stdio -- standard buffered input/output package


#include <stdio.h>

FILE *stdin, *stdout, *stderr;


The functions described in this manual entry constitute an efficient, user-level I/O buffering scheme. The in-line macros getc(S) and putc(S) handle characters quickly. The macros getchar and putchar, and the higher-level routines fgetc, fgets, fprintf, fputc, fputs, fread, fscanf, fwrite, gets, getw, printf, puts, putw, and scanf all use or act as if they use getc and putc; they can be freely intermixed.

If the program is compiled in the default mode, the following are defined only as macros: getc, getchar, putc, putchar, ferror, feof, clearerr, and fileno. If the program is compiled in -ansi, -posix, or -xpg3 mode, they exist as macros and functions. The function form can be used if the macro form is #undef'ed.

A file with associated buffering is called a stream and is declared to be a pointer to a defined type FILE. The fopen(S) function creates certain descriptive data for a stream and returns a pointer to designate the stream in all further transactions. Normally, there are three open streams with constant pointers declared in the <stdio.h> header file and associated with the standard open files:

stdin- standard input file

stdout- standard output file

stderr- standard error file

A constant NULL (0) designates a nonexistent pointer.

An integer-constant EOF (-1) is returned upon end-of-file or error by most integer functions that deal with streams (see the individual descriptions for details).

An integer constant BUFSIZ specifies the size of the buffers used by the particular implementation.

Any program that uses this package must include the header file of pertinent macro definitions, as follows:

   #include <stdio.h>
The constants and the following ``functions'' are implemented as macros (redeclaration of these names is perilous): getc, getchar, putc, putchar, ferror, feof, clearerr, and fileno.

Output streams, with the exception of the standard error stream stderr, are by default buffered if the output refers to a file, and line-buffered if the output refers to a terminal. The standard error output stream stderr is by default unbuffered, but use of freopen (see fopen(S)) cause it to become buffered or line-buffered. When an output stream is unbuffered, information is queued for writing on the destination file or terminal as soon as written. When it is buffered, many characters are saved up and written as a block. When it is line-buffered, each line of output is queued for writing on the destination terminal as soon as the line is completed (that is, as soon as a new-line character is written or terminal input is requested). The setbuf or setvbuf functions in setbuf(S) may be used to change the stream's buffering strategy.

The default setting for the open file limit (FOPEN_MAX) is 128. For code compiled to be iBCS2 conformant, the default is 60. The existing shared C library is iBCS2 conformant and, thus, is restricted to an open file limit of 60. The static archive (libc.a) is restricted to 128.


Invalid stream pointers usually cause disorders, possibly including program termination. Individual function descriptions describe the possible error conditions.

See also

close(S), ctermid(S), cuserid(S), fclose(S), ferror(S), fopen(S), fread(S), fseek(S), getc(S), gets(S), lseek(S), open(S), pipe(S), popen(S), printf(S), putc(S), puts(S), read(S), scanf(S), setbuf(S), system(S), tmpfile(S), tmpnam(S), ungetc(S) write(S),

Standards conformance

stdio is conformant with:

ANSI X3.159-1989 Programming Language -- C ;
IEEE POSIX Std 1003.1-1990 System Application Program Interface (API) [C Language] (ISO/IEC 9945-1) ;
X/Open Portability Guide, Issue 3, 1989 .

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