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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 $
30 .\" $DragonFly: src/lib/libc/sys/sigaltstack.2,v 1.2 2003/06/17 04:26:47 dillon Exp $
37 .Nd set and/or get signal stack context
43 typedef struct sigaltstack {
50 .Fn sigaltstack "const stack_t *ss" "stack_t *oss"
53 allows users to define an alternate stack on which signals
58 it specifies a pointer to and the size of a
60 on which to deliver signals,
61 and tells the system if the process is currently executing
63 When a signal's action indicates its handler
64 should execute on the signal stack (specified with a
66 call), the system checks to see
67 if the process is currently executing on that stack.
68 If the process is not currently executing on the signal stack,
69 the system arranges a switch to the signal stack for the
70 duration of the signal handler's execution.
79 are ignored and the signal stack will be disabled.
80 Trying to disable an active stack will cause
86 A disabled stack will cause all signals to be
87 taken on the regular user stack.
88 If the stack is later re-enabled then all signals that were specified
89 to be processed on an alternate stack will resume doing so.
93 is non-zero, the current signal stack state is returned.
96 field will contain the value
98 if the process is currently on a signal stack and
100 if the signal stack is currently disabled.
104 is defined to be the number of bytes/chars that would be used to cover
105 the usual case when allocating an alternate stack area.
106 The following code fragment is typically used to allocate an alternate stack.
107 .Bd -literal -offset indent
108 if ((sigstk.ss_sp = malloc(SIGSTKSZ)) == NULL)
110 sigstk.ss_size = SIGSTKSZ;
112 if (sigaltstack(&sigstk,0) < 0)
113 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
147 An attempt was made to disable an active stack.
149 Size of alternate stack area is less than or equal to
160 system call, appeared in