setbuf

Name

setbuf, setbuffer, setlinebuf, setvbuf
- stream buffering operations

Library

libc.lib

Synopsis

#include <stdio.h>

void setbuf (FILE * restrict stream, char * restrict buf);

void setbuffer (FILE *stream, char *buf, int size);

int setlinebuf (FILE *stream);

int setvbuf (FILE * restrict stream, char * restrict buf, int mode, size_t size);
Sh RETURN VALUES The setvbuf function returns 0 on success, or EOF if the request cannot be honoured (note that the stream is still functional in this case).

The setlinebuf function returns what the equivalent setvbuf would have returned.

Detailed 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). The function fflush may be used to force the block out early.

Normally all files are block buffered. When the first I/O operation occurs on a file, malloc is called, and an optimally-sized buffer is obtained. If a stream refers to a terminal (as stdout normally does) it is line buffered. The standard error stream stderr is always unbuffered.

The setvbuf function may be used to alter the buffering behavior of a stream. The mode argument must be one of the following three macros:

`_IONBF`
	unbuffered
`_IOLBF`
	line buffered
`_IOFBF`
	fully buffered

The size argument 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 buf argument should point to a buffer at least size bytes long; this buffer will be used instead of the current buffer. If buf is not NULL, it is the caller’s responsibility to free this buffer after closing the stream.

The 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 I/O is performed.

The other three calls are, in effect, simply aliases for calls to setvbuf. Except for the lack of a return value, the setbuf function is exactly equivalent to the call

     setvbuf(stream, buf, buf ? _IOFBF : _IONBF, BUFSIZ);

The setbuffer function is the same, except that the size of the buffer is up to the caller, rather than being determined by the default BUFSIZ. The setlinebuf function is exactly equivalent to the call:

     setvbuf(stream, (char *)NULL, _IOLBF, 0);

Examples

/****************** this program shows setting up a buffer using setbuf **************/
#include <stdio.h>
int main()
{
        FILE *fp;
        char FullBuf[100];
        char msg[100];
        char * rptr;
        char name[20] = "c:\\setbuf1.txt";
        fp = fopen(name, "w+");
        if (fp == NULL)
                {
                printf ("fopen failed\n");
                return -1;
                }
        setbuf(fp, FullBuf);  // Fully buffered
        if (ferror(fp))
                {
                printf ("setbuf failed\n");
                fclose(fp);
                unlink(name);
                return -1;
                }
        fprintf(fp, "we are trying to buffer 20 characters at once\n ");
        
        fclose(fp);
        fp=fopen(name,"r");
        rptr = fgets(msg,100,fp);
        if(rptr == NULL)
                {
                printf("fgets failed\n");
                fclose(fp);
                return -1;
                }
        printf("file reading returned \"%s\"\n",msg);
        fclose(fp);
        
        unlink(name);
        return 0;
}

Output

file reading returned "we are trying to buffer 20 characters at once"

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