fwrite(3)

1fread(3)                   Library Functions Manual                   fread(3)
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NAME

6       fread, fwrite - binary stream input/output
7

LIBRARY

9       Standard C library (libc, -lc)
10

SYNOPSIS

12       #include <stdio.h>
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14       size_t fread(void ptr[restrict .size * .nmemb],
15                    size_t size, size_t nmemb,
16                    FILE *restrict stream);
17       size_t fwrite(const void ptr[restrict .size * .nmemb],
18                    size_t size, size_t nmemb,
19                    FILE *restrict stream);
20

DESCRIPTION

22       The  function  fread() reads nmemb items of data, each size bytes long,
23       from the stream pointed to by stream,  storing  them  at  the  location
24       given by ptr.
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26       The function fwrite() writes nmemb items of data, each size bytes long,
27       to the stream pointed to by stream, obtaining them  from  the  location
28       given by ptr.
29
30       For nonlocking counterparts, see unlocked_stdio(3).
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RETURN VALUE

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

45       For an  explanation  of  the  terms  used  in  this  section,  see  at‐
46       tributes(7).
47
48       ┌────────────────────────────────────────────┬───────────────┬─────────┐
49       │Interface                                   │ Attribute     │ Value   │
50       ├────────────────────────────────────────────┼───────────────┼─────────┤
51       │fread(), fwrite()                           │ Thread safety │ MT-Safe │
52       └────────────────────────────────────────────┴───────────────┴─────────┘
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STANDARDS

55       C11, POSIX.1-2008.
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HISTORY

58       POSIX.1-2001, C89.
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EXAMPLES

61       The  program  below  demonstrates the use of fread() by parsing /bin/sh
62       ELF executable in binary mode and printing its magic and class:
63
64           $ ./a.out
65           ELF magic: 0x7f454c46
66           Class: 0x02
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68   Program source
69
70       #include <stdio.h>
71       #include <stdlib.h>
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73       #define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]))
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75       int
76       main(void)
77       {
78           FILE           *fp;
79           size_t         ret;
80           unsigned char  buffer[4];
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82           fp = fopen("/bin/sh", "rb");
83           if (!fp) {
84               perror("fopen");
85               return EXIT_FAILURE;
86           }
87
88           ret = fread(buffer, sizeof(*buffer), ARRAY_SIZE(buffer), fp);
89           if (ret != ARRAY_SIZE(buffer)) {
90               fprintf(stderr, "fread() failed: %zu\n", ret);
91               exit(EXIT_FAILURE);
92           }
93
94           printf("ELF magic: %#04x%02x%02x%02x\n", buffer[0], buffer[1],
95                  buffer[2], buffer[3]);
96
97           ret = fread(buffer, 1, 1, fp);
98           if (ret != 1) {
99               fprintf(stderr, "fread() failed: %zu\n", ret);
100               exit(EXIT_FAILURE);
101           }
102
103           printf("Class: %#04x\n", buffer[0]);
104
105           fclose(fp);
106
107           exit(EXIT_SUCCESS);
108       }
109