Linux / Unix Command: sigpending

Command Library

NAME

SYNOPSIS

int sigaction(int signum, const struct sigaction *act, struct sigaction *oldact);

int sigprocmask(int how, const sigset_t *set, sigset_t *oldset);

int sigpending(sigset_t *set);

int sigsuspend(const sigset_t *mask);  

DESCRIPTION

signum specifies the signal and can be any valid signal except SIGKILL and SIGSTOP.

If act is non-null, the new action for signal signum is installed from act. If oldact is non-null, the previous action is saved in oldact.

The sigaction structure is defined as something like

struct sigaction {
    void (*sa_handler)(int);
    void (*sa_sigaction)(int, siginfo_t *, void *);
    sigset_t sa_mask;
    int sa_flags;
    void (*sa_restorer)(void);
}

On some architectures a union is involved - do not assign to both sa_handler and sa_sigaction.

The sa_restorer element is obsolete and should not be used. POSIX does not specify a sa_restorer element.

sa_handler specifies the action to be associated with signum and may be SIG_DFL for the default action, SIG_IGN to ignore this signal, or a pointer to a signal handling function.

sa_mask gives a mask of signals which should be blocked during execution of the signal handler. In addition, the signal which triggered the handler will be blocked, unless the SA_NODEFER or SA_NOMASK flags are used.

sa_flags specifies a set of flags which modify the behaviour of the signal handling process. It is formed by the bitwise OR of zero or more of the following:

SA_NOCLDSTOP

If signum is SIGCHLD, do not receive notification when child processes stop (i.e., when child processes receive one of SIGSTOP, SIGTSTP, SIGTTIN or SIGTTOU).

SA_ONESHOT or SA_RESETHAND

Restore the signal action to the default state once the signal handler has been called.

SA_ONSTACK

Call the signal handler on an alternate signal stack provided by sigaltstack(2). If an alternate stack is not available, the default stack will be used.

SA_RESTART

Provide behaviour compatible with BSD signal semantics by making certain system calls restartable across signals.

SA_NOMASK or SA_NODEFER

Do not prevent the signal from being received from within its own signal handler.

SA_SIGINFO

The signal handler takes 3 arguments, not one. In this case, sa_sigaction should be set instead of sa_handler. (The sa_sigaction field was added in Linux 2.1.86.)

The siginfo_t parameter to sa_sigaction is a struct with the following elements

siginfo_t {
    int      si_signo;  /* Signal number */
    int      si_errno;  /* An errno value */
    int      si_code;   /* Signal code */
    pid_t    si_pid;    /* Sending process ID */
    uid_t    si_uid;    /* Real user ID of sending process */
    int      si_status; /* Exit value or signal */
    clock_t  si_utime;  /* User time consumed */
    clock_t  si_stime;  /* System time consumed */
    sigval_t si_value;  /* Signal value */
    int      si_int;    /* POSIX.1b signal */
    void *   si_ptr;    /* POSIX.1b signal */
    void *   si_addr;   /* Memory location which caused fault */
    int      si_band;   /* Band event */
    int      si_fd;     /* File descriptor */
}

si_signo, si_errno and si_code are defined for all signals. The rest of the struct may be a union, so that one should only read the fields that are meaningful for the given signal. kill(2), POSIX.1b signals and SIGCHLD fill in si_pid and si_uid. SIGCHLDalsofillsin si_status, si_utime and si_stime. si_int and si_ptr are specified by the sender of the POSIX.1b signal. SIGILL, SIGFPE, SIGSEGV and SIGBUS fill in si_addr with the address of the fault. SIGPOLL fills in si_band and si_fd.

si_code indicates why this signal was sent. It is a value, not a bitmask. The values which are possible for any signal are listed in this table:

The sigprocmask call is used to change the list of currently blocked signals. The behaviour of the call is dependent on the value of how, as follows.

SIG_BLOCK

The set of blocked signals is the union of the current set and the set argument.

SIG_UNBLOCK

The signals in set are removed from the current set of blocked signals. It is legal to attempt to unblock a signal which is not blocked.

SIG_SETMASK

The set of blocked signals is set to the argument set.

If oldset is non-null, the previous value of the signal mask is stored in oldset.

The sigpending call allows the examination of pending signals (ones which have been raised while blocked). The signal mask of pending signals is stored in set.

The sigsuspend call temporarily replaces the signal mask for the process with that given by mask and then suspends the process until a signal is received.

RETURN VALUE

ERRORS

EINVAL

An invalid signal was specified. This will also be generated if an attempt is made to change the action for SIGKILL or SIGSTOP, which cannot be caught.

EFAULT

act, oldact, set, oldset or mask point to memory which is not a valid part of the process address space.

EINTR

System call was interrupted.

SEE ALSO

Important: Use the man command (% man) to see how a command is used on your particular computer.

>> Linux/Unix Command Library

>> Shell Command Library