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1.6. Functions Associated with the COPY Command

The COPY command in Postgres has options to read from or write to the network connection used by libpq. Therefore, functions are necessary to access this network connection directly so applications may take advantage of this capability.

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

  • PQgetline Reads a newline-terminated line of characters (transmitted by the backend server) into a buffer string of size length.

    int PQgetline(PGconn *conn,
                  char *string,
                  int 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. If the application might receive lines that are more than length-1 characters long, care is needed to be sure one recognizes the "\." line correctly (and does not, for example, mistake the end of a long data line for a terminator line). The code in src/bin/psql/copy.c contains example routines that correctly handle the copy protocol.

  • PQgetlineAsync Reads a newline-terminated line of characters (transmitted by the backend server) into a buffer without blocking.

    int PQgetlineAsync(PGconn *conn,
                       char *buffer,
                       int bufsize)
    
    This routine is similar to PQgetline, but it can be used by applications that must read COPY data asynchronously, that is without blocking. Having issued the COPY command and gotten a PGRES_COPY_OUT response, the application should call PQconsumeInput and PQgetlineAsync until the end-of-data signal is detected. Unlike PQgetline, this routine takes responsibility for detecting end-of-data. On each call, PQgetlineAsync will return data if a complete newline- terminated data line is available in libpq's input buffer, or if the incoming data line is too long to fit in the buffer offered by the caller. Otherwise, no data is returned until the rest of the line arrives.

    The routine returns -1 if the end-of-copy-data marker has been recognized, or 0 if no data is available, or a positive number giving the number of bytes of data returned. If -1 is returned, the caller must next call PQendcopy, and then return to normal processing. The data returned will not extend beyond a newline character. If possible a whole line will be returned at one time. But if the buffer offered by the caller is too small to hold a line sent by the backend, then a partial data line will be returned. This can be detected by testing whether the last returned byte is "\n" or not. The returned string is not null-terminated. (If you want to add a terminating null, be sure to pass a bufsize one smaller than the room actually available.)

  • PQputline Sends a null-terminated string to the backend server. Returns 0 if OK, EOF if unable to send the string.

    int PQputline(PGconn *conn,
                  const char *string);
    
    Note the application must explicitly send the two characters "\." on a final line to indicate to the backend that it has finished sending its data.
  • PQputnbytes Sends a non-null-terminated string to the backend server. Returns 0 if OK, EOF if unable to send the string.

    int PQputnbytes(PGconn *conn,
                    const char *buffer,
                    int nbytes);
    
    This is exactly like PQputline, except that the data buffer need not be null-terminated since the number of bytes to send is specified directly.
  • PQendcopy 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.

    int PQendcopy(PGconn *conn);
    

    As an example:

    PQexec(conn, "create table foo (a int4, b char(16), d double precision)");
    PQexec(conn, "copy foo from stdin");
    PQputline(conn, "3\thello world\t4.5\n");
    PQputline(conn,"4\tgoodbye world\t7.11\n");
    ...
    PQputline(conn,"\\.\n");
    PQendcopy(conn);
    

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.

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