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NAME |LIBRARY |SYNOPSIS |DESCRIPTION |STANDARDS |HISTORY |NOTES |SEE ALSO |COLOPHON | |
stdin(3) Library Functions Manualstdin(3)stdin, stdout, stderr - standard I/O streams
Standard C library (libc,-lc)
#include <stdio.h>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 (seetty(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 ofsh(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 symbols used to refer to these files, namelystdin,stdout, andstderr. Each of these symbols is astdio(3) macro of type pointer toFILE, and can be used with functions likefprintf(3) orfread(3). SinceFILEs 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 likeread(2) andlseek(2). On program startup, the integer file descriptors associated with the streamsstdin,stdout, andstderr are 0, 1, and 2, respectively. The preprocessor symbolsSTDIN_FILENO,STDOUT_FILENO, andSTDERR_FILENOare defined with these values in<unistd.h>. (Applyingfreopen(3) to one of these streams can change the file descriptor number associated with the stream.) Note that mixing use ofFILEs 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 anexec(3), the child inherits all open file descriptors, but all old streams have become inaccessible. Since the symbolsstdin,stdout, andstderr 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 functionfreopen(3), specially introduced to make it possible to reassignstdin,stdout, andstderr. The standard streams are closed by a call toexit(3) and by normal program termination.
C11, POSIX.1-2008. The standards also stipulate that these three streams shall be open at program startup.
C89, POSIX.1-2001.
The streamstderr is unbuffered. The streamstdout is line- buffered when it points to a terminal. Partial lines will not appear untilfflush(3) orexit(3) is called, or a newline is printed. This can produce unexpected results, especially with debugging output. The buffering mode of the standard streams (or any other stream) can be changed using thesetbuf(3) orsetvbuf(3) call. Note that in casestdin is associated with a terminal, there may also be input buffering in the terminal driver, entirely unrelated to stdio buffering. (Indeed, normally terminal input is line buffered in the kernel.) This kernel input handling can be modified using calls liketcsetattr(3); see alsostty(1), andtermios(3).
csh(1),sh(1),open(2),fopen(3),stdio(3)
This page is part of theman-pages (Linux kernel and C library user-space interface documentation) project. Information about the project can be found at ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report for this manual page, see ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩. This page was obtained from the tarball man-pages-6.15.tar.gz fetched from ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on 2025-08-11. If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up- to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which isnot part of the original manual page), send a mail to man-pages@man7.orgLinux man-pages 6.15 2025-05-06stdin(3)Pages that refer to this page:intro(1), FILE(3type), stdio(3), pam_exec(8)
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HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. | |