fread

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



PROLOG
       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.


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.1‐2008 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 a Stream
       The following example reads a single element 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.1‐2008, <stdio.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
       -- Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, Copyright (C) 2013 by the Institute of Electrical
       and Electronics Engineers, Inc and The Open Group.  (This is POSIX.1-2008
       with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/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                   2013                         FREAD(3POSIX)