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10.16. sigsuspend FunctionWe have seen how we can change the signal mask for a process to block and unblock selected signals. We can use this technique to protect critical regions of code that we don't want interrupted by a signal. What if we want to unblock a signal and then pause, waiting for the previously blocked signal to occur? Assuming that the signal is SIGINT, the incorrect way to do this is
sigset_t newmask, oldmask;
sigemptyset(&newmask);
sigaddset(&newmask, SIGINT);
/* block SIGINT and save current signal mask */
if (sigprocmask(SIG_BLOCK, &newmask, &oldmask) < 0)
err_sys("SIG_BLOCK error");
/* critical region of code */
/* reset signal mask, which unblocks SIGINT */
if (sigprocmask(SIG_SETMASK, &oldmask, NULL) < 0)
err_sys("SIG_SETMASK error");
/* window is open */
pause(); /* wait for signal to occur */
/* continue processing */
If the signal is sent to the process while it is blocked, the signal delivery will be deferred until the signal is unblocked. To the application, this can look as if the signal occurs between the unblocking and the pause (depending on how the kernel implements signals). If this happens, or if the signal does occur between the unblocking and the pause, we have a problem. Any occurrence of the signal in this window of time is lost in the sense that we might not see the signal again, in which case the pause will block indefinitely. This is another problem with the earlier unreliable signals. To correct this problem, we need a way to both reset the signal mask and put the process to sleep in a single atomic operation. This feature is provided by the sigsuspend function.
The signal mask of the process is set to the value pointed to by sigmask. Then the process is suspended until a signal is caught or until a signal occurs that terminates the process. If a signal is caught and if the signal handler returns, then sigsuspend returns, and the signal mask of the process is set to its value before the call to sigsuspend. Note that there is no successful return from this function. If it returns to the caller, it always returns 1 with errno set to EINTR (indicating an interrupted system call). ExampleFigure 10.22 shows the correct way to protect a critical region of code from a specific signal. Note that when sigsuspend returns, it sets the signal mask to its value before the call. In this example, the SIGINT signal will be blocked. We therefore reset the signal mask to the value that we saved earlier (oldmask). Running the program from Figure 10.22 produces the following output: $ ./a.out program start: in critical region: SIGINT ^? type the interrupt character in sig_int: SIGINT SIGUSR1 after return from sigsuspend: SIGINT program exit: We added SIGUSR1 to the mask installed when we called sigsuspend so that when the signal handler ran, we could tell that the mask had actually changed. We can see that when sigsuspend returns, it restores the signal mask to its value before the call. Figure 10.22. Protecting a critical region from a signal
#include "apue.h"
static void sig_int(int);
int
main(void)
{
sigset_t newmask, oldmask, waitmask;
pr_mask("program start: ");
if (signal(SIGINT, sig_int) == SIG_ERR)
err_sys("signal(SIGINT) error");
sigemptyset(&waitmask);
sigaddset(&waitmask, SIGUSR1);
sigemptyset(&newmask);
sigaddset(&newmask, SIGINT);
/*
* Block SIGINT and save current signal mask.
*/
if (sigprocmask(SIG_BLOCK, &newmask, &oldmask) < 0)
err_sys("SIG_BLOCK error");
/*
* Critical region of code.
*/
pr_mask("in critical region: ");
/*
* Pause, allowing all signals except SIGUSR1.
*/
if (sigsuspend(&waitmask) != -1)
err_sys("sigsuspend error");
pr_mask("after return from sigsuspend: ");
/*
* Reset signal mask which unblocks SIGINT.
*/
if (sigprocmask(SIG_SETMASK, &oldmask, NULL) < 0)
err_sys("SIG_SETMASK error");
/*
* And continue processing ...
*/
pr_mask("program exit: ");
exit(0);
}
static void
sig_int(int signo)
{
pr_mask("\nin sig_int: ");
}
ExampleAnother use of sigsuspend is to wait for a signal handler to set a global variable. In the program shown in Figure 10.23, we catch both the interrupt signal and the quit signal, but want to wake up the main routine only when the quit signal is caught. Sample output from this program is $ ./a.out ^? type the interrupt character interrupt ^? type the interrupt character again interrupt ^? and again interrupt ^? and again interrupt ^? and again interrupt ^? and again interrupt ^? and again interrupt ^\ $ now terminate with quit character Figure 10.23. Using sigsuspend to wait for a global variable to be set
#include "apue.h"
volatile sig_atomic_t quitflag; /* set nonzero by signal handler */
static void
sig_int(int signo) /* one signal handler for SIGINT and SIGQUIT */
{
if (signo == SIGINT)
printf("\ninterrupt\n");
else if (signo == SIGQUIT)
quitflag = 1; /* set flag for main loop */
}
int
main(void)
{
sigset_t newmask, oldmask, zeromask;
if (signal(SIGINT, sig_int) == SIG_ERR)
err_sys("signal(SIGINT) error");
if (signal(SIGQUIT, sig_int) == SIG_ERR)
err_sys("signal(SIGQUIT) error");
sigemptyset(&zeromask);
sigemptyset(&newmask);
sigaddset(&newmask, SIGQUIT);
/*
* Block SIGQUIT and save current signal mask.
*/
if (sigprocmask(SIG_BLOCK, &newmask, &oldmask) < 0)
err_sys("SIG_BLOCK error");
while (quitflag == 0)
sigsuspend(&zeromask);
/*
* SIGQUIT has been caught and is now blocked; do whatever.
*/
quitflag = 0;
/*
* Reset signal mask which unblocks SIGQUIT.
*/
if (sigprocmask(SIG_SETMASK, &oldmask, NULL) < 0)
err_sys("SIG_SETMASK error");
exit(0);
}
ExampleAs another example of signals, we show how signals can be used to synchronize a parent and child. Figure 10.24 shows implementations of the five routines TELL_WAIT, TELL_PARENT, TELL_CHILD, WAIT_PARENT, and WAIT_CHILD from Section 8.9. We use the two user-defined signals: SIGUSR1 is sent by the parent to the child, and SIGUSR2 is sent by the child to the parent. In Figure 15.7, we show another implementation of these five functions using pipes. Figure 10.24. Routines to allow a parent and child to synchronize
#include "apue.h"
static volatile sig_atomic_t sigflag; /* set nonzero by sig handler */
static sigset_t newmask, oldmask, zeromask;
static void
sig_usr(int signo) /* one signal handler for SIGUSR1 and SIGUSR2 */
{
sigflag = 1;
}
void
TELL_WAIT(void)
{
if (signal(SIGUSR1, sig_usr) == SIG_ERR)
err_sys("signal(SIGUSR1) error");
if (signal(SIGUSR2, sig_usr) == SIG_ERR)
err_sys("signal(SIGUSR2) error");
sigemptyset(&zeromask);
sigemptyset(&newmask);
sigaddset(&newmask, SIGUSR1);
sigaddset(&newmask, SIGUSR2);
/*
* Block SIGUSR1 and SIGUSR2, and save current signal mask.
*/
if (sigprocmask(SIG_BLOCK, &newmask, &oldmask) < 0)
err_sys("SIG_BLOCK error");
}
void
TELL_PARENT(pid_t pid)
{
kill(pid, SIGUSR2); /* tell parent we're done */
}
void
WAIT_PARENT(void)
{
while (sigflag == 0)
sigsuspend(&zeromask); /* and wait for parent */
sigflag = 0;
/*
* Reset signal mask to original value.
*/
if (sigprocmask(SIG_SETMASK, &oldmask, NULL) < 0)
err_sys("SIG_SETMASK error");
}
void
TELL_CHILD(pid_t pid)
{
kill(pid, SIGUSR1); /* tell child we're done */
}
void
WAIT_CHILD(void)
{
while (sigflag == 0)
sigsuspend(&zeromask); /* and wait for child */
sigflag = 0;
/*
* Reset signal mask to original value.
*/
if (sigprocmask(SIG_SETMASK, &oldmask, NULL) < 0)
err_sys("SIG_SETMASK error");
}
The sigsuspend function is fine if we want to go to sleep while waiting for a signal to occur (as we've shown in the previous two examples), but what if we want to call other system functions while we're waiting? Unfortunately, this problem has no bulletproof solution unless we use multiple threads and dedicate a separate thread to handling signals, as we discuss in Section 12.8. Without using threads, the best we can do is to set a global variable in the signal handler when the signal occurs. For example, if we catch both SIGINT and SIGALRM and install the signal handlers using the signal_intr function, the signals will interrupt any slow system call that is blocked. The signals are most likely to occur when we're blocked in a call to the select function (Section 14.5.1), waiting for input from a slow device. (This is especially true for SIGALRM, since we set the alarm clock to prevent us from waiting forever for input.) The code to handle this looks similar to the following:
if (intr_flag) /* flag set by our SIGINT handler */
handle_intr();
if (alrm_flag) /* flag set by our SIGALRM handler */
handle_alrm();
/* signals occurring in here are lost */
while (select( ... ) < 0) {
if (errno == EINTR) {
if (alrm_flag)
handle_alrm();
else if (intr_flag)
handle_intr();
} else {
/* some other error */
}
}
We test each of the global flags before calling select and again if select returns an interrupted system call error. The problem occurs if either signal is caught between the first two if statements and the subsequent call to select. Signals occurring in here are lost, as indicated by the code comment. The signal handlers are called, and they set the appropriate global variable, but the select never returns (unless some data is ready to be read). What we would like to be able to do is the following sequence of steps, in order.
The sigsuspend function helps us only if step 3 is a pause operation. |
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