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fread(3) — Linux manual page

NAME \|LIBRARY \|SYNOPSIS \|DESCRIPTION \|RETURN VALUE \|ATTRIBUTES \|STANDARDS \|HISTORY \|EXAMPLES \|SEE ALSO \|COLOPHON

fread(3)                 Library Functions Manualfread(3)

NAME top

       fread, fwrite - binary stream input/output

LIBRARY top

       Standard C library (libc,-lc)

SYNOPSIS top

#include <stdio.h>size_t fread(size_t size, size_t n;voidptr[restrictsize*n],size_tsize, size_tn,FILE *restrictstream);size_t fwrite(size_t size, size_t n;const voidptr[restrictsize*n],size_tsize, size_tn,FILE *restrictstream);

DESCRIPTION top

       The functionfread() readsn items of data, eachsize bytes long,       from the stream pointed to bystream, storing them at the location       given byptr.       The functionfwrite() writesn items of data, eachsize bytes       long, to the stream pointed to bystream, obtaining them from the       location given byptr.       For nonlocking counterparts, seeunlocked_stdio(3).

RETURN VALUE top

       On success,fread() andfwrite() return the number of items read       or written.  This number equals the number of bytes transferred       only whensize 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 usefeof(3) andferror(3) to determine which       occurred.

ATTRIBUTES top

       For an explanation of the terms used in this section, seeattributes(7).       ┌──────────────────────────────────────┬───────────────┬─────────┐       │Interface│Attribute│Value│       ├──────────────────────────────────────┼───────────────┼─────────┤       │fread(),fwrite()                    │ Thread safety │ MT-Safe │       └──────────────────────────────────────┴───────────────┴─────────┘

STANDARDS top

       C11, POSIX.1-2008.

HISTORY top

       POSIX.1-2001, C89.

EXAMPLES top

       The program below demonstrates the use offread() by parsing       /bin/sh ELF executable in binary mode and printing its magic and       class:           $./a.out;           ELF magic: 0x7f454c46           Class: 0x02Program 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);       }

COLOPHON top

       This page is part of theman-pages (Linux kernel and C library       user-space interface documentation) project.  Information about       the project can be found at        ⟨https://www.kernel.org/doc/man-pages/⟩.  If you have a bug report       for this manual page, see       ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.       This page was obtained from the tarball man-pages-6.15.tar.gz       fetched from       ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on       2025-08-11.  If you discover any rendering problems in this HTML       version of the page, or you believe there is a better or more up-       to-date source for the page, or you have corrections or       improvements to the information in this COLOPHON (which isnot       part of the original manual page), send a mail to       man-pages@man7.orgLinux man-pages 6.15            2025-06-28fread(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)