NAME

fread, fwrite — binary stream input/output

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#include <stdio.h>

size_t
fread(void * restrict ptr, size_t size, size_t nmemb, FILE * restrict stream);

size_t
fwrite(const void * restrict ptr, size_t size, size_t nmemb, FILE * restrict stream);

DESCRIPTION

The function fread() reads nmemb objects, each size bytes long, from the stream pointed to by stream, storing them at the location given by ptr.

The function fwrite() writes nmemb objects, each size bytes long, to the stream pointed to by stream, obtaining them from the location given by ptr.

Mixing fread() and fwrite() calls without setting the file position explicitly using fsetpos(3) between read and write or write and read operations will lead to unexpected results because of buffering the file pointer not being set to the expected position after each operation completes. This behavior is allowed by ANSI C for efficiency and it will not be changed.

RETURN VALUES

The functions fread() and fwrite() advance the file position indicator for the stream by the number of bytes read or written. They return the number of objects read or written. If size or nmemb is 0, the functions return 0 and the state of stream remains unchanged.

If the product of size and nmemb results in size_t overflow, 0 is returned and errno is set to EOVERFLOW.

If an error occurs, or the end-of-file is reached, the return value is a short object count (or zero).

The function fread() does not distinguish between end-of-file and error, and callers must use feof(3) and ferror(3) to determine which occurred. The function fwrite() returns a value less than nmemb only if a write error has occurred.

SEE ALSO

read(2), write(2)

STANDARDS

The functions fread() and fwrite() conform to ANSI X3.159-1989 (“ANSI C89”).