compiled-in default.
          This value is typically taken from a  command-line
          switch.
<ProgramListing>
void fe_setauthsvc(char *name,
                   char* errorMessage)
</ProgramListing>
          Any   error   messages   from  the  authentication
          attempts are returned in  the  errorMessage  argument.
</Para>
</ListItem>
</ItemizedList>
</Para>

</Sect1>

<Sect1>
<Title>BUGS</Title>

<Para>
     The  query  buffer is 8192 bytes long, and queries over
     that length will be rejected.
</Para>
</Sect1>

<Sect1>
<Title>Sample Programs</Title>

<Sect2>
<Title>Sample Program 1</Title>

<Para>
<ProgramListing>
         /*
          * testlibpq.c
          *   Test the C version of LIBPQ, the <ProductName>Postgres</ProductName> frontend library.
          *
          *
          */
         #include &lt;stdio.h&gt;
         #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 '&percnt;s' failed.\n", dbName);
             fprintf(stderr,"&percnt;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 &lt; nFields; i++) {
             printf("&percnt;-15s",PQfname(res,i));
           }
           printf("\n");

           /* next, print out the instances */
           for (i=0; i &lt; PQntuples(res); i++) {
             for (j=0  ; j &lt; nFields; j++) {
               printf("&percnt;-15s", PQgetvalue(res,i,j));
             }
             printf("\n");
           }

           PQclear(res);

           /* close the cursor */
           res = PQexec(conn, "CLOSE mycursor");
           PQclear(res);

           /* end the transaction */
           res = PQexec(conn, "END");
           PQclear(res);

           /* close the connection to the database and cleanup */
           PQfinish(conn);

         /*   fclose(debug); */
         }
</ProgramListing>
</Para>
</Sect2>

<Sect2>
<Title>Sample Program 2</Title>

<Para>
<ProgramListing>
         /*
          * 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 &lt;stdio.h&gt;
         #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 '&percnt;s' failed.\n", dbName);
             fprintf(stderr,"&percnt;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 '&percnt;s' from backend pid '&percnt;d' received\n",
                     notify-&gt;relname, notify-&gt;be_pid);
                free(notify);
               }
           }

           /* close the connection to the database and cleanup */
           PQfinish(conn);

         }
</ProgramListing>
</Para>
</Sect2>

<Sect2>
<Title>Sample Program 3</Title>

<Para>
<ProgramListing>
         /*
          * testlibpq3.c
          *   Test the C version of LIBPQ, the <ProductName>Postgres</ProductName> 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 &lt;stdio.h&gt;
         #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 '&percnt;s' failed.\n", dbName);
             fprintf(stderr,"&percnt;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&lt;3;i++) {
               printf("type[&percnt;d] = &percnt;d, size[&percnt;d] = &percnt;d\n",
                   i, PQftype(res,i),
                   i, PQfsize(res,i));
           }
           for (i=0; i &lt; PQntuples(res); i++) {
             int *ival;
             float *dval;
             int plen;
             POLYGON* pval;
             /*/
             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-&gt;size = plen;
             memmove((char*)&amp;pval-&gt;npts, PQgetvalue(res,i,p_fnum), plen);
             printf("tuple &percnt;d: got\n", i);
             printf(" i = (&percnt;d bytes) &percnt;d,\n",
                 PQgetlength(res,i,i_fnum), *ival);
             printf(" d = (&percnt;d bytes) &percnt;f,\n",
                 PQgetlength(res,i,d_fnum), *dval);
             printf(" p = (&percnt;d bytes) &percnt;d points boundbox = (hi=&percnt;f/&percnt;f, lo = &percnt;f,&percnt;f)\n",
                 PQgetlength(res,i,d_fnum),
                 pval-&gt;npts,
                 pval-&gt;boundbox.xh,
                 pval-&gt;boundbox.yh,
                 pval-&gt;boundbox.xl,
                 pval-&gt;boundbox.yl);
           }

           PQclear(res);

           /* close the cursor */
           res = PQexec(conn, "CLOSE mycursor");
           PQclear(res);

           /* end the transaction */
           res = PQexec(conn, "END");
           PQclear(res);

           /* close the connection to the database and cleanup */
           PQfinish(conn);

         }
</ProgramListing>
<Para>

</Sect2>
</Sect1>
</Chapter>