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NAME |LIBRARY |SYNOPSIS |DESCRIPTION |RETURN VALUE |ATTRIBUTES |STANDARDS |HISTORY |CAVEATS |BUGS |SEE ALSO |COLOPHON | |
setbuf(3) Library Functions Manualsetbuf(3)setbuf, setbuffer, setlinebuf, setvbuf - stream buffering operations
Standard C library (libc,-lc)
#include <stdio.h>int setvbuf(size_t size;FILE *restrictstream, charbuf[restrictsize],intmode, size_tsize);void setbuf(FILE *restrictstream, char *restrictbuf);void setbuffer(size_t size;FILE *restrictstream, charbuf[restrictsize],size_tsize);void setlinebuf(FILE *stream); Feature Test Macro Requirements for glibc (seefeature_test_macros(7)):setbuffer(),setlinebuf(): Since glibc 2.19: _DEFAULT_SOURCE glibc 2.19 and earlier: _BSD_SOURCE
The three types of buffering available are unbuffered, block buffered, and line buffered. When an output stream is unbuffered, information appears on the destination file or terminal as soon as written; when it is block buffered, many characters are saved up and written as a block; when it is line buffered, characters are saved up until a newline is output or input is read from any stream attached to a terminal device (typicallystdin). The functionfflush(3) may be used to force the block out early. (Seefclose(3).) Normally all files are block buffered. If a stream refers to a terminal (asstdout normally does), it is line buffered. The standard error streamstderr is always unbuffered by default. Thesetvbuf() function may be used on any open stream to change its buffer. Themode argument must be one of the following three macros:_IONBFunbuffered_IOLBFline buffered_IOFBFfully buffered Except for unbuffered files, thebuf argument should point to a buffer at leastsize bytes long; this buffer will be used instead of the current buffer. If the argumentbuf is NULL, only the mode is affected; a new buffer will be allocated on the next read or write operation. Thesetvbuf() function may be used only after opening a stream and before any other operations have been performed on it. The other three calls are, in effect, simply aliases for calls tosetvbuf(). Thesetbuf() function is exactly equivalent to the call setvbuf(stream, buf, buf ? _IOFBF : _IONBF, BUFSIZ); Thesetbuffer() function is the same, except that the size of the buffer is up to the caller, rather than being determined by the defaultBUFSIZ. Thesetlinebuf() function is exactly equivalent to the call: setvbuf(stream, NULL, _IOLBF, 0);
The functionsetvbuf() returns 0 on success. It returns nonzero on failure (mode is invalid or the request cannot be honored). It may seterrno on failure. The other functions do not return a value.
For an explanation of the terms used in this section, seeattributes(7). ┌──────────────────────────────────────┬───────────────┬─────────┐ │Interface│Attribute│Value│ ├──────────────────────────────────────┼───────────────┼─────────┤ │setbuf(),setbuffer(),setlinebuf(), │ Thread safety │ MT-Safe │ │setvbuf() │ │ │ └──────────────────────────────────────┴───────────────┴─────────┘
setbuf()setvbuf() C11, POSIX.1-2008.
setbuf()setvbuf() C89, POSIX.1-2001.
POSIX notes that the value oferrno is unspecified after a call tosetbuf() and further notes that, since the value oferrno is not required to be unchanged after a successful call tosetbuf(), applications should instead usesetvbuf() in order to detect errors.
You must make sure that the space thatbuf points to still exists by the timestream is closed, which also happens at program termination. For example, the following is invalid: #include <stdio.h> int main(void) { char buf[BUFSIZ]; setbuf(stdout, buf); printf("Hello, world!\n"); return 0; }stdbuf(1),fclose(3),fflush(3),fopen(3),fread(3),malloc(3),printf(3),puts(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 2025-06-28setbuf(3)Pages that refer to this page:fclose(3), fcloseall(3), fflush(3), fpurge(3), open_memstream(3), procio(3), stdin(3), stdio(3)
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. | |