fread

FREAD(3)                   Linux Programmer's Manual                  FREAD(3)

NAME
       fread, fwrite - binary stream input/output

SYNOPSIS
       #include <stdio.h>

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

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

DESCRIPTION
       The  function  fread() reads nmemb 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 nmemb 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
       written.  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  at-
       tributes(7).

       +------------------+---------------+---------+
       |Interface         | Attribute     | Value   |
       +------------------+---------------+---------+
       |fread(), fwrite() | Thread safety | MT-Safe |
       +------------------+---------------+---------+
CONFORMING TO
       POSIX.1-2001, POSIX.1-2008, 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 <stdio.h>
       #include <stdlib.h>

       #define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]))

       int
       main(void)
       {
           FILE *fp = fopen("/bin/sh", "rb");
           if (!fp) {
               perror("fopen");
               return EXIT_FAILURE;
           }

           unsigned char buffer[4];

           size_t ret = fread(buffer, ARRAY_SIZE(buffer), sizeof(*buffer), fp);
           if (ret != sizeof(*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)

COLOPHON
       This page is part of release 5.10 of the Linux  man-pages  project.   A
       description  of  the project, information about reporting bugs, and the
       latest    version    of    this    page,    can     be     found     at
       https://www.kernel.org/doc/man-pages/.

GNU                               2020-08-13                          FREAD(3)