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(void ptr[restrict .size * .nmemb],
                    size_t size, size_t nmemb,
                    FILE *restrict stream);
       size_t fwrite(const void ptr[restrict .size * .nmemb],
                    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 trans‐
       ferred 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 │
       └─────────────────────────────────────────────────────────────────────────────────────────────────┴───────────────┴─────────┘

STANDARDS
       POSIX.1-2001, POSIX.1-2008, C99.

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;
           size_t         ret;
           unsigned char  buffer[4];

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

           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)

Linux man-pages 6.03                                         2022-12-29                                                    fread(3)