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 st_atime field of the file
associated with stream for update. The st_atime field shall be
marked for update by the first successful execution of fgetc(),
fgets(), fgetwc(), fgetws(), fread(),
fscanf(), getc(), getchar(), gets(), or
scanf() using stream that returns data not supplied by a prior
call to ungetc() or ungetwc().
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.
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.
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 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