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@(#)setbuf.3 8.1 (Berkeley) 6/4/93
.Dd June 4, 1993 .Dt SETBUF 3 .Os .Sh NAME .Nm setbuf , .Nm setbuffer , .Nm setlinebuf , .Nm setvbuf .Nd stream buffering operations .Sh LIBRARY .Lb libc .Sh SYNOPSIS n stdio.h .Ft void .Fn setbuf "FILE * restrict stream" "char * restrict buf" .Ft void .Fn setbuffer "FILE *stream" "char *buf" "size_t size" .Ft int .Fn setlinebuf "FILE *stream" .Ft int .Fn setvbuf "FILE * restrict stream" "char * restrict buf" "int mode" "size_t size" .Sh DESCRIPTION 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 (typically stdin).
p The default buffer settings can be overwritten per descriptor .Dv ( STDBUFn ) where .Dv n is the numeric value of the file descriptor represented by the stream, or for all descriptors .Dv ( STDBUF ) . The environment variable value is a letter followed by an optional numeric value indicating the size of the buffer. Valid sizes range from 0B to 1MB. Valid letters are: l -tag -width X -indent t Dv Li U Unbuffered. t Dv Li L Line-buffered. t Dv Li F Fully-buffered. .El
p The function .Xr fflush 3 may be used to force the block out early. (See .Xr fclose 3 . )
p Normally all files are block buffered. When the first .Tn I/O operation occurs on a file, .Xr malloc 3 is called, and an optimally-sized buffer is obtained. If a stream refers to a terminal (as .Em stdout normally does) it is line buffered. The standard error stream .Em stderr is initially unbuffered.
p The .Fn setvbuf function may be used to alter the buffering behavior of a stream. The .Fa mode parameter must be one of the following three macros: l -tag -width _IOFBF -offset indent t Dv _IONBF unbuffered t Dv _IOLBF line buffered t Dv _IOFBF fully buffered .El
p The .Fa size parameter may be given as zero to obtain deferred optimal-size buffer allocation as usual. If it is not zero, then except for unbuffered files, the .Fa buf argument should point to a buffer at least .Fa size bytes long; this buffer will be used instead of the current buffer. (If the .Fa size argument is not zero but .Fa buf is .Dv NULL , a buffer of the given size will be allocated immediately, and released on close. This is an extension to ANSI C; portable code should use a size of 0 with any .Dv NULL buffer.)
p The .Fn setvbuf function may be used at any time, but may have peculiar side effects (such as discarding input or flushing output) if the stream is ``active''. Portable applications should call it only once on any given stream, and before any .Tn I/O is performed.
p The other three calls are, in effect, simply aliases for calls to .Fn setvbuf . Except for the lack of a return value, the .Fn setbuf function is exactly equivalent to the call
p .Dl "setvbuf(stream, buf, buf ? _IOFBF : _IONBF, BUFSIZ);"
p The .Fn setbuffer function is the same, except that the size of the buffer is up to the caller, rather than being determined by the default .Dv BUFSIZ . The .Fn setlinebuf function is exactly equivalent to the call:
p .Dl "setvbuf(stream, (char *)NULL, _IOLBF, 0);" .Sh RETURN VALUES The .Fn setvbuf function returns 0 on success, or .Dv EOF if the request cannot be honored (note that the stream is still functional in this case).
p The .Fn setlinebuf function returns what the equivalent .Fn setvbuf would have returned. .Sh SEE ALSO .Xr fclose 3 , .Xr fopen 3 , .Xr fread 3 , .Xr malloc 3 , .Xr printf 3 , .Xr puts 3 .Sh STANDARDS The .Fn setbuf and .Fn setvbuf functions conform to .St -ansiC . .Sh BUGS The .Fn setbuffer and .Fn setlinebuf functions are not portable to versions of x before x 4.2 . On x 4.2 and x 4.3 systems, .Fn setbuf always uses a suboptimal buffer size and should be avoided.
Indexes created Mon Nov 03 11:09:54 GMT 2025