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