fread(3) — Linux manual page

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 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 │
       └─────────────────────────────────────┴───────────────┴─────────┘

STANDARDS

       C11, POSIX.1-2008.

HISTORY

       POSIX.1-2001, 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;
           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)

COLOPHON

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Linux man-pages 6.9.1          2024-06-15                       fread(3)

Pages that refer to this page: pmlogger(1), read(2), write(2), fgetc(3), FILE(3type), getline(3), gets(3), getw(3), puts(3), setbuf(3), size_t(3type), stdin(3), stdio(3)

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