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 attributes(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

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