Question

We can get a file descriptor from a FILE* stream by using the fileno() function. Is there a function for getting a FILE* stream from the file descriptor without reopening the file?

Answer 1

   FILE *fdopen(int fd, const char *mode);

See fdopen(3), but it should be on the same page as fopen(3):

The fdopen() function associates a stream with the existing
file descriptor, fd. The mode of the stream (one of the values
"r", "r+", "w", "w+", "a", "a+") must be compatible
with the mode of the file descriptor. The file position indicator
of the new stream is set to that belonging to fd, and the error and
end-of-file indicators are cleared. Modes "w" or "w+" do not
cause truncation of the file. The file descriptor is not dup’ed,
and will be closed when the stream created by fdopen() is
closed. The result of applying fdopen() to a shared memory object
is undefined.