fread(3p) — Linux manual page

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

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.1‐2017 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(3p).

       The following sections are informative.

EXAMPLES

   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.

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(3p), ferror(3p),
       fgetc(3p), fopen(3p), fscanf(3p), getc(3p), gets(3p)

       The Base Definitions volume of POSIX.1‐2017, stdio.h(0p)

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(3P)

Pages that refer to this page: stdio.h(0p), fgetc(3p), fgets(3p), fgetws(3p), stdin(3p)

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