PostgreSQL 8.4.22 Documentation
Prev | Fast Backward | Chapter 30. libpq - C Library | Fast Forward | Next

# 30.8. Functions Associated with the `COPY` Command

The `COPY` command in PostgreSQL has options to read from or write to the network connection used by libpq. The functions described in this section allow applications to take advantage of this capability by supplying or consuming copied data.

The overall process is that the application first issues the SQL `COPY` command via `PQexec` or one of the equivalent functions. The response to this (if there is no error in the command) will be a `PGresult` object bearing a status code of `PGRES_COPY_OUT` or `PGRES_COPY_IN` (depending on the specified copy direction). The application should then use the functions of this section to receive or transmit data rows. When the data transfer is complete, another `PGresult` object is returned to indicate success or failure of the transfer. Its status will be `PGRES_COMMAND_OK` for success or `PGRES_FATAL_ERROR` if some problem was encountered. At this point further SQL commands can be issued via `PQexec`. (It is not possible to execute other SQL commands using the same connection while the `COPY` operation is in progress.)

If a `COPY` command is issued via `PQexec` in a string that could contain additional commands, the application must continue fetching results via `PQgetResult` after completing the `COPY` sequence. Only when `PQgetResult` returns `NULL` is it certain that the `PQexec` command string is done and it is safe to issue more commands.

The functions of this section should be executed only after obtaining a result status of `PGRES_COPY_OUT` or `PGRES_COPY_IN` from `PQexec` or `PQgetResult`.

A `PGresult` object bearing one of these status values carries some additional data about the `COPY` operation that is starting. This additional data is available using functions that are also used in connection with query results:

`PQnfields`

  Returns the number of columns (fields) to be copied.

`PQbinaryTuples`

  0 indicates the overall copy format is textual (rows separated by newlines, columns separated by separator characters, etc). 1 indicates the overall copy format is binary. See COPY for more information.

`PQfformat`

  Returns the format code (0 for text, 1 for binary) associated with each column of the copy operation. The per-column format codes will always be zero when the overall copy format is textual, but the binary format can support both text and binary columns. (However, as of the current implementation of `COPY`, only binary columns appear in a binary copy; so the per-column formats always match the overall format at present.)

> Note: These additional data values are only available when using protocol 3.0. When using protocol 2.0, all these functions will return 0.

## 30.8.1. Functions for Sending `COPY` Data

These functions are used to send data during `COPY FROM STDIN`. They will fail if called when the connection is not in `COPY_IN` state.

`PQputCopyData`

  Sends data to the server during `COPY_IN` state.

              int PQputCopyData(PGconn *conn,
                                const char *buffer,
                                int nbytes);

  Transmits the `COPY` data in the specified `buffer`, of length `nbytes`, to the server. The result is 1 if the data was sent, zero if it was not sent because the attempt would block (this case is only possible if the connection is in nonblocking mode), or -1 if an error occurred. (Use `PQerrorMessage` to retrieve details if the return value is -1. If the value is zero, wait for write-ready and try again.)

  The application can divide the `COPY` data stream into buffer loads of any convenient size. Buffer-load boundaries have no semantic significance when sending. The contents of the data stream must match the data format expected by the `COPY` command; see COPY for details.

`PQputCopyEnd`

  Sends end-of-data indication to the server during `COPY_IN` state.

              int PQputCopyEnd(PGconn *conn,
                               const char *errormsg);

  Ends the `COPY_IN` operation successfully if `errormsg` is `NULL`. If `errormsg` is not `NULL` then the `COPY` is forced to fail, with the string pointed to by `errormsg` used as the error message. (One should not assume that this exact error message will come back from the server, however, as the server might have already failed the `COPY` for its own reasons. Also note that the option to force failure does not work when using pre-3.0-protocol connections.)

  The result is 1 if the termination data was sent, zero if it was not sent because the attempt would block (this case is only possible if the connection is in nonblocking mode), or -1 if an error occurred. (Use `PQerrorMessage` to retrieve details if the return value is -1. If the value is zero, wait for write-ready and try again.)

  After successfully calling `PQputCopyEnd`, call `PQgetResult` to obtain the final result status of the `COPY` command. One can wait for this result to be available in the usual way. Then return to normal operation.

## 30.8.2. Functions for Receiving `COPY` Data

These functions are used to receive data during `COPY TO STDOUT`. They will fail if called when the connection is not in `COPY_OUT` state.

`PQgetCopyData`

  Receives data from the server during `COPY_OUT` state.

              int PQgetCopyData(PGconn *conn,
                                char **buffer,
                                int async);

  Attempts to obtain another row of data from the server during a `COPY`. Data is always returned one data row at a time; if only a partial row is available, it is not returned. Successful return of a data row involves allocating a chunk of memory to hold the data. The `buffer` parameter must be non-`NULL`. `*buffer` is set to point to the allocated memory, or to `NULL` in cases where no buffer is returned. A non-`NULL` result buffer must be freed using `PQfreemem` when no longer needed.

  When a row is successfully returned, the return value is the number of data bytes in the row (this will always be greater than zero). The returned string is always null-terminated, though this is probably only useful for textual `COPY`. A result of zero indicates that the `COPY` is still in progress, but no row is yet available (this is only possible when `async` is true). A result of -1 indicates that the `COPY` is done. A result of -2 indicates that an error occurred (consult `PQerrorMessage` for the reason).

  When `async` is true (not zero), `PQgetCopyData` will not block waiting for input; it will return zero if the `COPY` is still in progress but no complete row is available. (In this case wait for read-ready and then call `PQconsumeInput` before calling `PQgetCopyData` again.) When `async` is false (zero), `PQgetCopyData` will block until data is available or the operation completes.

  After `PQgetCopyData` returns -1, call `PQgetResult` to obtain the final result status of the `COPY` command. One can wait for this result to be available in the usual way. Then return to normal operation.

## 30.8.3. Obsolete Functions for `COPY`

These functions represent older methods of handling `COPY`. Although they still work, they are deprecated due to poor error handling, inconvenient methods of detecting end-of-data, and lack of support for binary or nonblocking transfers.

`PQgetline`

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

              int PQgetline(PGconn *conn,
                            char *buffer,
                            int length);

  This function copies up to `length`-1 characters into the buffer and converts the terminating newline into a zero byte. `PQgetline` returns `EOF` at the end of input, 0 if the entire line has been read, and 1 if the buffer is full but the terminating newline has not yet been read.

  Note that the application must check to see if a new line consists of the two characters `\.`, which indicates that the 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 it recognizes the `\.` line correctly (and does not, for example, mistake the end of a long data line for a terminator line).

`PQgetlineAsync`

  Reads a row of `COPY` data (transmitted by the server) into a buffer without blocking.

              int PQgetlineAsync(PGconn *conn,
                                 char *buffer,
                                 int bufsize);

  This function 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 function takes responsibility for detecting end-of-data.

  On each call, `PQgetlineAsync` will return data if a complete data row is available in libpq's input buffer. Otherwise, no data is returned until the rest of the row arrives. The function 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 data-row boundary. If possible a whole row will be returned at one time. But if the buffer offered by the caller is too small to hold a row sent by the server, then a partial data row will be returned. With textual data this can be detected by testing whether the last returned byte is `\n` or not. (In a binary `COPY`, actual parsing of the `COPY` data format will be needed to make the equivalent determination.) 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 server. Returns 0 if OK and `EOF` if unable to send the string.

              int PQputline(PGconn *conn,
                            const char *string);

  The `COPY` data stream sent by a series of calls to `PQputline` has the same format as that returned by `PQgetlineAsync`, except that applications are not obliged to send exactly one data row per `PQputline` call; it is okay to send a partial line or multiple lines per call.

  > Note: Before PostgreSQL protocol 3.0, it was necessary for the application to explicitly send the two characters `\.` as a final line to indicate to the server that it had finished sending `COPY` data. While this still works, it is deprecated and the special meaning of `\.` can be expected to be removed in a future release. It is sufficient to call `PQendcopy` after having sent the actual data.

`PQputnbytes`

  Sends a non-null-terminated string to the server. Returns 0 if OK and `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. Use this procedure when sending binary data.

`PQendcopy`

  Synchronizes with the server.

              int PQendcopy(PGconn *conn);

  This function waits until the server has finished the copying. It should either be issued when the last string has been sent to the server using `PQputline` or when the last string has been received from the server using `PGgetline`. It must be issued or the server will get "out of sync" with the client. Upon return from this function, the server is ready to receive the next SQL command. The return value is 0 on successful completion, nonzero otherwise. (Use `PQerrorMessage` to retrieve details if the return value is nonzero.)

  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 a null pointer. 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` command embedded in a series of SQL commands will be executed correctly.

  Older applications are likely to submit a `COPY` via `PQexec` and assume that the transaction is done after `PQendcopy`. This will work correctly only if the `COPY` is the only SQL command in the command string.

Prev | Home | Next
Asynchronous Notification | Up | Control Functions