fread, fwrite - binary stream input/output
Standard C library (
libc,
-lc)
#include <stdio.h>
size_t fread(void ptr[restrict .size * .nmemb],
size_t size, size_t nmemb,
FILE *restrict stream);
size_t fwrite(const void ptr[restrict .size * .nmemb],
size_t size, size_t nmemb,
FILE *restrict stream);
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).
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.
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, C99.
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
#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);
}
read(2),
write(2),
feof(3),
ferror(3),
unlocked_stdio(3)