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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.
Preliminary: | MT-Safe | AS-Unsafe heap lock | AC-Unsafe mem lock | See POSIX Safety Concepts.
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 systems make 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 descriptors 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.
Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.
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.
Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.
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.