1FREAD(3P) POSIX Programmer's Manual FREAD(3P)
2
6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
10
13 fread — binary input
14
16 #include <stdio.h>
17 size_t fread(void *restrict ptr, size_t size, size_t nitems,
19 FILE *restrict stream);
20
22 The functionality described on this reference page is aligned with the
23 ISO C standard. Any conflict between the requirements described here
24 and the ISO C standard is unintentional. This volume of POSIX.1‐2008
25 defers to the ISO C standard.
26 The fread() function shall read into the array pointed to by ptr up to
28 nitems elements whose size is specified by size in bytes, from the
29 stream pointed to by stream. For each object, size calls shall be made
30 to the fgetc() function and the results stored, in the order read, in
31 an array of unsigned char exactly overlaying the object. The file posi‐
32 tion indicator for the stream (if defined) shall be advanced by the
33 number of bytes successfully read. If an error occurs, the resulting
34 value of the file position indicator for the stream is unspecified. If
35 a partial element is read, its value is unspecified.
36 The fread() function may mark the last data access timestamp of the
38 file associated with stream for update. The last data access timestamp
39 shall be marked for update by the first successful execution of
40 fgetc(), fgets(), fread(), fscanf(), getc(), getchar(), getdelim(),
41 getline(), gets(), or scanf() using stream that returns data not sup‐
42 plied by a prior call to ungetc().
43
45 Upon successful completion, fread() shall return the number of elements
46 successfully read which is less than nitems only if a read error or
47 end-of-file is encountered. If size or nitems is 0, fread() shall
48 return 0 and the contents of the array and the state of the stream
49 remain unchanged. Otherwise, if a read error occurs, the error indica‐
50 tor for the stream shall be set, and errno shall be set to indicate the
51 error.
52
54 Refer to fgetc().
55 The following sections are informative.
57
59 Reading from a Stream
60 The following example reads a single element from the fp stream into
61 the array pointed to by buf.
62 #include <stdio.h>
64 ...
65 size_t elements_read;
66 char buf[100];
67 FILE *fp;
68 ...
69 elements_read = fread(buf, sizeof(buf), 1, fp);
70 ...
71 If a read error occurs, elements_read will be zero but the number of
73 bytes read from the stream could be anything from zero to
74 sizeof(buf)−1.
75 The following example reads multiple single-byte elements from the fp
77 stream into the array pointed to by buf.
78 #include <stdio.h>
80 ...
81 size_t bytes_read;
82 char buf[100];
83 FILE *fp;
84 ...
85 bytes_read = fread(buf, 1, sizeof(buf), fp);
86 ...
87 If a read error occurs, bytes_read will contain the number of bytes
89 read from the stream.
90
92 The ferror() or feof() functions must be used to distinguish between an
93 error condition and an end-of-file condition.
94 Because of possible differences in element length and byte ordering,
96 files written using fwrite() are application-dependent, and possibly
97 cannot be read using fread() by a different application or by the same
98 application on a different processor.
99
101 None.
102
104 None.
105
107 Section 2.5, Standard I/O Streams, feof(), ferror(), fgetc(), fopen(),
108 fscanf(), getc(), gets()
109 The Base Definitions volume of POSIX.1‐2008, <stdio.h>
111
113 Portions of this text are reprinted and reproduced in electronic form
114 from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
115 -- Portable Operating System Interface (POSIX), The Open Group Base
116 Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
117 cal and Electronics Engineers, Inc and The Open Group. (This is
118 POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
119 event of any discrepancy between this version and the original IEEE and
120 The Open Group Standard, the original IEEE and The Open Group Standard
121 is the referee document. The original Standard can be obtained online
122 at http://www.unix.org/online.html .
123 Any typographical or formatting errors that appear in this page are
125 most likely to have been introduced during the conversion of the source
126 files to man page format. To report such errors, see https://www.ker‐
127 nel.org/doc/man-pages/reporting_bugs.html .
128IEEE/The Open Group 2013 FREAD(3P)