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.
fdopenfunction returns a new stream for the file descriptor filedes.
The opentype argument is interpreted in the same way as for the
fopenfunction (see Opening Streams), except that the ‘b’ option is not permitted; this is because GNU systems make no distinction between text and binary files. Also,
"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,
fdopenmay 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
see Creating a Pipe.
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_unlockedfunction is equivalent to the
filenofunction except that it does not implicitly lock the stream if the state is
This function is a GNU extension.
There are also symbolic constants defined in unistd.h for the
file descriptors belonging to the standard streams
stderr; see Standard Streams.
0, which is the file descriptor for standard input.
1, which is the file descriptor for standard output.
2, which is the file descriptor for standard error output.