GNU C Library (libc) Programming Guide - Descriptors and Streams

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13.4 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 (see 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 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 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.