fread(3) - Linux manual page (original) (raw)

fread(3) Library Functions Manual fread(3)

NAME top

   fread, fwrite - binary stream input/output

LIBRARY top

   Standard C library (_libc_, _-lc_)

SYNOPSIS top

   **#include <stdio.h>**

   **size_t fread(void** _ptr_**[restrict .**_size_ *** .**_n_**],**
                **size_t** _size_**, size_t** _n_**,**
                **FILE *restrict** _stream_**);**
   **size_t fwrite(const void** _ptr_**[restrict .**_size_ *** .**_n_**],**
                **size_t** _size_**, size_t** _n_**,**
                **FILE *restrict** _stream_**);**

DESCRIPTION top

   The function **fread**() reads _n_ 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 _n_ 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)](../man3/unlocked%5Fstdio.3.html).

RETURN VALUE top

   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)](../man3/feof.3.html) and [ferror(3)](../man3/ferror.3.html) to determine which
   occurred.

ATTRIBUTES top

   For an explanation of the terms used in this section, see
   [attributes(7)](../man7/attributes.7.html).
   ┌──────────────────────────────────────┬───────────────┬─────────┐
   │ **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 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 top

   [read(2)](../man2/read.2.html), [write(2)](../man2/write.2.html), [feof(3)](../man3/feof.3.html), [ferror(3)](../man3/ferror.3.html), [unlocked_stdio(3)](../man3/unlocked%5Fstdio.3.html)

COLOPHON top

   This page is part of the _man-pages_ (Linux kernel and C library
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   part of the original manual page), send a mail to
   man-pages@man7.org

Linux man-pages 6.10 2024-11-17 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)