19.9 为何Ctrl-C未能产生SIGINT(2)

https://scz.617.cn/unix/200904231116.txt

Q: plank@SMTH 2009-04-22 02:02

一个前台进程，从stdin输入，但这个进程不响应Ctrl-C。查看了termios结构的c_cc [VINTR]，等于3，正常。程序里没有安装SIGINT信号处理函数。如果从另一个终端上 "kill -2 "，可以杀掉该进程。这说明之前按Ctrl-C并未将SIGINT投递到该进程。为什么？

A: scz@nsfocus 2009-04-23 11:16

至少有一种可能，c_lflag的ISIG被复位了，此时Ctrl-C不会产生SIGINT信号。下面这个Disable_Ctrl_C.c演示了这种效果。

/ * For x86/Linux Kernel 2.6.18-4 * gcc-3.3 -Wall -pipe -O3 -o Disable_Ctrl_C Disable_Ctrl_C.c /

include

static struct termios original_term; / * for signal handlers / typedef void Sigfunc ( int );

static void Atexit ( void ( func ) ( void ) ) { if ( 0 != atexit( func ) ) { / * perror( "atexit error" ); / exit( EXIT_FAILURE ); } return; } / end of Atexit */

static void on_sigint ( int signo ) { / * 演示用，不推荐在信号处理函数中使用fprintf() / fprintf( stderr, "\nsigno = %d\n", signo ); / * 这次我们使用atexit()函数 / exit( EXIT_SUCCESS ); } / end of on_sigint /

static Sigfunc * PrivateSignal ( int signo, Sigfunc *func ) { struct sigaction act, oact;

act.sa_handler  = func;
sigemptyset( &act.sa_mask );
act.sa_flags    = 0;
if ( signo == SIGALRM )
{

ifdef SA_INTERRUPT

    /*
     * SunOS 4.x
     */
    act.sa_flags   |= SA_INTERRUPT;

endif

}
else
{

ifdef SA_RESTART

    /*
     * SVR4, 4.4BSD
     */
    act.sa_flags   |= SA_RESTART;

endif

}
if ( sigaction( signo, &act, &oact ) < 0 )
{
    return( SIG_ERR );
}
return( oact.sa_handler );

} / end of PrivateSignal /

static Sigfunc * Signal ( int signo, Sigfunc func ) { Sigfunc sigfunc;

if ( SIG_ERR == ( sigfunc = PrivateSignal( signo, func ) ) )
{
    perror( "signal" );
    exit( EXIT_FAILURE );
}
return( sigfunc );

} / end of Signal /

static void terminate ( void ) { / * 恢复原始终端属性 / tcsetattr( STDIN_FILENO, TCSANOW, &original_term ); _exit( EXIT_SUCCESS ); } / end of terminate /

int main ( int argc, char * argv[] ) { int ret = EXIT_FAILURE; struct termios current_term;

if ( isatty( STDIN_FILENO ) == 0 )
{
    fprintf( stderr, "standard input is not a terminal device\n" );
    goto main_exit;
}
/*
 * 保存原始终端属性
 */
if ( tcgetattr( STDIN_FILENO, &original_term ) < 0 )
{
    perror( "tcgetattr( STDIN_FILENO, ... ) failed" );
    goto main_exit;
}
/*
 * 安装一些回调函数
 */
Atexit( terminate );
Signal( SIGINT, on_sigint );
current_term            = original_term;
/*
 * When any of the characters INTR, QUIT, SUSP, or DSUSP are received,
 * generate the corresponding signal.
 */
current_term.c_lflag   &= ~ISIG;
/*
 * 也可以直接调用cfmakeraw()，这包括将ISIG复位的操作。
 *
 * cfmakeraw( &current_term );
 */
if ( tcsetattr( STDIN_FILENO, TCSANOW, &current_term ) < 0 )
{
    perror( "tcsetattr( STDIN_FILENO, TCSANOW, ... ) failed" );
    goto main_exit;
}
/*
 * 产生阻塞
 */
getchar();
ret                     = EXIT_SUCCESS;
printf( "ok\n" );

main_exit:

return( ret );

} / end of main /

$ ./Disable_Ctrl_C ^C^C^C // getchar()产生阻塞，在此狂按Ctrl-C，没反应 signo = 2 // 从另一个终端上"kill -2 "，on_sigint()被执行 $

从另一个终端上执行如下命令投递SIGINT信号:

$ kill -INT ps auwx | grep Disable_Ctrl_C | grep -v grep | awk '{print $2;}'

参看termios(3)手册页了解更多信息。