Q32813: The fopen Function Fails to Open Printer in Bound Application

Article: Q32813
Product(s): See article
Version(s): 5.10   | 5.10
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | | mspl13_c
Last Modified: 12-JAN-1990

Attempting to use the C run-time function fopen() to open the printer
will fail in real mode if the application calling the function is
bound. The "invalid argument" error is returned.

This problem is caused by the Family API DosOpen function. It will
fail to open the printer when called with a flag that specifies
"truncate if the file exists," which is the flag used by fopen().

The following program demonstrates a work around for this problem
using the C run-time library functions open() and fdopen():
The program below uses open() to get a file handle for the printer.
Note that the oflag used is O_WRONLY. ORing in O_TRUNC would fail in
real mode. The device name "lpt1" or "lpt2" could be used instead of
"prn", depending on how the computer is set up. The generic name for
the printer is "prn".

Then the fdopen() function is used to create a stream for the printer,
and the printer is written to by the fprintf() function.

The following is the sample program:

#include <fcntl.h>      /* include files for open() */
#include <sys\types.h>
#include <sys\stat.h>
#include <io.h>
#include <stdio.h>      /* include file for printf() and
fdopen() */

int   fh;     /* file handle for printer */
FILE *stream; /* stream for printer     */

main() {
/* open file handle for printer, check for open failure */
    if ((fh = open("prn",O_WRONLY)) == -1 )
        printf("Opening file handle failed.\n");

/* associate stream for fh above, check for failure */
    else {
        if((stream = fdopen(fh,"w")) == NULL)
            printf("Creation of stream from file handle
failed.\n");

/* if previous function calls succeeded, print "Hello" */
        else
            fprintf(stream,"Hello\n");
        }
    }

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