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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ATTRIBUTES | CONFORMING TO | EXAMPLES | SEE ALSO | COLOPHON |
FREAD(3) Linux Programmer's Manual FREAD(3)
fread, fwrite - binary stream input/output
#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);
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, C89.
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>
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, sizeof(buffer) / sizeof(*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)
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latest version of this page, can be found at
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GNU 2020-08-13 FREAD(3)
Pages that refer to this page: read(2) , fgetc(3) , fgets(3) , getc(3) , getchar(3) , getdelim(3) , getline(3) , gets(3) , getw(3) , putw(3) , setbuf(3) , setbuffer(3) , setlinebuf(3) , setvbuf(3) , stderr(3) , stdin(3) , stdio(3) , stdout(3) , ungetc(3)
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