FREAD(3) Linux Programmer's Manual FREAD(3)
NAME
 fread, fwrite - binary stream input/output
SYNOPSIS
 #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);
DESCRIPTION
 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).
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 end-of-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 at-
 tributes(7).
 +------------------+---------------+---------+
 |Interface | Attribute | Value |
 +------------------+---------------+---------+
 |fread(), fwrite() | Thread safety | MT-Safe |
 +------------------+---------------+---------+
CONFORMING TO
 POSIX.1-2001, POSIX.1-2008, 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 <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);
 }
SEE ALSO
 read(2), write(2), feof(3), ferror(3), unlocked_stdio(3)
COLOPHON
 This page is part of release 5.10 of the Linux man-pages project. A
 description of the project, information about reporting bugs, and the
 latest version of this page, can be found at
 https://www.kernel.org/doc/man-pages/.
GNU 2020年08月13日 FREAD(3)