fwrite - write bytes to a file

fread, fwrite - binary stream input/output


#include <stdio.h>
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).


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).

fread() does not distinguish between end-of-file and error, and callers must use feof(3) and ferror(3) to determine which occurred.


#include <stdio.h>
int main(void) { FILE *input = fopen("input.txt", "r"); if (input == NULL) { return 1; } FILE *output = fopen("output.txt", "w"); if (output == NULL) { fclose(input); return 1; } char c; while (fread(&c, sizeof(char), 1, input)) { fwrite(&c, sizeof(char), 1, output); } fclose(input); fclose(output); }


For an explanation of the terms used in this section, see attributes(7).

Interface Attribute Value
fread(), fwrite() Thread safety MT-Safe


POSIX.1-2001, POSIX.1-2008, C89.


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


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