I was wondering the difference between stdout
and STDOUT_FILENO
in Linux C.
After some searching work, I draw the following conclusion. Could you help me review it and correct any mistake in it? Thanks
stdout
belongs to standard I/O stream of C language; whose type is FILE* and defined in stdio.h
STDOUT_FILENO
, possessing an int type, is defined at unistd.h
. It's a file descriptor of LINUX system. In unistd.h
, it's explained as below:
The following symbolic constants shall be defined for file streams: STDERR_FILENO File number of stderr; 2. STDIN_FILENO File number of stdin; 0. STDOUT_FILENO File number of stdout; 1.
So, in my opinion, the STDOUT_FILENO
belongs system-level calling and, to some extent, like a system API. STDOUT_FILENO
can be used to describe any device in system.
The stdout
locates in a higher level (user level?) and actually encapsulate the details of STDOUT_FILENO
. stdout
has I/O buffer.
That's my understand about their difference. Any comment or correction is appreciated, thanks.
stdout
is a FILE*
"constant" giving the standard outout stream. So obviously fprintf(stdout, "x=%d\n", x);
has the same behavior as printf("x=%d\n", x);
; you use stdout
for <stdio.h>
functions like fprintf
, fputs
etc..
STDOUT_FILENO
is an integer file descriptor (actually, the integer 1). You might use it for write
syscall.
The relation between the two is STDOUT_FILENO == fileno(stdout)
(Except after you do weird things like fclose(stdout);
, or perhaps some freopen
after some fclose(stdin)
, which you should almost never do! See this, as commented by J.F.Sebastian)
You usually prefer the FILE*
things, because they are buffered (so usually perform well). Sometimes, you may want to call fflush
to flush buffers.
You could use file descriptor numbers for syscalls like write(2) (which is used by the stdio
library), or poll(2). But using syscalls is clumpsy. It may give you very good efficiency (but that is hard to code), but very often the stdio
library is good enough (and more portable).
(Of course you should #include <stdio.h>
for the stdio functions, and #include <unistd.h>
-and some other headers- for the syscalls like write
. And the stdio functions are implemented with syscalls, so fprintf
may call write
).