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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       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at https://www.kernel.org/doc/man-pages/.