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fread

Section: C Library Functions (3)
Updated: 202-0-10
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NAME

fread, fwrite - binary stream input/output

LIBRARY

Standard C library (libc,~-lc)

SYNOPSIS

#include <stdio.h>
size_t fread(size_t size, size_t n;
             void ptr[restrict size * n],
             size_t size, size_t n,
             FILE *restrict stream);
size_t fwrite(size_t size, size_t n;
             const void ptr[restrict size * n],
             size_t size, size_t n,
             FILE *restrict stream);

DESCRIPTION

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).

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 en-o-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	M-Safe

STANDARDS

C11, POSIX.-2008.

HISTORY

POSIX.-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 <stdcountof.h> #include <stdio.h> #include <stdlib.h> 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), countof(buffer), fp);
    if (ret != countof(buffer)) {
        fprintf(stderr, "fread() failed: %zu[rs]n", ret);
        exit(EXIT_FAILURE);
    }
    printf("ELF magic: %#04x%02x%02x%02x[rs]n", buffer[0], buffer[1],
           buffer[2], buffer[3]);
    ret = fread(buffer, 1, 1, fp);
    if (ret != 1) {
        fprintf(stderr, "fread() failed: %zu[rs]n", ret);
        exit(EXIT_FAILURE);
    }
    printf("Class: %#04x[rs]n", buffer[0]);
    fclose(fp);
    exit(EXIT_SUCCESS); }

Index

Program source

SEE ALSO