FREAD(3S)                            SysV                            FREAD(3S)



NAME
     fread, fwrite - binary input/output

SYNOPSIS
     #include <stdio.h>

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

     size_t fwrite(ptr, size, nmemb, stream)
     const void *ptr;
     size_t size;
     size_t nmemb;
     FILE *stream;

DESCRIPTION
     The fread function reads, into the array pointed to by ptr, up to nmemb
     elements whose size is specified by size, from the stream pointed to by
     stream.  The file position indicator for the stream (if defined) is
     advanced by the number of characters successfully read.  If an error
     occurs, the resulting value of the file position indicator for the stream
     is indeterminate.  If a partial element is read, its value is
     indeterminate.

     If stream is stdin or is unbuffered or line-buffered, then all active
     line-buffered output streams are flushed before any call to read(2) to
     satisfy the fread.

     fread returns the number of elements successfully read, which may be less
     than nmemb if a read error or EOF is encountered.  If size or nmemb is
     zero, fread returns zero and the contents of the array and the state of
     the stream remain unchanged. Otherwise, if a write error occurs, the
     error indicator for the stream is set and errno is set to indicate the
     error.

     The st_atime field is marked for update by the first successful execution
     of fread using stream that returns data not supplied by a prior call to
     ungetc.

     The fwrite function writes, from the array pointed to by ptr, up to nmemb
     elements whose size is specified by size, to the stream pointed to by
     stream.  The file position indicator for the stream (if defined) is
     advanced by the number of characters successfully written.  If an error
     occurs, the resulting value of the file position indicator for the stream
     is indeterminate.

     fwrite returns the number of items actually written, which will be less
     than nmemb only if a write error is encountered.

     If size or nmemb is zero, fwrite returns zero and the contents of the
     array and the state of the stream remain unchanged. Otherwise, if a write
     error occurs, the error indicator for the stream is set and errno is set
     to indicate the error.

     The st_ctime and st_mtime fields of the file are marked for update
     between the successful execution of fwrite and the next successful
     completion of a call to fflush(3S) or fclose(3S) on the same stream or a
     call to exit(2) or abort(3C).

ERRORS
     fread fails if either stream is unbuffered, or the data needs to be read
     into the stream's buffer and the call to fread caused an underlying
     read(2) or lseek(2) to be invoked, and:

     [EAGAIN]       The O_NONBLOCK flag is set for the file descriptor
                    underlying stream and the process would be delayed in the
                    write operation.

     [EBADF]        The file descriptor underlying stream is not a valid file
                    descriptor open for reading.

     [EINTR]        The read operation was interrupted by a signal which was
                    caught, and no data was transferred.

     [EIO]          The process is a member of a background process group
                    attempting to write to its controlling terminal, TOSTOP is
                    set, the process is neither ignoring nor blocking SIGTTOU
                    and the process group of the process is orphaned.

     fwrite fails if either stream is unbuffered, or the buffer needed to be
     flushed and the call to fwrite caused an underlying read or lseek to be
     invoked, and:

     [EAGAIN]       The O_NONBLOCK flag is set for the file descriptor
                    underlying stream and the process would be delayed in the
                    write operation.

     [EBADF]        The file descriptor underlying stream is not a valid file
                    descriptor open for writing.

     [EFBIG]        An attempt was made to write to a file that exceeds the
                    process' file size limit or the maximum file size.

     [EINTR]        The write operation was interrupted by a signal which was
                    caught, and no data was transferred.

     [EIO]          The process is a member of a background process group
                    attempting to write to its controlling terminal, TOSTOP is
                    set, the process is neither ignoring nor blocking SIGTTOU
                    and the process group of the process is orphaned.

     [ENOSPC]       There was no free space remaining on the device containing
                    the file.

     [EPIPE]        An attempt was made to write to a pipe or FIFO that is not
                    open for reading by any process.  A SIGPIPE signal will
                    also be sent to the process.

NOTES
     If the fwrite call indicates that less data was written than had been
     requested, an underlying error has ocrurred and errno is set.

     To be compatible with traditional C usage, make fread and fwrite return
     an int by compiling your program with the

          -A nansi

     option (see cc(1)) and inserting the directive

          #define _CLASSIC_TYPES

     before any

          #include

     directives.

SEE ALSO
     cc(1), read(2), write(2), fopen(3S), getc(3S), gets(3S), printf(3S),
     putc(3S), puts(3S), scanf(3S), stdio(3S).

DIAGNOSTICS
     fread and fwrite return the number of items read or written.  If nitems
     is non-positive, no characters are read or written and 0 is returned by
     both fread and fwrite.  Zero is returned on EOF or error.