fwrite - write bytes to a file

fread, fwrite - binary stream input/output

Header File

#include <stdio.h>

Prototype

size_t fwrite(void *ptr, size_t size, size_t nmemb, FILE *stream);

Think of void * as representing the address of the first byte of any type of data. Think of size_t as a long.

#include <stdio.h>

size_t fread(void *ptr, size_t size, size_t nmemb, FILE *stream);

size_t fwrite(const void *ptr, size_t size, size_t nmemb,
 FILE *stream);

This function writes data to a file that has been opened via fopen. It expects as input:

• ptr, which is the address (of the first byte) of memory from which to read the data,
• size, which is the size (in bytes) of the type of data to write,
• nmemb, which is the number of those types to write at once, and
• stream, which is the pointer to a FILE returned by fopen.

For instance, if writing one char at a time, size would be sizeof(char) (i.e., 1), and nmemb would be 1.

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).

This function returns the number of items written, which equals the number of bytes written when size is 1.

If an error occurs, or the end of the file is reached, this function might return a value smaller than nmemb or even 0.

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.

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, ARRAY_SIZE(buffer), sizeof(*buffer), fp);
    if (ret != sizeof(*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);
}

read(2), write(2), feof(3), ferror(3), unlocked_stdio(3)