fwrite(3)

1FREAD(3)                   Linux Programmer's Manual                  FREAD(3)
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NAME

6       fread, fwrite - binary stream input/output
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SYNOPSIS

9       #include <stdio.h>
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11       size_t fread(void *ptr, size_t size, size_t nmemb, FILE *stream);
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13       size_t fwrite(const void *ptr, size_t size, size_t nmemb,
14                     FILE *stream);
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DESCRIPTION

17       The  function  fread() reads nmemb items of data, each size bytes long,
18       from the stream pointed to by stream,  storing  them  at  the  location
19       given by ptr.
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21       The function fwrite() writes nmemb items of data, each size bytes long,
22       to the stream pointed to by stream, obtaining them  from  the  location
23       given by ptr.
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25       For nonlocking counterparts, see unlocked_stdio(3).
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RETURN VALUE

28       On  success,  fread()  and  fwrite() return the number of items read or
29       written.  This number equals the number of bytes transferred only  when
30       size  is 1.  If an error occurs, or the end of the file is reached, the
31       return value is a short item count (or zero).
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33       The file position indicator for the stream is advanced by the number of
34       bytes successfully read or written.
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36       fread() does not distinguish between end-of-file and error, and callers
37       must use feof(3) and ferror(3) to determine which occurred.
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ATTRIBUTES

40       For an  explanation  of  the  terms  used  in  this  section,  see  at‐
41       tributes(7).
42
43       ┌──────────────────┬───────────────┬─────────┐
44       │Interface         │ Attribute     │ Value   │
45       ├──────────────────┼───────────────┼─────────┤
46       │fread(), fwrite() │ Thread safety │ MT-Safe │
47       └──────────────────┴───────────────┴─────────┘

CONFORMING TO

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

52       The  program  below  demonstrates the use of fread() by parsing /bin/sh
53       ELF executable in binary mode and printing its magic and class:
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55           $ ./a.out
56           ELF magic: 0x7f454c46
57           Class: 0x02
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59   Program source
60
61       #include <stdio.h>
62       #include <stdlib.h>
63
64       #define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]))
65
66       int
67       main(void)
68       {
69           FILE *fp = fopen("/bin/sh", "rb");
70           if (!fp) {
71               perror("fopen");
72               return EXIT_FAILURE;
73           }
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75           unsigned char buffer[4];
76
77           size_t ret = fread(buffer, ARRAY_SIZE(buffer), sizeof(*buffer), fp);
78           if (ret != sizeof(*buffer)) {
79               fprintf(stderr, "fread() failed: %zu\n", ret);
80               exit(EXIT_FAILURE);
81           }
82
83           printf("ELF magic: %#04x%02x%02x%02x\n", buffer[0], buffer[1],
84                  buffer[2], buffer[3]);
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86           ret = fread(buffer, 1, 1, fp);
87           if (ret != 1) {
88               fprintf(stderr, "fread() failed: %zu\n", ret);
89               exit(EXIT_FAILURE);
90           }
91
92           printf("Class: %#04x\n", buffer[0]);
93
94           fclose(fp);
95
96           exit(EXIT_SUCCESS);
97       }
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COLOPHON

103       This page is part of release 5.10 of the Linux  man-pages  project.   A
104       description  of  the project, information about reporting bugs, and the
105       latest    version    of    this    page,    can     be     found     at
106       https://www.kernel.org/doc/man-pages/.
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110GNU                               2020-08-13                          FREAD(3)