FREAD(3)                   POSIX Programmer's Manual                  FREAD(3)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

NAME
       fread -- binary input

SYNOPSIS
       #include <stdio.h>

       size_t fread(void *restrict ptr, size_t size, size_t nitems,
           FILE *restrict stream);

DESCRIPTION
       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().

RETURN VALUE
       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.

ERRORS
       Refer to fgetc().

       The following sections are informative.

EXAMPLES
   Reading from aa 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.

APPLICATION USAGE
       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.

RATIONALE
       None.

FUTURE DIRECTIONS
       None.

SEE ALSO
       Section 2.5, Standard I/O Streams, feof(), ferror(), fgetc(), fopen(),
       fscanf(), getc(), gets()

       The Base Definitions volume of POSIX.1-2017, <stdio.h>

COPYRIGHT
       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(3)
