extern FILE *stdin;
extern FILE *stdout;
extern FILE *stderr;
Under normal circumstances every UNIX program has three streams opened
for it when it starts up, one for input, one for output, and one for
printing diagnostic or error messages. These are typically attached to
the user's terminal (see tty(4) but might instead refer to files or
other devices, depending on what the parent process chose to set up.
(See also the "Redirection" section of sh(1).)
The input stream is referred to as "standard input"; the output stream
is referred to as "standard output"; and the error stream is referred
to as "standard error". These terms are abbreviated to form the sym-
bols used to refer to these files, namely stdin, stdout, and stderr.
Each of these symbols is a stdio(3) macro of type pointer to FILE, and
can be used with functions like fprintf(3) or fread(3).
Since FILEs are a buffering wrapper around UNIX file descriptors, the
same underlying files may also be accessed using the raw UNIX file
interface, that is, the functions like read(2) and lseek(2).
On program startup, the integer file descriptors associated with the
streams stdin, stdout, and stderr are 0, 1, and 2, respectively. The
preprocessor symbols STDIN_FILENO, STDOUT_FILENO, and STDERR_FILENO are
defined with these values in <unistd.h>. (Applying freopen(3) to one
of these streams can change the file descriptor number associated with
Note that mixing use of FILEs and raw file descriptors can produce
unexpected results and should generally be avoided. (For the masochis-
tic among you: POSIX.1, section 8.2.3, describes in detail how this
interaction is supposed to work.) A general rule is that file descrip-
tors are handled in the kernel, while stdio is just a library. This
means for example, that after an exec(3), the child inherits all open
file descriptors, but all old streams have become inaccessible.
Since the symbols stdin, stdout, and stderr are specified to be macros,
assigning to them is nonportable. The standard streams can be made to
refer to different files with help of the library function freopen(3),
specially introduced to make it possible to reassign stdin, stdout, and
stderr. The standard streams are closed by a call to exit(3) and by
normal program termination.
The stdin, stdout, and stderr macros conform to C89 and this standard
also stipulates that these three streams shall be open at program
csh(1), sh(1), open(2), fopen(3), stdio(3)
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