sigaltstack - set and/or get signal stack context

#include <signal.h>
int sigaltstack(const stack_t *_Nullable restrict ss,
 stack_t *_Nullable restrict old_ss);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

    _XOPEN_SOURCE >= 500
        || /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
        || /* glibc <= 2.19: */ _BSD_SOURCE

sigaltstack() allows a thread to define a new alternate signal stack and/or retrieve the state of an existing alternate signal stack. An alternate signal stack is used during the execution of a signal handler if the establishment of that handler (see sigaction(2)) requested it.

The normal sequence of events for using an alternate signal stack is the following:

1.

Allocate an area of memory to be used for the alternate signal stack.

2.

Use sigaltstack() to inform the system of the existence and location of the alternate signal stack.

3.

When establishing a signal handler using sigaction(2), inform the system that the signal handler should be executed on the alternate signal stack by specifying the SA_ONSTACK flag.

The ss argument is used to specify a new alternate signal stack, while the old_ss argument is used to retrieve information about the currently established signal stack. If we are interested in performing just one of these tasks, then the other argument can be specified as NULL.

The stack_t type used to type the arguments of this function is defined as follows:

typedef struct {
    void  *ss_sp;     /* Base address of stack */
    int    ss_flags;  /* Flags */
    size_t ss_size;   /* Number of bytes in stack */
} stack_t;

To establish a new alternate signal stack, the fields of this structure are set as follows:

ss.ss_flags

This field contains either 0, or the following flag:

SS_AUTODISARM (since Linux 4.7)

Clear the alternate signal stack settings on entry to the signal handler. When the signal handler returns, the previous alternate signal stack settings are restored.

This flag was added in order to make it safe to switch away from the signal handler with swapcontext(3). Without this flag, a subsequently handled signal will corrupt the state of the switched-away signal handler. On kernels where this flag is not supported, sigaltstack() fails with the error EINVAL when this flag is supplied.

ss.ss_sp

This field specifies the starting address of the stack. When a signal handler is invoked on the alternate stack, the kernel automatically aligns the address given in ss.ss_sp to a suitable address boundary for the underlying hardware architecture.

ss.ss_size

This field specifies the size of the stack. The constant SIGSTKSZ is defined to be large enough to cover the usual size requirements for an alternate signal stack, and the constant MINSIGSTKSZ defines the minimum size required to execute a signal handler.

To disable an existing stack, specify ss.ss_flags as SS_DISABLE. In this case, the kernel ignores any other flags in ss.ss_flags and the remaining fields in ss.

If old_ss is not NULL, then it is used to return information about the alternate signal stack which was in effect prior to the call to sigaltstack(). The old_ss.ss_sp and old_ss.ss_size fields return the starting address and size of that stack. The old_ss.ss_flags may return either of the following values:

SS_ONSTACK

The thread is currently executing on the alternate signal stack. (Note that it is not possible to change the alternate signal stack if the thread is currently executing on it.)

SS_DISABLE

The alternate signal stack is currently disabled.

Alternatively, this value is returned if the thread is currently executing on an alternate signal stack that was established using the SS_AUTODISARM flag. In this case, it is safe to switch away from the signal handler with swapcontext(3). It is also possible to set up a different alternative signal stack using a further call to sigaltstack().

SS_AUTODISARM

The alternate signal stack has been marked to be autodisarmed as described above.

By specifying ss as NULL, and old_ss as a non-NULL value, one can obtain the current settings for the alternate signal stack without changing them.

sigaltstack() returns 0 on success, or -1 on failure with errno set to indicate the error.

The following code segment demonstrates the use of sigaltstack() (and sigaction(2)) to install an alternate signal stack that is employed by a handler for the SIGSEGV signal:

stack_t ss;
ss.ss_sp = malloc(SIGSTKSZ);
if (ss.ss_sp == NULL) {
    perror("malloc");
    exit(EXIT_FAILURE);
}
ss.ss_size = SIGSTKSZ;
ss.ss_flags = 0;
if (sigaltstack(&ss, NULL) == -1) {
    perror("sigaltstack");
    exit(EXIT_FAILURE);
}
sa.sa_flags = SA_ONSTACK;
sa.sa_handler = handler();      /* Address of a signal handler */
sigemptyset(&sa.sa_mask);
if (sigaction(SIGSEGV, &sa, NULL) == -1) {
    perror("sigaction");
    exit(EXIT_FAILURE);
}

execve(2), setrlimit(2), sigaction(2), siglongjmp(3), sigsetjmp(3), signal(7)