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sigaction(2) [osf1 man page]

sigaction(2)							System Calls Manual						      sigaction(2)

NAME
sigaction - Specifies the action to take upon delivery of a signal SYNOPSIS
#include <signal.h> int sigaction( int signal, const struct sigaction *action, struct sigaction *o_action); STANDARDS
Interfaces documented on this reference page conform to industry standards as follows: sigaction(): XPG4, XPG4-UNIX Refer to the standards(5) reference page for more information about industry standards and associated tags. PARAMETERS
Defines the signal. Points to a sigaction structure that describes the action to be taken upon receipt of the signal specified by the sig- nal parameter. Points to a sigaction structure. When the sigaction() function returns from a call, the action previously attached to the specified signal is stored in this structure. DESCRIPTION
When a process requests the sigaction() function, the process can both examine or specify what action is to be performed when the specified signal is delivered. The parameters determine the behavior of the sigaction() function as follows: Specifying the signal parameter identi- fies the signal that is to be affected. Specifying the action parameter, if not null, points to a sigaction structure that defines what action is to be performed when the signal is received. If the action parameter is null, signal handling remains unchanged; thus the call can be used to inquire about the current handling of the signal. Specifying the o_action parameter, if not null, points to a sigaction structure that contains the action previously attached to the specified signal. The X/Open Base Standard defines the following members for the sigaction structure: (*sa_handler)(int); sa_mask; sa_flags; [XSH4.2] The following additional member is defined in the sigaction structure for use by realtime and X/Open UNIX programs: sa_sigaction; The members are defined as follows: [XSH4.2] A pointer to a signal-handling function. This field can contain a value of SIG_DFL or SIG_IGN, or it can point to a function. A SIG_DFL value specifies the default action is to be taken when the signal is delivered. A value of SIG_IGN specifies that the signal has no effect on the receiving process. A pointer to a function requests that the signal be caught; that is, the signal should cause the function to be called. These actions are more fully described in the <signal.h> file. This field can request that individual signals, in addition to those in the current signal mask, be blocked from being delivered while the signal handler function specified by the sa_handler or sa_sigaction field is executing. This field can set the following flags to enable further control over the actions taken when a signal is delivered: [XSH4.2] Setting this bit causes the system to run the signal-catching function on the signal stack specified by the sigaltstack() and sigstack() functions. If this bit is not set, the function runs on the stack of the process to which the signal is delivered. [XSH4.2] Setting this bit causes the signal to be reset to SIG_DFL. Note that SIGILL, SIGTRAP, and SIGPWR cannot be automatically reset. [XSH4.2] Setting this bit causes the signal not to be blocked automatically by the kernel as it is being caught. [XSH4.2] Setting this bit enables a function that has been interrupted by the execution of this signal's handler to be restarted transparently by the system. The affected functions include wait(), and the read() and write() functions on a slow device (such as a terminal, but not a regular file). If this bit is not set and one of the previously mentioned functions is interrupted by a signal which is caught, the function returns the value -1 and sets errno to [EINTR]. [XSH4.2] If this bit is clear, the signal-catching function is entered as: void func(int signo); where signo is the only argument to the signal catching function. In this case, the sa_handler member must be used to describe the signal-catching function, and the application must not modify the sa_sigaction member. If this bit is set and the signal is caught, the signal-catching function is entered as: void func(int signo, siginfo_t *info, void *context); where two additional arguments are passed to the signal-catching function. If the second argument is not a null pointer, it points to an object of type siginfo_t explaining the reason why the signal was generated. The third argument can be cast to a pointer to an object of type ucontext_t to refer to the receiving process' context that was interrupted when the signal was delivered. In this case, the sa_sigaction member must be used to describe the signal catching function and the application must not modify the sa_han- dler member. The si_signo member contains the system-generated signal number. If non-zero, the si_errno member contains an error number identifying the condition that caused the signal to be generated. The si_code member contains a code identifying the cause of the signal. If the value of si_code is less than or equal to 0, then the signal was generated by a process, and si_pid and si_uid indicate the process ID and the real user ID of the sender. The values of si_pid and si_uid are otherwise meaningless. [Tru64 UNIX] In the backward-compatible version of sigaction(), the second and third arguments available to the signal are defined as follows: If SA_SIGINFO is clear, the second argument is an integer value providing additional error information for exception signals (see <machine/signal.h>). The third argument points to a sigcontext structure containing context information in a different format. If this bit is set and the signal parameter is equal to SIGCHLD, a SIGCHLD signal is not sent to the calling process when child processes terminate. [XSH4.2] If this bit is set and the signal parameter is equal to SIGCHLD, zombie processes are not cre- ated by the system when a child process of the calling process exits. If the wait function is subsequently issued by the calling process, it blocks until all of its child processes terminate. It then returns a value of -1 and errno is set to ECHILD to indicate the error. Note: when this flag is set, exiting child processes do not send SIGCHLD signals to the parent. [Tru64 UNIX] If this bit is set in combination with SA_SIGINFO, two SIGCHLD signals are queued to the parent for each abnormal termination of a child process that includes a core dump: an early notification signal and a final notification signal. The early notification signal delivers a siginfo structure with the si_signo field set to SIGCHLD, the si_code field set to CLD_SIGEXITING, and the si_pid field set to the process ID of the child process that is about to write a core file and then termi- nate abnormally. This signal tells the parent process that it can start a failover operation or take other appropriate action. The final notification signal delivers a siginfo structure with the si_signo field set to SIGCHLD, the si_code field set to CLD_DUMPED, and the si_pid field set to the process ID of the child that has terminated abnormally. This signal tells the parent process that the child process is now a zombie and can be cleaned up with a call to wait(2). When SA_CLDNOTIFY is used without SA_SIGINFO, the early notification signal is not sent to the parent. Once an action is installed for a specific signal, it remains installed until one of the following events occurs: A sigaction() call that explicitly requests a different action. [XSH4.2] A signal that causes the SA_RESETHAND flag to reset the signal handler. A call to one of the exec* functions. The signal parameter can be any one of the signal values defined in the signal.h header file, except SIGKILL and SIGSTOP. In a multithreaded environment, the sigaction() function should only be used for the synchronous signals. Use the sigwait() function for asynchronous signals. NOTES
[Tru64 UNIX] When compiled in the X/Open UNIX environment, calls to the sigaction() function are internally renamed by prepending _E to the function name. When you are debugging a module that includes the sigaction() function and for which _XOPEN_SOURCE_EXTENDED has been defined, use _Esigaction to refer to the sigaction() call. See standards(5) for information on when the _XOPEN_SOURCE_EXTENDED macro is defined. EXAMPLE
The following program illustrates the use of the SA_CLDNOTIFY flag: #include <stdio.h> #include <sys/types.h> #include <sys/signal.h> #include <sys/siginfo.h> volatile pid_t kid1, kid2; /* * Handler for SIGHCLD signal. Note: printf() calls * are technically unsupported from signal handlers * and are show for illustrative purposes only. */ void sigchld_handler(int sig, siginfo_t *sip, void *extra) { pid_t kid; kid = sip->si_pid; if (sip->si_code == CLD_SIGEXITING) { printf("SIGEXITING: Got signal %d, si_code %d" " for kid %d ", sip->si_signo, sip->si_code, kid); } else if (sip->si_code == CLD_DUMPED) { printf("EXITED: Got signal %d, si_code %d" " for kid %d ", sip->si_signo, sip->si_code, kid); kid = wait(0); printf("Parent got PID %d exiting ", kid); if (kid == kid1) kid1 = 0; else if (kid == kid2) kid2 = 0; } } main() { struct sigaction sa; int ret; /* * Set up SIGHCLD handler for early exit * notification. */ sa.sa_sigaction = sigchild_handler; sigemptyset(&sa.sa_mask); sa.sa_flags = SA_CLDNOTIFY|SA_SIGINFO; ret = sigaction(SIGCHLD, &sa, 0); if (ret) { perror("sigaction"); exit(1); } /* * Create 2 children to die with SIGABRT * and create core files. */ kid1 = fork(); if (kid1) printf("Parent forked %d ", kid1); else { /* * First child... */ sleep(1); printf("Kid %d will now die with core file... ", getpid()); abort(); /*NOTREACHED*/ } kid2 = fork(); if (kid2) printf("Parent forked %d ", kid2); else { /* * Second child... */ sleep(1); printf("Kid %d will now die with core file... ", getpid()); abort(); /*NOTREACHED*/ } /* * Parent: keep busy until children exit. */ while (kid1 || kid2) ; /* * Children have exit: verify by printing zeros. */ printf("Parent: done -- kid1 %d, kid2 %d ", kid1, kid2); ; /* * Children have exit: verify by printing zeros. */ printf("Parent: done -- kid1 %d, kid2 %d ", kid1, kid2); } RETURN VALUES
Upon successful completion of the sigaction() function, a value of zero (0) is returned. If the sigaction() function fails, a value of -1 is returned and errno is set to indicate the error. ERRORS
If the sigaction() function fails, no new signal handler is installed and errno may be set to one of the following values: [Tru64 UNIX] The action or o_action parameter points to a location outside of the allocated address space of the process. The signal parameter is not a valid signal number. An attempt was made to ignore or supply a handler for the SIGKILL and SIGSTOP signals. RELATED INFORMATION
Functions: acct(2), exit(2), kill(2), ptrace(2), sigblock(2), sigprocmask(2), sigstack(2), sigsuspend(2), sigvec(2), umask(2), wait(2), pause(3), setjmp(3), sigpause(3) Commands: kill(1) Files: signal(4) Standards: standards(5) delim off sigaction(2)

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