fread

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 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, 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)

COLOPHON
       This page is part of release 5.13 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                                2021-03-22                           FREAD(3)