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28 .\" @(#)sigaltstack.2 8.2 (Berkeley) 5/1/95
29 .\" $FreeBSD: src/lib/libc/sys/sigaltstack.2,v 1.11.2.7 2002/09/15 00:32:41 archie Exp $
36 .Nd set and/or get signal stack context
42 typedef struct sigaltstack {
49 .Fn sigaltstack "const stack_t *ss" "stack_t *oss"
52 allows users to define an alternate stack on which signals
57 it specifies a pointer to and the size of a
59 on which to deliver signals,
60 and tells the system if the process is currently executing
62 When a signal's action indicates its handler
63 should execute on the signal stack (specified with a
65 call), the system checks to see
66 if the process is currently executing on that stack.
67 If the process is not currently executing on the signal stack,
68 the system arranges a switch to the signal stack for the
69 duration of the signal handler's execution.
78 are ignored and the signal stack will be disabled.
79 Trying to disable an active stack will cause
85 A disabled stack will cause all signals to be
86 taken on the regular user stack.
87 If the stack is later re-enabled then all signals that were specified
88 to be processed on an alternate stack will resume doing so.
92 is non-zero, the current signal stack state is returned.
95 field will contain the value
97 if the process is currently on a signal stack and
99 if the signal stack is currently disabled.
103 is defined to be the number of bytes/chars that would be used to cover
104 the usual case when allocating an alternate stack area.
105 The following code fragment is typically used to allocate an alternate stack.
106 .Bd -literal -offset indent
107 if ((sigstk.ss_sp = malloc(SIGSTKSZ)) == NULL)
109 sigstk.ss_size = SIGSTKSZ;
111 if (sigaltstack(&sigstk,0) < 0)
112 perror("sigaltstack");
115 An alternative approach is provided for programs with signal handlers
116 that require a specific amount of stack space other than the default size.
119 is defined to be the number of bytes/chars that is required by
120 the operating system to implement the alternate stack feature.
121 In computing an alternate stack size,
124 to their stack requirements to allow for the operating system overhead.
126 Signal stacks are automatically adjusted for the direction of stack
127 growth and alignment requirements.
128 Signal stacks may or may not be protected by the hardware and
129 are not ``grown'' automatically as is done for the normal stack.
130 If the stack overflows and this space is not protected
131 unpredictable results may occur.
136 will fail and the signal stack context will remain unchanged
137 if one of the following occurs.
144 points to memory that is not a valid part of the process
149 argument is not a null pointer, and the
153 contains flags other than
156 Size of alternate stack area is less than or equal to
158 An attempt was made to modify an active stack.
169 system call, appeared in