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

NAME
       fread, fwrite - binary stream input/output

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 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 er‐
       ror 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, sizeof(*buffer), ARRAY_SIZE(buffer), fp);
           if (ret != ARRAY_SIZE(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)

GNU                                                                            2021-03-22                                                                       FREAD(3)