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28 .\" @(#)sigaltstack.2 8.2 (Berkeley) 5/1/95
36 .Nd set and/or get signal stack context
42 typedef struct sigaltstack {
49 .Fn sigaltstack "const stack_t * restrict ss" "stack_t * restrict oss"
54 allows users to define an alternate stack on which signals
59 it specifies a pointer to and the size of a
61 on which to deliver signals,
62 and tells the system if the process is currently executing
64 When a signal's action indicates its handler
65 should execute on the signal stack (specified with a
67 system call), the system checks to see
68 if the process is currently executing on that stack.
69 If the process is not currently executing on the signal stack,
70 the system arranges a switch to the signal stack for the
71 duration of the signal handler's execution.
80 are ignored and the signal stack will be disabled.
81 Trying to disable an active stack will cause
87 A disabled stack will cause all signals to be
88 taken on the regular user stack.
89 If the stack is later re-enabled then all signals that were specified
90 to be processed on an alternate stack will resume doing so.
94 is non-zero, the current signal stack state is returned.
97 field will contain the value
99 if the process is currently on a signal stack and
101 if the signal stack is currently disabled.
105 is defined to be the number of bytes/chars that would be used to cover
106 the usual case when allocating an alternate stack area.
107 The following code fragment is typically used to allocate an alternate stack.
108 .Bd -literal -offset indent
109 if ((sigstk.ss_sp = malloc(SIGSTKSZ)) == NULL)
111 sigstk.ss_size = SIGSTKSZ;
113 if (sigaltstack(&sigstk,0) < 0)
114 perror("sigaltstack");
116 An alternative approach is provided for programs with signal handlers
117 that require a specific amount of stack space other than the default size.
120 is defined to be the number of bytes/chars that is required by
121 the operating system to implement the alternate stack feature.
122 In computing an alternate stack size,
125 to their stack requirements to allow for the operating system overhead.
127 Signal stacks are automatically adjusted for the direction of stack
128 growth and alignment requirements.
129 Signal stacks may or may not be protected by the hardware and
130 are not ``grown'' automatically as is done for the normal stack.
131 If the stack overflows and this space is not protected
132 unpredictable results may occur.
139 will fail and the signal stack context will remain unchanged
140 if one of the following occurs.
147 points to memory that is not a valid part of the process
150 An attempt was made to disable an active stack.
152 Size of alternate stack area is less than or equal to
163 system call, appeared in