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(libc.info)Descriptors and Streams

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Descriptors and Streams
=======================

   Given an open file descriptor, you can create a stream for it with
the `fdopen' function.  You can get the underlying file descriptor for
an existing stream with the `fileno' function.  These functions are
declared in the header file `stdio.h'.

 - Function: FILE * fdopen (int FILEDES, const char *OPENTYPE)
     The `fdopen' function returns a new stream for the file descriptor
     FILEDES.

     The OPENTYPE argument is interpreted in the same way as for the
     `fopen' function (Note: Opening Streams), except that the `b'
     option is not permitted; this is because GNU makes no distinction
     between text and binary files.  Also, `"w"' and `"w+"' do not
     cause truncation of the file; these have an effect only when
     opening a file, and in this case the file has already been opened.
     You must make sure that the OPENTYPE argument matches the actual
     mode of the open file descriptor.

     The return value is the new stream.  If the stream cannot be
     created (for example, if the modes for the file indicated by the
     file descriptor do not permit the access specified by the OPENTYPE
     argument), a null pointer is returned instead.

     In some other systems, `fdopen' may fail to detect that the modes
     for file descriptor do not permit the access specified by
     `opentype'.  The GNU C library always checks for this.

   For an example showing the use of the `fdopen' function, see Note:
Creating a Pipe.

 - Function: int fileno (FILE *STREAM)
     This function returns the file descriptor associated with the
     stream STREAM.  If an error is detected (for example, if the STREAM
     is not valid) or if STREAM does not do I/O to a file, `fileno'
     returns -1.

 - Function: int fileno_unlocked (FILE *STREAM)
     The `fileno_unlocked' function is equivalent to the `fileno'
     function except that it does not implicitly lock the stream if the
     state is `FSETLOCKING_INTERNAL'.

     This function is a GNU extension.

   There are also symbolic constants defined in `unistd.h' for the file
descriptors belonging to the standard streams `stdin', `stdout', and
`stderr'; see Note: Standard Streams.

`STDIN_FILENO'
     This macro has value `0', which is the file descriptor for
     standard input.

`STDOUT_FILENO'
     This macro has value `1', which is the file descriptor for
     standard output.

`STDERR_FILENO'
     This macro has value `2', which is the file descriptor for
     standard error output.