fread(3)		    Library Functions Manual			fread(3)

NAME
     fread, fwrite - binary stream input/output

LIBRARY
     Standard C library (libc, -lc)

SYNOPSIS
     #include <stdio.h>

     size_t fread(size_t size, size_t n;
		  void ptr[restrict size * n],
		  size_t size, size_t n,
		  FILE *restrict stream);
     size_t fwrite(size_t size, size_t n;
		  const void ptr[restrict size * n],
		  size_t size, size_t n,
		  FILE *restrict stream);

DESCRIPTION
     The  function fread() reads n items of data, each size bytes long, from the
     stream pointed to by stream, storing them at the location given by ptr.

     The function fwrite() writes n items of data, each size bytes long, to  the
     stream pointed to by stream, obtaining them from the location given by ptr.

     For nonlocking counterparts, see unlocked_stdio(3).

RETURN VALUE
     On	 success,  fread() and fwrite() return the number of items read or writ-
     ten.  This number equals the number of bytes transferred only when size  is
     1.	 If an error occurs, or the end of the file is reached, the return value
     is a short item count (or zero).

     The  file	position  indicator  for the stream is advanced by the number of
     bytes successfully read or written.

     fread() does not distinguish between end-of-file  and  error,  and	 callers
     must use feof(3) and ferror(3) to determine which occurred.

ATTRIBUTES
     For  an  explanation  of the terms used in this section, see attributes(7).
     allbox; lbx lb lb l l l.  Interface Attribute Value T{ fread(), fwrite()
     T}	  Thread safety	  MT-Safe

STANDARDS
     C11, POSIX.1-2008.

HISTORY
     POSIX.1-2001, C89.

EXAMPLES
     The program below demonstrates the use of fread() by  parsing  /bin/sh  ELF
     executable in binary mode and printing its magic and class:

	 $ ./a.out;
	 ELF magic: 0x7f454c46
	 Class: 0x02

   Program source

     #include <stdcountof.h>
     #include <stdio.h>
     #include <stdlib.h>

     int
     main(void)
     {
	 FILE		*fp;
	 size_t		ret;
	 unsigned char	buffer[4];

	 fp = fopen("/bin/sh", "rb");
	 if (!fp) {
	     perror("fopen");
	     return EXIT_FAILURE;
	 }

	 ret = fread(buffer, sizeof(*buffer), countof(buffer), fp);
	 if (ret != countof(buffer)) {
	     fprintf(stderr, "fread() failed: %zu\n", ret);
	     exit(EXIT_FAILURE);
	 }

	 printf("ELF magic: %#04x%02x%02x%02x\n", buffer[0], buffer[1],
		buffer[2], buffer[3]);

	 ret = fread(buffer, 1, 1, fp);
	 if (ret != 1) {
	     fprintf(stderr, "fread() failed: %zu\n", ret);
	     exit(EXIT_FAILURE);
	 }

	 printf("Class: %#04x\n", buffer[0]);

	 fclose(fp);

	 exit(EXIT_SUCCESS);
     }

SEE ALSO
     read(2), write(2), feof(3), ferror(3), unlocked_stdio(3)

Linux man-pages 6.18		   2026-02-10				fread(3)