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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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FREAD(3P) POSIX Programmer's Manual FREAD(3P)
This manual page is part of the POSIX Programmer's Manual. The
Linux implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
fread — binary input
#include <stdio.h>
size_t fread(void *restrict ptr, size_t size, size_t nitems,
FILE *restrict stream);
The functionality described on this reference page is aligned with
the ISO C standard. Any conflict between the requirements
described here and the ISO C standard is unintentional. This
volume of POSIX.1‐2017 defers to the ISO C standard.
The fread() function shall read into the array pointed to by ptr
up to nitems elements whose size is specified by size in bytes,
from the stream pointed to by stream. For each object, size calls
shall be made to the fgetc() function and the results stored, in
the order read, in an array of unsigned char exactly overlaying
the object. The file position indicator for the stream (if
defined) shall be advanced by the number of bytes successfully
read. If an error occurs, the resulting value of the file position
indicator for the stream is unspecified. If a partial element is
read, its value is unspecified.
The fread() function may mark the last data access timestamp of
the file associated with stream for update. The last data access
timestamp shall be marked for update by the first successful
execution of fgetc(), fgets(), fread(), fscanf(), getc(),
getchar(), getdelim(), getline(), gets(), or scanf() using stream
that returns data not supplied by a prior call to ungetc().
Upon successful completion, fread() shall return the number of
elements successfully read which is less than nitems only if a
read error or end-of-file is encountered. If size or nitems is 0,
fread() shall return 0 and the contents of the array and the state
of the stream remain unchanged. Otherwise, if a read error occurs,
the error indicator for the stream shall be set, and errno shall
be set to indicate the error.
Refer to fgetc(3p).
The following sections are informative.
Reading from a Stream
The following example transfers a single 100-byte fixed length
record from the fp stream into the array pointed to by buf.
#include <stdio.h>
...
size_t elements_read;
char buf[100];
FILE *fp;
...
elements_read = fread(buf, sizeof(buf), 1, fp);
...
If a read error occurs, elements_read will be zero but the number
of bytes read from the stream could be anything from zero to
sizeof(buf)-1.
The following example reads multiple single-byte elements from the
fp stream into the array pointed to by buf.
#include <stdio.h>
...
size_t bytes_read;
char buf[100];
FILE *fp;
...
bytes_read = fread(buf, 1, sizeof(buf), fp);
...
If a read error occurs, bytes_read will contain the number of
bytes read from the stream.
The ferror() or feof() functions must be used to distinguish
between an error condition and an end-of-file condition.
Because of possible differences in element length and byte
ordering, files written using fwrite() are application-dependent,
and possibly cannot be read using fread() by a different
application or by the same application on a different processor.
None.
None.
Section 2.5, Standard I/O Streams, feof(3p), ferror(3p),
fgetc(3p), fopen(3p), fscanf(3p), getc(3p), gets(3p)
The Base Definitions volume of POSIX.1‐2017, stdio.h(0p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 by the Institute of Electrical and Electronics Engineers,
Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard,
the original IEEE and The Open Group Standard is the referee
document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page
are most likely to have been introduced during the conversion of
the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 FREAD(3P)
Pages that refer to this page: stdio.h(0p), fgetc(3p), fgets(3p), fgetws(3p), stdin(3p)
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HTML rendering created 2025-02-02 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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