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28 .\" @(#)sigaltstack.2 8.2 (Berkeley) 5/1/95
36 .Nd set and/or get signal stack context
49 .Fn sigaltstack "const stack_t * restrict ss" "stack_t * restrict oss"
54 allows defining an alternate stack on which signals
55 are to be processed for the current thread.
59 it specifies a pointer to and the size of a
61 on which to deliver signals.
62 When a signal's action indicates its handler
63 should execute on the signal stack (specified with a
65 system call), the system checks to see
66 if the thread is currently executing on that stack.
67 If the thread 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.
71 An active stack cannot be modified.
80 are ignored and the signal stack will be disabled.
81 A disabled stack will cause all signals to be
82 taken on the regular user stack.
83 If the stack is later re-enabled then all signals that were specified
84 to be processed on an alternate stack will resume doing so.
88 is non-zero, the current signal stack state is returned.
91 field will contain the value
93 if the thread is currently on a signal stack and
95 if the signal stack is currently disabled.
99 is defined to be the number of bytes/chars that would be used to cover
100 the usual case when allocating an alternate stack area.
101 The following code fragment is typically used to allocate an alternate stack.
102 .Bd -literal -offset indent
103 if ((sigstk.ss_sp = malloc(SIGSTKSZ)) == NULL)
105 sigstk.ss_size = SIGSTKSZ;
107 if (sigaltstack(&sigstk, NULL) < 0)
108 perror("sigaltstack");
110 An alternative approach is provided for programs with signal handlers
111 that require a specific amount of stack space other than the default size.
114 is defined to be the number of bytes/chars that is required by
115 the operating system to implement the alternate stack feature.
116 In computing an alternate stack size,
119 to their stack requirements to allow for the operating system overhead.
121 Signal stacks are automatically adjusted for the direction of stack
122 growth and alignment requirements.
123 Signal stacks may or may not be protected by the hardware and
124 are not ``grown'' automatically as is done for the normal stack.
125 If the stack overflows and this space is not protected
126 unpredictable results may occur.
133 will fail and the signal stack context will remain unchanged
134 if one of the following occurs.
141 points to memory that is not a valid part of the process
144 An attempt was made to modify an active stack.
150 Size of alternate stack area is less than or equal to
161 system call, appeared in