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 *ss, stack_t *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 versions <= 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 make it safe to switch away
65 from the signal handler with swapcontext(3). Without
66 this flag, a subsequently handled signal will corrupt the
67 state of the switched-away signal handler. On kernels
68 where this flag is not supported, sigaltstack() fails
69 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 └──────────────┴───────────────┴─────────┘
144 POSIX.1-2001, POSIX.1-2008, SUSv2, SVr4.
145 The SS_AUTODISARM flag is a Linux extension.
147
149 The most common usage of an alternate signal stack is to handle the
150 SIGSEGV signal that is generated if the space available for the stan‐
151 dard stack is exhausted: in this case, a signal handler for SIGSEGV
152 cannot be invoked on the standard stack; if we wish to handle it, we
153 must use an alternate signal stack.
154 Establishing an alternate signal stack is useful if a thread expects
156 that it may exhaust its standard stack. This may occur, for example,
157 because the stack grows so large that it encounters the upwardly grow‐
158 ing heap, or it reaches a limit established by a call to setr‐
159 limit(RLIMIT_STACK, &rlim). If the standard stack is exhausted, the
160 kernel sends the thread a SIGSEGV signal. In these circumstances the
161 only way to catch this signal is on an alternate signal stack.
162 On most hardware architectures supported by Linux, stacks grow down‐
164 ward. sigaltstack() automatically takes account of the direction of
165 stack growth.
166 Functions called from a signal handler executing on an alternate signal
168 stack will also use the alternate signal stack. (This also applies to
169 any handlers invoked for other signals while the thread is executing on
170 the alternate signal stack.) Unlike the standard stack, the system
171 does not automatically extend the alternate signal stack. Exceeding
172 the allocated size of the alternate signal stack will lead to unpre‐
173 dictable results.
174 A successful call to execve(2) removes any existing alternate signal
176 stack. A child process created via fork(2) inherits a copy of its par‐
177 ent's alternate signal stack settings. The same is also true for a
178 child process created using clone(2), unless the clone flags include
179 CLONE_VM and do not include CLONE_VFORK, in which case any alternate
180 signal stack that was established in the parent is disabled in the
181 child process.
182 sigaltstack() supersedes the older sigstack() call. For backward com‐
184 patibility, glibc also provides sigstack(). All new applications
185 should be written using sigaltstack().
186 History
188 4.2BSD had a sigstack() system call. It used a slightly different
189 struct, and had the major disadvantage that the caller had to know the
190 direction of stack growth.
191
193 In Linux 2.2 and earlier, the only flag that could be specified in
194 ss.sa_flags was SS_DISABLE. In the lead up to the release of the Linux
195 2.4 kernel, a change was made to allow sigaltstack() to allow
196 ss.ss_flags==SS_ONSTACK with the same meaning as ss.ss_flags==0 (i.e.,
197 the inclusion of SS_ONSTACK in ss.ss_flags is a no-op). On other im‐
198 plementations, and according to POSIX.1, SS_ONSTACK appears only as a
199 reported flag in old_ss.ss_flags. On Linux, there is no need ever to
200 specify SS_ONSTACK in ss.ss_flags, and indeed doing so should be
201 avoided on portability grounds: various other systems give an error if
202 SS_ONSTACK is specified in ss.ss_flags.
203
205 The following code segment demonstrates the use of sigaltstack() (and
206 sigaction(2)) to install an alternate signal stack that is employed by
207 a handler for the SIGSEGV signal:
208 stack_t ss;
210 ss.ss_sp = malloc(SIGSTKSZ);
212 if (ss.ss_sp == NULL) {
213 perror("malloc");
214 exit(EXIT_FAILURE);
215 }
216 ss.ss_size = SIGSTKSZ;
218 ss.ss_flags = 0;
219 if (sigaltstack(&ss, NULL) == -1) {
220 perror("sigaltstack");
221 exit(EXIT_FAILURE);
222 }
223 sa.sa_flags = SA_ONSTACK;
225 sa.sa_handler = handler(); /* Address of a signal handler */
226 sigemptyset(&sa.sa_mask);
227 if (sigaction(SIGSEGV, &sa, NULL) == -1) {
228 perror("sigaction");
229 exit(EXIT_FAILURE);
230 }
231
233 execve(2), setrlimit(2), sigaction(2), siglongjmp(3), sigsetjmp(3),
234 signal(7)
235
237 This page is part of release 5.10 of the Linux man-pages project. A
238 description of the project, information about reporting bugs, and the
239 latest version of this page, can be found at
240 https://www.kernel.org/doc/man-pages/.
241Linux 2020-12-21 SIGALTSTACK(2)