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# sigaltstack() - Unix, Linux System Call

# NAME

sigaltstack - get or set alternate signal stack content

# SYNOPSIS

#include <signal.h>

int sigaltstack(const stack_t *ss, stack_t *oss);

where:

Tag | Description
--- | ---
ss | points to a signalstack structure defined in <signal.h> containing stack content after the call.
oss | if not NULL, points to a signalstack structure containing stack content before the call.

# DESCRIPTION

sigaction(2) may indicate that a signal should execute on an alternate stack. Where this is the case, sigaltstack(2) stores the signal in an alternate stack structure ss where its execution status may be examined prior to processing.

The sigaltstack struct is defined in <signal.h> as follows:

        void       *ss_sp     /* SVID3 uses caddr_t ss_sp
        int        ss_flags
        size_t     ss_size

where:

Tag | Description
--- | ---
ss_sp | points to the stack structure.
ss_flags
| specifies the stack state to SS_DISABLE or SS_ONSTACK as follows: If ss is not NULL,the new state may be set to SS_DISABLE, which specifies that the stack is to be disabled and ss_sp and ss_size are ignored. If SS_DISABLE is not set, the stack will be enabled.

If oss is not NULL, the stack state may be either SS_ONSTACK or SS_DISABLE. The value SS_ONSTACK indicates that the process is currently executing on the alternate stack and that any attempt to modify it during execution will fail. The value SS_DISABLE indicates that the current signal stack is disabled.

ss_size
| specifies the size of the stack.

The value SIGSTKSZ defines the average number of bytes used when allocating an alternate stack area. The value MINSIGSTKSZ defines the minimum stack size for a signal handler. When processing an alternate stack size, your program should include these values in the stack requirement to plan for the overhead of the operating system.

# RETURN VALUES

sigaltstack(2) returns 0 on success and -1 on failure.

# ERRORS

sigaltstack(2) sets errno for the following conditions:

Tag | Description
--- | ---
EINVAL | ss is not a null pointer the ss_flags member pointed to by ss contains flags other than SS_DISABLE.
ENOMEM | The size of the alternate stack area is less than MINSIGSTKSZ.
EPERM | An attempt was made to modify an active stack.

# STANDARDS

This function comforms to: XPG4-UNIX.

# SEE ALSO

getcontext(2), sigaction(2), sigsetjmp(3).

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