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man pages section 3: Basic Library Functions

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Updated: Wednesday, July 27, 2022



sigstack - set and/or get alternate signal stack context


#include <signal.h>

int sigstack(struct sigstack *ss, struct sigstack *oss);


The sigstack() function allows the calling process to indicate to the system an area of its address space to be used for processing signals received by the process.

If the ss argument is not a null pointer, it must point to a sigstack structure. The length of the application-supplied stack must be at least SIGSTKSZ bytes. If the alternate signal stack overflows, the resulting behavior is undefined. (See USAGE below.)

  • The value of the ss_onstack member indicates whether the process wants the system to use an alternate signal stack when delivering signals.

  • The value of the ss_sp member indicates the desired location of the alternate signal stack area in the process' address space.

  • If the ss argument is a null pointer, the current alternate signal stack context is not changed.

If the oss argument is not a null pointer, it points to a sigstack structure in which the current alternate signal stack context is placed. The value stored in the ss_onstack member of oss will be non-zero if the process is currently executing on the alternate signal stack. If the oss argument is a null pointer, the current alternate signal stack context is not returned.

When a signal's action indicates its handler should execute on the alternate signal stack (specified by calling sigaction(2)), sigstack() checks to see if the process is currently executing on that stack. If the process is not currently executing on the alternate signal stack, the system arranges a switch to the alternate signal stack for the duration of the signal handler's execution.

After a successful call to one of the exec functions, there are no alternate signal stacks in the new process image.

Return Values

Upon successful completion, sigstack() returns 0. Otherwise, it returns −1 and sets errno to indicate the error.


The sigstack() function will fail if:


An attempt was made to modify an active stack.


A portable application, when being written or rewritten, should use sigaltstack(2) instead of sigstack().

The direction of stack growth is not indicated in the historical definition of struct sigstack. The only way to portably establish a stack pointer is for the application to determine stack growth direction, or to allocate a block of storage and set the stack pointer to the middle. sigstack() may assume that the size of the signal stack is SIGSTKSZ as found in <signal.h>. An application that would like to specify a signal stack size other than SIGSTKSZ should use sigaltstack(2).

Applications should not use longjmp(3C) to leave a signal handler that is running on a stack established with sigstack(). Doing so may disable future use of the signal stack. For abnormal exit from a signal handler, siglongjmp(3C), setcontext(2), or swapcontext(3C) may be used. These functions fully support switching from one stack to another.

The sigstack() function requires the application to have knowledge of the underlying system's stack architecture. For this reason, sigaltstack(2) is recommended over this function.

See Also

sigaltstack(2), fork(2), _longjmp(3C), longjmp(3C), setjmp(3C), siglongjmp(3C), sigsetjmp(3C)