sigaltstack(2)

💖 sigaltstack(2) — the set-and/or-get signal stack context page, bestie 💖

Okay so this is sigaltstack(2), the system call manual page, and honestly? It's giving "quietly saves your program from a segfault death spiral" energy. Let's get into it. 🫶

✨ NAME ✨

sigaltstack — set and/or get signal stack context

📚 LIBRARY 📚

Standard C library (libc, -lc)

🌸 SYNOPSIS 🌸

#include <signal.h>

int sigaltstack(const stack_t *_Nullable restrict ss,
                stack_t *_Nullable restrict old_ss);

Feature Test Macro Requirements for glibc (peep feature_test_macros(7) for the lore):

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

👑 DESCRIPTION 👑

sigaltstack() lets a thread define a new alternate signal stack and/or retrieve the state of an existing alternate signal stack. And like — what even is an alternate signal stack? It's the stack that gets used while a signal handler runs, if the handler was set up (via sigaction(2), we love her) to request it. 💅

So the normal sequence of events for using an alternate signal stack goes like this:

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 what you use to specify a new alternate signal stack, and the old_ss argument is what you use to retrieve info about the signal stack that's currently established. If you literally only wanna do one of those two things, the other argument can just be NULL. No pressure. 🙈

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

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, you set the fields of that structure like so:

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. (The kernel is doing the fiddly bits for you, periodt. ✨)

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. Bye bestie, we simply don't look at them. 💀

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: it is not possible to change the alternate signal stack if the thread is currently executing on it. So like, don't try 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. Peek, don't touch. 👀

🎀 RETURN VALUE 🎀

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

🌈 ERRORS 🌈

• EFAULT — Either ss or old_ss is not NULL and points to an area outside of the process's address space.
• EINVAL — ss is not NULL and the ss_flags field contains an invalid flag.
• ENOMEM — The specified size of the new alternate signal stack ss.ss_size was less than MINSIGSTKSZ.
• EPERM — An attempt was made to change the alternate signal stack while it was active (i.e., the thread was already executing on the current alternate signal stack).

🦋 ATTRIBUTES 🦋

For an explanation of the terms used in this section, see attributes(7).

• Interface: sigaltstack()
• Attribute: Thread safety
• Value: MT-Safe

(Yes she's MT-Safe, we stan a thread-safe icon. 👑)

⭐ STANDARDS ⭐

POSIX.1-2024.

SS_AUTODISARM is a Linux extension.

💜 HISTORY 💜

POSIX.1-2001, SUSv2, SVr4.

🔥 NOTES 🔥

The most common usage of an alternate signal stack is to handle the SIGSEGV signal that is generated if the space available for the standard stack is exhausted: in this case, a signal handler for SIGSEGV cannot be invoked on the standard stack; if we wish to handle it, we must use an alternate signal stack. Like, the standard stack is literally gone, so obviously we need a backup. 😭

Establishing an alternate signal stack is useful if a thread expects that it may exhaust its standard stack. This may occur, for example, because the stack grows so large that it encounters the upwardly growing heap, or it reaches a limit established by a call to setrlimit(RLIMIT_STACK, &rlim). If the standard stack is exhausted, the kernel sends the thread a SIGSEGV signal. In these circumstances the only way to catch this signal is on an alternate signal stack.

On most hardware architectures supported by Linux, stacks grow downward. sigaltstack() automatically takes account of the direction of stack growth. (She's doing the math so you don't have to. 🧚)

Functions called from a signal handler executing on an alternate signal stack will also use the alternate signal stack. (This also applies to any handlers invoked for other signals while the thread is executing on the alternate signal stack.) Unlike the standard stack, the system does not automatically extend the alternate signal stack.

Exceeding the allocated size of the alternate signal stack will lead to unpredictable results. Not the cute kind of unpredictable. The bad kind. 💀

A successful call to execve(2) removes any existing alternate signal stack. A child process created via fork(2) inherits a copy of its parent's alternate signal stack settings. The same is also true for a child process created using clone(2), unless the clone flags include CLONE_VM and do not include CLONE_VFORK, in which case any alternate signal stack that was established in the parent is disabled in the child process.

sigaltstack() supersedes the older sigstack() call. For backward compatibility, glibc also provides sigstack(). All new applications should be written using sigaltstack(). Basically: new code uses the new one. Old code stays grandfathered in. 🫶

📜 History 📜

4.2BSD had a sigstack() system call. It used a slightly different struct, and had the major disadvantage that the caller had to know the direction of stack growth. Imagine having to know that yourself. Could not be me. 🙈

💅 BUGS 💅

In Linux 2.2 and earlier, the only flag that could be specified in ss.sa_flags was SS_DISABLE. In the lead up to the release of the Linux 2.4 kernel, a change was made to allow sigaltstack() to allow ss.ss_flags==SS_ONSTACK with the same meaning as ss.ss_flags==0 (i.e., the inclusion of SS_ONSTACK in ss.ss_flags is a no-op). On other implementations, and according to POSIX.1, SS_ONSTACK appears only as a reported flag in old_ss.ss_flags.

On Linux, there is no need ever to specify SS_ONSTACK in ss.ss_flags, and indeed doing so should be avoided on portability grounds: various other systems give an error if SS_ONSTACK is specified in ss.ss_flags.

✨ EXAMPLES ✨

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);
}

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🌟 SEE ALSO 🌟

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

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And that's the whole page, bestie! 🎀 This one's from Linux man-pages 6.20, dated 2026-02-08, and it's filed under sigaltstack(2). Go forth and stack your signals alternately. You got this. 💖✨🦋