setvbuf

 

C

Language

Standard Library

Type support

Dynamic memory management

Error handling

Program utilities

Date and time utilities

Strings library

Algorithms

Numerics

Input/output support

Localization support

Thread support (C11)

Atomic operations (C11)

 

File input/output

Functions
File access
fopen freopen fclose fflush	fwide setbuf setvbuf
Direct input/output
fread
fwrite
Unformatted input/output
fgetc fgets fputc fputs getchar gets putchar puts ungetc	fgetwc fgetws fputwc fputws getwchar putwchar ungetwc
Formatted input/output
scanffscanfsscanf vscanfvfscanfvsscanf (C99)(C99)(C99) printffprintfsprintfsnprintf (C99) vprintfvfprintfvsprintfvsnprintf (C99)	wscanffwscanfswscanf vwscanfvfwscanfvswscanf (C99)(C99)(C99) wprintffwprintfswprintf vwprintfvfwprintfvswprintf
File positioning
ftell
fgetpos
fseek
fsetpos
rewind
Error handling
clearerr
feof
ferror
perror
Operations on files
remove
rename
tmpfile
tmpnam

 

Defined in header <stdio.h>
int setvbuf( FILE *         stream, char *         buffer,              int mode, size_t size );		(until C99)
int setvbuf( FILE *restrict stream, char *restrict buffer,              int mode, size_t size );		(since C99)

Sets the internal buffer of the given file stream stream.

Parameters

stream	-	the file stream to set the buffer to
buffer	-	pointer to a buffer for the stream to use
mode	-	buffering mode to use. It can be one of the following values: _IOFBF full buffering _IOLBF line buffering _IONBF no buffering
size	-	size of the buffer

Return value

​0​ on success or nonzero on failure.

Notes

This function may only be used after stream has been associated with an open file, but before any other operation.

buffer may be null, in which case this call only resizes the internal buffer.

Example

setvbuf with error checking

Run this code

#include <stdio.h>
#include <stdlib.h>
 
int main(void)
{
    int file_size;
    char buffer[BUFSIZ];
    FILE * fp = fopen("test.txt","w+");
    if (setvbuf(fp,buffer,_IOFBF,BUFSIZ) != 0)
    {
       perror("setvbuf()");
       fprintf(stderr,"setvbuf() failed in file %s at line # %d\n", __FILE__,__LINE__-3);
       exit(EXIT_FAILURE);
    }
 
    /* Exhibit the contents of buffer. */
    fputs ("aaa",fp);
    printf("%s\n", buffer);
    fputs ("bbb",fp);
    printf("%s\n", buffer);
    fputs ("ccc",fp);
    printf("%s\n", buffer);
    file_size = ftell(fp);
    printf("file_size = %d\n", file_size);
    fflush (fp);              /* flush buffer */
    printf("%s\n", buffer);
    fputs ("ddd",fp);
    printf("%s\n", buffer);
    fputs ("eee",fp);
    printf("%s\n", buffer);
 
    rewind(fp);               /* flush buffer and rewind file */
    char buf[20];
    fgets(buf,sizeof buf,fp);
    printf("%s\n", buf);
 
    fclose(fp);
 
    return 0;
}

Output:

aaa
aaabbb
aaabbbccc
file_size = 9
aaabbbccc
dddbbbccc
dddeeeccc
aaabbbcccdddeee

See also

setbuf	sets the buffer for a file stream (function)
C++ documentation for setvbuf