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From: cpp@cci632.UUCP (Carl P. Petito)
Newsgroups: comp.os.msdos.programmer
Subject: Aborting program with ^C
Keywords: signal, ^C
Message-ID: <39616@cci632.UUCP>
Date: 2 Sep 90 17:18:42 GMT
Distribution: usa
Organization: Computer Consoles Inc. An STC Company, Rochester NY
Lines: 90

I have a program that I would like to be able to terminate from the
keyboard.  QC2.5 has an example using 'signal' that I used; however, it appears
that the only time ^C is effective is when the program is doing i/o to
the console.  To confirm this, I modified the example program as shown
below, and I found that if I hit ^C nothing happens until the string
starts to print; I get the first two characters followed by ^C.
What's worse, if output is redirected to a file, ^C doesn't do anything
while the program is running.  What am I doing wrong?  Any pointers would
be appreciated.  By the the way I'm using MS-DOS 3.3.

Carl Petito



#include <stdio.h>
#include <conio.h>
#include <signal.h>
#include <process.h>
#include <setjmp.h>
#include <stdlib.h>
#include <float.h>
#include <dos.h>
#include <bios.h>

void ctrlchandler( void );          /* Prototypes */
void safeout( char *str );
int safein( void );

void main()
{
    long i;

    /* Modify CTRL+C behavior. */
    if( signal( SIGINT, ctrlchandler ) == SIG_ERR )
    {
	fprintf( stderr, "Couldn't set SIGINT\n" );
	abort();
    }

    /* loop illustrates results. */

    for (i = 0; i < 999999l; i++)
	;
    printf("First loop done\n");

    for (i = 0; i < 999999l; i++)
	;
    printf("Second loop done\n");

}

/* Handles SIGINT (CTRL+C) interrupt. */
void ctrlchandler()
{
    int ch;

    /* Disallow CTRL+C during handler. */
    signal( SIGINT, SIG_IGN );

    safeout( "Abort processing? " );
    ch = safein();
    safeout( "\r\n" );
    if( (ch == 'y') || (ch == 'Y') )
	abort();
    else

	/* The CTRL+C interrupt must be reset to our handler since by
	 * default it is reset to the system handler.
	 */
	signal( SIGINT, ctrlchandler );
}

/* Outputs a string using system level calls. */
void safeout( char *str )
{
    union REGS inregs, outregs;

    inregs.h.ah = 0x0e;
    while( *str )
    {
	inregs.h.al = *str++;
	int86( 0x10, &inregs, &outregs );
    }
}

/* Inputs a character using system level calls. */
int safein()
{
    return (_bios_keybrd( _KEYBRD_READ ) & 0xff );
}