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STDIO (3) | C library functions | Unix Manual Pages | :man


stdio - standard input/output library functions


See Also
List Of Functions


.Lb libc


.In stdio.h
.Vt FILE *stdin ;
.Vt FILE *stdout ;
.Vt FILE *stderr ;


The standard I/O library provides a simple and efficient buffered stream I/O interface. Input and output is mapped into logical data streams and the physical I/O characteristics are concealed. The functions and macros are listed below; more information is available from the individual man pages.

A stream is associated with an external file (which may be a physical device) by opening a file, which may involve creating a new file. Creating an existing file causes its former contents to be discarded. If a file can support positioning requests (such as a disk file, as opposed to a terminal) then a file position indicator associated with the stream is positioned at the start of the file (byte zero), unless the file is opened with append mode. If append mode is used, the position indicator will be placed at the end-of-file. The position indicator is maintained by subsequent reads, writes and positioning requests. All input occurs as if the characters were read by successive calls to the fgetc(3) function; all output takes place as if all characters were written by successive calls to the fputc(3) function.

A file is disassociated from a stream by closing the file. Output streams are flushed (any unwritten buffer contents are transferred to the host environment) before the stream is disassociated from the file. The value of a pointer to a FILE object is indeterminate (garbage) after a file is closed.

A file may be subsequently reopened, by the same or another program execution, and its contents reclaimed or modified (if it can be repositioned at the start). If the main function returns to its original caller, or the exit(3) function is called, all open files are closed (hence all output streams are flushed) before program termination. Other methods of program termination may not close files properly and hence buffered output may be lost. In particular, _exit(2) does not flush stdio files. Neither does an exit due to a signal. Buffers are flushed by abort(3) as required by POSIX, although previous implementations did not.

This implementation makes no distinction between "text" and "binary" streams. In effect, all streams are binary. No translation is performed and no extra padding appears on any stream.

At program startup, three streams are predefined and need not be opened explicitly:

  • standard input (for reading conventional input),
  • standard output (for writing conventional output), and
  • standard error (for writing diagnostic output).
These streams are abbreviated stdin, stdout and stderr. Initially, the standard error stream is unbuffered; the standard input and output streams are fully buffered if and only if the streams do not refer to an interactive or "terminal" device, as determined by the isatty(3) function. In fact, all freshly-opened streams that refer to terminal devices default to line buffering, and pending output to such streams is written automatically whenever such an input stream is read. Note that this applies only to ""true reads""; if the read request can be satisfied by existing buffered data, no automatic flush will occur. In these cases, or when a large amount of computation is done after printing part of a line on an output terminal, it is necessary to fflush(3) the standard output before going off and computing so that the output will appear. Alternatively, these defaults may be modified via the setvbuf(3) function.

The stdio library is a part of the library libc and routines are automatically loaded as needed by the C compiler. The SYNOPSIS sections of the following manual pages indicate which include files are to be used, what the compiler declaration for the function looks like and which external variables are of interest.

The following are defined as macros; these names may not be re-used without first removing their current definitions with #undef: BUFSIZ, EOF, FILENAME_MAX, FOPEN_MAX, L_ctermid, L_cuserid, L_tmpnam, NULL, P_tmpdir, SEEK_CUR, SEEK_END, SEEK_SET, TMP_MAX, clearerr, clearerr_unlocked, feof, feof_unlocked, ferror, ferror_unlocked, fileno, fileno_unlocked, fropen, fwopen, getc, getc_unlocked, getchar, getchar_unlocked, putc, putc_unlocked, putchar, putchar_unlocked, stderr, stdin and stdout. Function versions of the macro functions clearerr, clearerr_unlocked, feof, feof_unlocked, ferror, ferror_unlocked, fileno, fileno_unlocked, getc, getc_unlocked, getchar, getchar_unlocked, putc, putc_unlocked, putchar, and putchar_unlocked exist and will be used if the macro definitions are explicitly removed.


close(2), open(2), read(2), write(2)





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