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sigaltstack(2) [freebsd man page]

SIGALTSTACK(2)						      BSD System Calls Manual						    SIGALTSTACK(2)

NAME
sigaltstack -- set and/or get signal stack context LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <signal.h> typedef struct { char *ss_sp; size_t ss_size; int ss_flags; } stack_t; int sigaltstack(const stack_t * restrict ss, stack_t * restrict oss); DESCRIPTION
The sigaltstack() system call allows defining an alternate stack on which signals are to be processed for the current thread. If ss is non- zero, it specifies a pointer to and the size of a signal stack on which to deliver signals. When a signal's action indicates its handler should execute on the signal stack (specified with a sigaction(2) system call), the system checks to see if the thread is currently executing on that stack. If the thread is not currently executing on the signal stack, the system arranges a switch to the signal stack for the dura- tion of the signal handler's execution. An active stack cannot be modified. If SS_DISABLE is set in ss_flags, ss_sp and ss_size are ignored and the signal stack will be disabled. A disabled stack will cause all sig- nals to be taken on the regular user stack. If the stack is later re-enabled then all signals that were specified to be processed on an alternate stack will resume doing so. If oss is non-zero, the current signal stack state is returned. The ss_flags field will contain the value SS_ONSTACK if the thread is cur- rently on a signal stack and SS_DISABLE if the signal stack is currently disabled. NOTES
The value SIGSTKSZ is defined to be the number of bytes/chars that would be used to cover the usual case when allocating an alternate stack area. The following code fragment is typically used to allocate an alternate stack. if ((sigstk.ss_sp = malloc(SIGSTKSZ)) == NULL) /* error return */ sigstk.ss_size = SIGSTKSZ; sigstk.ss_flags = 0; if (sigaltstack(&sigstk, NULL) < 0) perror("sigaltstack"); An alternative approach is provided for programs with signal handlers that require a specific amount of stack space other than the default size. The value MINSIGSTKSZ is defined to be the number of bytes/chars that is required by the operating system to implement the alternate stack feature. In computing an alternate stack size, programs should add MINSIGSTKSZ to their stack requirements to allow for the operating system overhead. Signal stacks are automatically adjusted for the direction of stack growth and alignment requirements. Signal stacks may or may not be pro- tected by the hardware and are not ``grown'' automatically as is done for the normal stack. If the stack overflows and this space is not protected unpredictable results may occur. RETURN VALUES
The sigaltstack() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error. ERRORS
The sigaltstack() system call will fail and the signal stack context will remain unchanged if one of the following occurs. [EFAULT] Either ss or oss points to memory that is not a valid part of the process address space. [EPERM] An attempt was made to modify an active stack. [EINVAL] The ss_flags field was invalid. [ENOMEM] Size of alternate stack area is less than or equal to MINSIGSTKSZ. SEE ALSO
sigaction(2), setjmp(3) HISTORY
The predecessor to sigaltstack(), the sigstack() system call, appeared in 4.2BSD. BSD
May 6, 2010 BSD

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SIGALTSTACK(2)						     Linux Programmer's Manual						    SIGALTSTACK(2)

NAME
sigaltstack - get or set alternate signal stack content SYNOPSIS
#include <signal.h> int sigaltstack(const stack_t *ss, stack_t *oss); where: 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: 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 cur- rently executing on the alternate stack and that any attempt to modify it during execution will fail. The value SS_DISABLE indi- cates 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: 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). Red Hat Linux 6.1 20 September 1999 SIGALTSTACK(2)

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