1SIGALTSTACK(2) Linux Programmer's Manual SIGALTSTACK(2)
2
6 sigaltstack - set and/or get signal stack context
7
9 #include <signal.h>
10 int sigaltstack(const stack_t *restrict ss, stack_t *restrict old_ss);
12 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
14 sigaltstack():
16 _XOPEN_SOURCE >= 500
17 || /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
18 || /* Glibc <= 2.19: */ _BSD_SOURCE
19
21 sigaltstack() allows a thread to define a new alternate signal stack
22 and/or retrieve the state of an existing alternate signal stack. An
23 alternate signal stack is used during the execution of a signal handler
24 if the establishment of that handler (see sigaction(2)) requested it.
25 The normal sequence of events for using an alternate signal stack is
27 the following:
28 1. Allocate an area of memory to be used for the alternate signal
30 stack.
31 2. Use sigaltstack() to inform the system of the existence and location
33 of the alternate signal stack.
34 3. When establishing a signal handler using sigaction(2), inform the
36 system that the signal handler should be executed on the alternate
37 signal stack by specifying the SA_ONSTACK flag.
38 The ss argument is used to specify a new alternate signal stack, while
40 the old_ss argument is used to retrieve information about the currently
41 established signal stack. If we are interested in performing just one
42 of these tasks, then the other argument can be specified as NULL.
43 The stack_t type used to type the arguments of this function is defined
45 as follows:
46 typedef struct {
48 void *ss_sp; /* Base address of stack */
49 int ss_flags; /* Flags */
50 size_t ss_size; /* Number of bytes in stack */
51 } stack_t;
52 To establish a new alternate signal stack, the fields of this structure
54 are set as follows:
55 ss.ss_flags
57 This field contains either 0, or the following flag:
58 SS_AUTODISARM (since Linux 4.7)
60 Clear the alternate signal stack settings on entry to the
61 signal handler. When the signal handler returns, the
62 previous alternate signal stack settings are restored.
63 This flag was added in order to make it safe to switch
65 away from the signal handler with swapcontext(3). With‐
66 out this flag, a subsequently handled signal will corrupt
67 the state of the switched-away signal handler. On ker‐
68 nels where this flag is not supported, sigaltstack()
69 fails with the error EINVAL when this flag is supplied.
70 ss.ss_sp
72 This field specifies the starting address of the stack. When a
73 signal handler is invoked on the alternate stack, the kernel au‐
74 tomatically aligns the address given in ss.ss_sp to a suitable
75 address boundary for the underlying hardware architecture.
76 ss.ss_size
78 This field specifies the size of the stack. The constant
79 SIGSTKSZ is defined to be large enough to cover the usual size
80 requirements for an alternate signal stack, and the constant
81 MINSIGSTKSZ defines the minimum size required to execute a sig‐
82 nal handler.
83 To disable an existing stack, specify ss.ss_flags as SS_DISABLE. In
85 this case, the kernel ignores any other flags in ss.ss_flags and the
86 remaining fields in ss.
87 If old_ss is not NULL, then it is used to return information about the
89 alternate signal stack which was in effect prior to the call to sigalt‐
90 stack(). The old_ss.ss_sp and old_ss.ss_size fields return the start‐
91 ing address and size of that stack. The old_ss.ss_flags may return ei‐
92 ther of the following values:
93 SS_ONSTACK
95 The thread is currently executing on the alternate signal stack.
96 (Note that it is not possible to change the alternate signal
97 stack if the thread is currently executing on it.)
98 SS_DISABLE
100 The alternate signal stack is currently disabled.
101 Alternatively, this value is returned if the thread is currently
103 executing on an alternate signal stack that was established us‐
104 ing the SS_AUTODISARM flag. In this case, it is safe to switch
105 away from the signal handler with swapcontext(3). It is also
106 possible to set up a different alternative signal stack using a
107 further call to sigaltstack().
108 SS_AUTODISARM
110 The alternate signal stack has been marked to be autodisarmed as
111 described above.
112 By specifying ss as NULL, and old_ss as a non-NULL value, one can ob‐
114 tain the current settings for the alternate signal stack without chang‐
115 ing them.
116
118 sigaltstack() returns 0 on success, or -1 on failure with errno set to
119 indicate the error.
120
122 EFAULT Either ss or old_ss is not NULL and points to an area outside of
123 the process's address space.
124 EINVAL ss is not NULL and the ss_flags field contains an invalid flag.
126 ENOMEM The specified size of the new alternate signal stack ss.ss_size
128 was less than MINSIGSTKSZ.
129 EPERM An attempt was made to change the alternate signal stack while
131 it was active (i.e., the thread was already executing on the
132 current alternate signal stack).
133
135 For an explanation of the terms used in this section, see at‐
136 tributes(7).
137 ┌────────────────────────────────────────────┬───────────────┬─────────┐
139 │Interface │ Attribute │ Value │
140 ├────────────────────────────────────────────┼───────────────┼─────────┤
141 │sigaltstack() │ Thread safety │ MT-Safe │
142 └────────────────────────────────────────────┴───────────────┴─────────┘
143
145 POSIX.1-2001, POSIX.1-2008, SUSv2, SVr4.
146 The SS_AUTODISARM flag is a Linux extension.
148
150 The most common usage of an alternate signal stack is to handle the
151 SIGSEGV signal that is generated if the space available for the stan‐
152 dard stack is exhausted: in this case, a signal handler for SIGSEGV
153 cannot be invoked on the standard stack; if we wish to handle it, we
154 must use an alternate signal stack.
155 Establishing an alternate signal stack is useful if a thread expects
157 that it may exhaust its standard stack. This may occur, for example,
158 because the stack grows so large that it encounters the upwardly grow‐
159 ing heap, or it reaches a limit established by a call to setrlim‐
160 it(RLIMIT_STACK, &rlim). If the standard stack is exhausted, the ker‐
161 nel sends the thread a SIGSEGV signal. In these circumstances the only
162 way to catch this signal is on an alternate signal stack.
163 On most hardware architectures supported by Linux, stacks grow down‐
165 ward. sigaltstack() automatically takes account of the direction of
166 stack growth.
167 Functions called from a signal handler executing on an alternate signal
169 stack will also use the alternate signal stack. (This also applies to
170 any handlers invoked for other signals while the thread is executing on
171 the alternate signal stack.) Unlike the standard stack, the system
172 does not automatically extend the alternate signal stack. Exceeding
173 the allocated size of the alternate signal stack will lead to unpre‐
174 dictable results.
175 A successful call to execve(2) removes any existing alternate signal
177 stack. A child process created via fork(2) inherits a copy of its par‐
178 ent's alternate signal stack settings. The same is also true for a
179 child process created using clone(2), unless the clone flags include
180 CLONE_VM and do not include CLONE_VFORK, in which case any alternate
181 signal stack that was established in the parent is disabled in the
182 child process.
183 sigaltstack() supersedes the older sigstack() call. For backward com‐
185 patibility, glibc also provides sigstack(). All new applications
186 should be written using sigaltstack().
187 History
189 4.2BSD had a sigstack() system call. It used a slightly different
190 struct, and had the major disadvantage that the caller had to know the
191 direction of stack growth.
192
194 In Linux 2.2 and earlier, the only flag that could be specified in
195 ss.sa_flags was SS_DISABLE. In the lead up to the release of the Linux
196 2.4 kernel, a change was made to allow sigaltstack() to allow
197 ss.ss_flags==SS_ONSTACK with the same meaning as ss.ss_flags==0 (i.e.,
198 the inclusion of SS_ONSTACK in ss.ss_flags is a no-op). On other im‐
199 plementations, and according to POSIX.1, SS_ONSTACK appears only as a
200 reported flag in old_ss.ss_flags. On Linux, there is no need ever to
201 specify SS_ONSTACK in ss.ss_flags, and indeed doing so should be avoid‐
202 ed on portability grounds: various other systems give an error if
203 SS_ONSTACK is specified in ss.ss_flags.
204
206 The following code segment demonstrates the use of sigaltstack() (and
207 sigaction(2)) to install an alternate signal stack that is employed by
208 a handler for the SIGSEGV signal:
209 stack_t ss;
211 ss.ss_sp = malloc(SIGSTKSZ);
213 if (ss.ss_sp == NULL) {
214 perror("malloc");
215 exit(EXIT_FAILURE);
216 }
217 ss.ss_size = SIGSTKSZ;
219 ss.ss_flags = 0;
220 if (sigaltstack(&ss, NULL) == -1) {
221 perror("sigaltstack");
222 exit(EXIT_FAILURE);
223 }
224 sa.sa_flags = SA_ONSTACK;
226 sa.sa_handler = handler(); /* Address of a signal handler */
227 sigemptyset(&sa.sa_mask);
228 if (sigaction(SIGSEGV, &sa, NULL) == -1) {
229 perror("sigaction");
230 exit(EXIT_FAILURE);
231 }
232
234 execve(2), setrlimit(2), sigaction(2), siglongjmp(3), sigsetjmp(3),
235 signal(7)
236
238 This page is part of release 5.13 of the Linux man-pages project. A
239 description of the project, information about reporting bugs, and the
240 latest version of this page, can be found at
241 https://www.kernel.org/doc/man-pages/.
242Linux 2021-03-22 SIGALTSTACK(2)