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32 .\" @(#)sigaltstack.2 8.2 (Berkeley) 5/1/95
40 .Nd set and/or get signal stack context
46 typedef struct sigaltstack {
53 .Fn sigaltstack "const stack_t * restrict ss" "stack_t * restrict oss"
58 allows users to define an alternate stack on which signals
63 it specifies a pointer to and the size of a
65 on which to deliver signals,
66 and tells the system if the process is currently executing
68 When a signal's action indicates its handler
69 should execute on the signal stack (specified with a
71 system call), the system checks to see
72 if the process is currently executing on that stack.
73 If the process is not currently executing on the signal stack,
74 the system arranges a switch to the signal stack for the
75 duration of the signal handler's execution.
84 are ignored and the signal stack will be disabled.
85 Trying to disable an active stack will cause
91 A disabled stack will cause all signals to be
92 taken on the regular user stack.
93 If the stack is later re-enabled then all signals that were specified
94 to be processed on an alternate stack will resume doing so.
98 is non-zero, the current signal stack state is returned.
101 field will contain the value
103 if the process is currently on a signal stack and
105 if the signal stack is currently disabled.
109 is defined to be the number of bytes/chars that would be used to cover
110 the usual case when allocating an alternate stack area.
111 The following code fragment is typically used to allocate an alternate stack.
112 .Bd -literal -offset indent
113 if ((sigstk.ss_sp = malloc(SIGSTKSZ)) == NULL)
115 sigstk.ss_size = SIGSTKSZ;
117 if (sigaltstack(&sigstk,0) < 0)
118 perror("sigaltstack");
120 An alternative approach is provided for programs with signal handlers
121 that require a specific amount of stack space other than the default size.
124 is defined to be the number of bytes/chars that is required by
125 the operating system to implement the alternate stack feature.
126 In computing an alternate stack size,
129 to their stack requirements to allow for the operating system overhead.
131 Signal stacks are automatically adjusted for the direction of stack
132 growth and alignment requirements.
133 Signal stacks may or may not be protected by the hardware and
134 are not ``grown'' automatically as is done for the normal stack.
135 If the stack overflows and this space is not protected
136 unpredictable results may occur.
143 will fail and the signal stack context will remain unchanged
144 if one of the following occurs.
151 points to memory that is not a valid part of the process
154 An attempt was made to disable an active stack.
156 Size of alternate stack area is less than or equal to
167 system call, appeared in