#include <stdio.h> extern FILE *stdin; extern FILE *stdout; extern FILE *stderr;
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 symbols used to refer to these files, namely stdin, stdout, and stderr.
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 the stream.)
Note that mixing use of FILEs and raw file descriptors can produce unexpected results and should generally be avoided. (For the masochistic among you: POSIX.1, section 8.2.3, describes in detail how this interaction is supposed to work.) A general rule is that file descriptors 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.