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1 .\" $NetBSD: sigaltstack.2,v 1.3 1995/02/27 10:41:52 cgd Exp $
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34 .\" @(#)sigaltstack.2 8.1 (Berkeley) 6/4/93
35 .\"
36 .Dd June 4, 1993
37 .Dt SIGALTSTACK 2
38 .Os BSD 4.2
39 .Sh NAME
40 .Nm sigaltstack
41 .Nd set and/or get signal stack context
42 .Sh SYNOPSIS
43 .Fd #include <signal.h>
44 .Ft int
45 .Fo sigaltstack
46 .Fa "const stack_t *restrict ss"
47 .Fa "stack_t *restrict oss"
48 .Fc
49 .Sh DESCRIPTION
50 .Fn sigaltstack
51 allows users to define an alternate stack on which signals
52 are to be processed.
53 If
54 .Fa ss
55 is non-zero,
56 it specifies a pointer to and the size of a
57 .Em "signal stack"
58 on which to deliver signals,
59 and tells the system if the process is currently executing
60 on that stack.
61 When a signal's action indicates its handler
62 should execute on the signal stack (specified with a
63 .Xr sigaction 2
64 call), the system checks to see
65 if the process is currently executing on that stack.
66 If the process 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.
69 .Pp
70 If
71 .Dv SS_DISABLE
72 is set in
73 .Fa ss_flags ,
74 .Fa ss_sp
75 and
76 .Fa ss_size
77 are ignored and the signal stack will be disabled.
78 Trying to disable an active stack will cause
79 .Nm
80 to return -1 with
81 .Va errno
82 set to
83 .Dv EINVAL .
84 A disabled stack will cause all signals to be
85 taken on the regular user stack.
86 If the stack is later re-enabled then all signals that were specified
87 to be processed on an alternate stack will resume doing so.
88 .Pp
89 If
90 .Fa oss
91 is non-zero, the current signal stack state is returned.
92 The
93 .Fa ss_flags
94 field will contain the value
95 .Dv SA_ONSTACK
96 if the process is currently on a signal stack and
97 .Dv SS_DISABLE
98 if the signal stack is currently disabled.
99 .Sh NOTES
100 The value
101 .Dv SIGSTKSZ
102 is defined to be the number of bytes/chars that would be used to cover
103 the usual case when allocating an alternate stack area.
104 The following code fragment is typically used to allocate an alternate stack.
105 .Bd -literal -offset indent
106 if ((sigstk.ss_sp = malloc(SIGSTKSZ)) == NULL)
107 /* error return */
108 sigstk.ss_size = SIGSTKSZ;
109 sigstk.ss_flags = 0;
110 if (sigaltstack(&sigstk,0) < 0)
111 perror("sigaltstack");
112 .Ed
113 An alternative approach is provided for programs with signal handlers
114 that require a specific amount of stack space other than the default size.
115 The value
116 .Dv MINSIGSTKSZ
117 is defined to be the number of bytes/chars that is required by
118 the operating system to implement the alternate stack feature.
119 In computing an alternate stack size,
120 programs should add
121 .Dv MINSIGSTKSZ
122 to their stack requirements to allow for the operating system overhead.
123 .Pp
124 Signal stacks are automatically adjusted for the direction of stack
125 growth and alignment requirements.
126 Signal stacks may or may not be protected by the hardware and
127 are not ``grown'' automatically as is done for the normal stack.
128 If the stack overflows and this space is not protected
129 unpredictable results may occur.
130 .Sh RETURN VALUES
131 Upon successful completion, a value of 0 is returned.
132 Otherwise, a value of -1 is returned and
133 .Va errno
134 is set to indicate the error.
135 .Sh ERRORS
136 .Fn sigaltstack
137 will fail and the signal stack context will remain unchanged
138 if one of the following occurs.
139 .Bl -tag -width [ENOMEM]
140 .\" ===========
141 .It Bq Er EFAULT
142 Either
143 .Fa ss
144 or
145 .Fa oss
146 points to memory that is not a valid part of the process
147 address space.
148 .\" ===========
149 .It Bq Er EINVAL
150 An attempt is made to disable an active stack.
151 .\" ===========
152 .It Bq Er EINVAL
153 The
154 .Fa ss
155 argument is not a null pointer, and the ss_flags member
156 pointed to by
157 .Fa ss
158 contains flags other than SS_DISABLE.
159 .\" ===========
160 .It Bq Er ENOMEM
161 The size of the alternate stack area is less than or equal to
162 .Dv MINSIGSTKSZ .
163 .\" ===========
164 .It Bq Er EPERM
165 An attempt was made to modify an active stack.
166 .El
167 .Sh LEGACY SYNOPSIS
168 .Fd #include <sys/types.h>
169 .Fd #include <signal.h>
170 .Pp
171 The include file
172 .In sys/types.h
173 is necessary.
174 .Pp
175 .Bd -literal
176 struct sigaltstack {
177 char *ss_sp;
178 int ss_size;
179 int ss_flags;
180 };
181 .Ed
182 .Pp
183 .Ft int
184 .br
185 .Fo sigaltstack
186 .Fa "const struct sigaltstack *ss"
187 .Fa "struct sigaltstack *oss"
188 .Fc ;
189 .Pp
190 The variable types have changed.
191 Specifically, the
192 .Vt sigaltstack
193 struct is no longer used.
194 .Sh COMPATIBILITY
195 Use of the (obsolete)
196 .Vt sigaltstack
197 struct will cause compiler diagnostics.
198 Use
199 .Vt stack_t ,
200 defined in
201 .In signal.h .
202 .Sh SEE ALSO
203 .Xr sigaction 2 ,
204 .Xr setjmp 3 ,
205 .Xr compat 5
206 .Sh HISTORY
207 The predecessor to
208 .Nm sigaltstack ,
209 the
210 .Fn sigstack
211 system call, appeared in
212 .Bx 4.2 .