1sigaltstack(2)                System Calls Manual               sigaltstack(2)
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NAME

6       sigaltstack - set and/or get signal stack context
7

LIBRARY

9       Standard C library (libc, -lc)
10

SYNOPSIS

12       #include <signal.h>
13
14       int sigaltstack(const stack_t *_Nullable restrict ss,
15                       stack_t *_Nullable restrict old_ss);
16
17   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
18
19       sigaltstack():
20           _XOPEN_SOURCE >= 500
21               || /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
22               || /* glibc <= 2.19: */ _BSD_SOURCE
23

DESCRIPTION

25       sigaltstack()  allows  a  thread to define a new alternate signal stack
26       and/or retrieve the state of an existing alternate  signal  stack.   An
27       alternate signal stack is used during the execution of a signal handler
28       if the establishment of that handler (see sigaction(2)) requested it.
29
30       The normal sequence of events for using an alternate  signal  stack  is
31       the following:
32
33       1. Allocate  an  area  of  memory  to  be used for the alternate signal
34          stack.
35
36       2. Use sigaltstack() to inform the system of the existence and location
37          of the alternate signal stack.
38
39       3. When  establishing  a  signal handler using sigaction(2), inform the
40          system that the signal handler should be executed on  the  alternate
41          signal stack by specifying the SA_ONSTACK flag.
42
43       The  ss argument is used to specify a new alternate signal stack, while
44       the old_ss argument is used to retrieve information about the currently
45       established  signal stack.  If we are interested in performing just one
46       of these tasks, then the other argument can be specified as NULL.
47
48       The stack_t type used to type the arguments of this function is defined
49       as follows:
50
51           typedef struct {
52               void  *ss_sp;     /* Base address of stack */
53               int    ss_flags;  /* Flags */
54               size_t ss_size;   /* Number of bytes in stack */
55           } stack_t;
56
57       To establish a new alternate signal stack, the fields of this structure
58       are set as follows:
59
60       ss.ss_flags
61              This field contains either 0, or the following flag:
62
63              SS_AUTODISARM (since Linux 4.7)
64                     Clear the alternate signal stack settings on entry to the
65                     signal  handler.   When  the  signal handler returns, the
66                     previous alternate signal stack settings are restored.
67
68                     This flag was added in order to make it  safe  to  switch
69                     away  from the signal handler with swapcontext(3).  With‐
70                     out this flag, a subsequently handled signal will corrupt
71                     the  state  of the switched-away signal handler.  On ker‐
72                     nels where this  flag  is  not  supported,  sigaltstack()
73                     fails with the error EINVAL when this flag is supplied.
74
75       ss.ss_sp
76              This  field specifies the starting address of the stack.  When a
77              signal handler is invoked on the alternate stack, the kernel au‐
78              tomatically  aligns  the address given in ss.ss_sp to a suitable
79              address boundary for the underlying hardware architecture.
80
81       ss.ss_size
82              This field specifies  the  size  of  the  stack.   The  constant
83              SIGSTKSZ  is  defined to be large enough to cover the usual size
84              requirements for an alternate signal  stack,  and  the  constant
85              MINSIGSTKSZ  defines the minimum size required to execute a sig‐
86              nal handler.
87
88       To disable an existing stack, specify ss.ss_flags  as  SS_DISABLE.   In
89       this  case,  the  kernel ignores any other flags in ss.ss_flags and the
90       remaining fields in ss.
91
92       If old_ss is not NULL, then it is used to return information about  the
93       alternate signal stack which was in effect prior to the call to sigalt‐
94       stack().  The old_ss.ss_sp and old_ss.ss_size fields return the  start‐
95       ing address and size of that stack.  The old_ss.ss_flags may return ei‐
96       ther of the following values:
97
98       SS_ONSTACK
99              The thread is currently executing on the alternate signal stack.
100              (Note  that  it  is  not possible to change the alternate signal
101              stack if the thread is currently executing on it.)
102
103       SS_DISABLE
104              The alternate signal stack is currently disabled.
105
106              Alternatively, this value is returned if the thread is currently
107              executing  on an alternate signal stack that was established us‐
108              ing the SS_AUTODISARM flag.  In this case, it is safe to  switch
109              away  from  the  signal handler with swapcontext(3).  It is also
110              possible to set up a different alternative signal stack using  a
111              further call to sigaltstack().
112
113       SS_AUTODISARM
114              The alternate signal stack has been marked to be autodisarmed as
115              described above.
116
117       By specifying ss as NULL, and old_ss as a non-NULL value, one  can  ob‐
118       tain the current settings for the alternate signal stack without chang‐
119       ing them.
120

RETURN VALUE

122       sigaltstack() returns 0 on success, or -1 on failure with errno set  to
123       indicate the error.
124

ERRORS

126       EFAULT Either ss or old_ss is not NULL and points to an area outside of
127              the process's address space.
128
129       EINVAL ss is not NULL and the ss_flags field contains an invalid flag.
130
131       ENOMEM The specified size of the new alternate signal stack  ss.ss_size
132              was less than MINSIGSTKSZ.
133
134       EPERM  An  attempt  was made to change the alternate signal stack while
135              it was active (i.e., the thread was  already  executing  on  the
136              current alternate signal stack).
137

ATTRIBUTES

139       For  an  explanation  of  the  terms  used  in  this  section,  see at‐
140       tributes(7).
141
142       ┌────────────────────────────────────────────┬───────────────┬─────────┐
143Interface                                   Attribute     Value   
144       ├────────────────────────────────────────────┼───────────────┼─────────┤
145sigaltstack()                               │ Thread safety │ MT-Safe │
146       └────────────────────────────────────────────┴───────────────┴─────────┘
147

STANDARDS

149       POSIX.1-2008.
150
151       SS_AUTODISARM is a Linux extension.
152

HISTORY

154       POSIX.1-2001, SUSv2, SVr4.
155

NOTES

157       The most common usage of an alternate signal stack  is  to  handle  the
158       SIGSEGV  signal  that  is  generated  if  the  space  available for the
159       standard stack is exhausted: in this case, a signal handler for SIGSEGV
160       cannot  be  invoked  on the standard stack; if we wish to handle it, we
161       must use an alternate signal stack.
162
163       Establishing an alternate signal stack is useful if  a  thread  expects
164       that  it  may exhaust its standard stack.  This may occur, for example,
165       because the stack grows  so  large  that  it  encounters  the  upwardly
166       growing  heap,  or  it  reaches  a  limit  established  by  a  call  to
167       setrlimit(RLIMIT_STACK, &rlim).  If the standard  stack  is  exhausted,
168       the  kernel  sends the thread a SIGSEGV signal.  In these circumstances
169       the only way to catch this signal is on an alternate signal stack.
170
171       On  most  hardware  architectures  supported  by  Linux,  stacks   grow
172       downward.   sigaltstack()  automatically takes account of the direction
173       of stack growth.
174
175       Functions called from a signal handler executing on an alternate signal
176       stack  will also use the alternate signal stack.  (This also applies to
177       any handlers invoked for other signals while the thread is executing on
178       the  alternate  signal  stack.)   Unlike the standard stack, the system
179       does not automatically extend the alternate  signal  stack.   Exceeding
180       the  allocated  size  of  the  alternate  signal  stack  will  lead  to
181       unpredictable results.
182
183       A successful call to execve(2) removes any  existing  alternate  signal
184       stack.   A  child  process  created  via fork(2) inherits a copy of its
185       parent's alternate signal stack settings.  The same is also true for  a
186       child  process  created  using clone(2), unless the clone flags include
187       CLONE_VM and do not include CLONE_VFORK, in which  case  any  alternate
188       signal  stack  that  was  established  in the parent is disabled in the
189       child process.
190
191       sigaltstack() supersedes  the  older  sigstack()  call.   For  backward
192       compatibility,  glibc  also  provides sigstack().  All new applications
193       should be written using sigaltstack().
194
195   History
196       4.2BSD had a sigstack() system call.   It  used  a  slightly  different
197       struct,  and had the major disadvantage that the caller had to know the
198       direction of stack growth.
199

BUGS

201       In Linux 2.2 and earlier, the only flag  that  could  be  specified  in
202       ss.sa_flags was SS_DISABLE.  In the lead up to the release of the Linux
203       2.4  kernel,  a  change  was  made  to  allow  sigaltstack()  to  allow
204       ss.ss_flags==SS_ONSTACK  with the same meaning as ss.ss_flags==0 (i.e.,
205       the inclusion of SS_ONSTACK in  ss.ss_flags  is  a  no-op).   On  other
206       implementations, and according to POSIX.1, SS_ONSTACK appears only as a
207       reported flag in old_ss.ss_flags.  On Linux, there is no need  ever  to
208       specify  SS_ONSTACK  in  ss.ss_flags,  and  indeed  doing  so should be
209       avoided on portability grounds: various other systems give an error  if
210       SS_ONSTACK is specified in ss.ss_flags.
211

EXAMPLES

213       The  following  code segment demonstrates the use of sigaltstack() (and
214       sigaction(2)) to install an alternate signal stack that is employed  by
215       a handler for the SIGSEGV signal:
216
217           stack_t ss;
218
219           ss.ss_sp = malloc(SIGSTKSZ);
220           if (ss.ss_sp == NULL) {
221               perror("malloc");
222               exit(EXIT_FAILURE);
223           }
224
225           ss.ss_size = SIGSTKSZ;
226           ss.ss_flags = 0;
227           if (sigaltstack(&ss, NULL) == -1) {
228               perror("sigaltstack");
229               exit(EXIT_FAILURE);
230           }
231
232           sa.sa_flags = SA_ONSTACK;
233           sa.sa_handler = handler();      /* Address of a signal handler */
234           sigemptyset(&sa.sa_mask);
235           if (sigaction(SIGSEGV, &sa, NULL) == -1) {
236               perror("sigaction");
237               exit(EXIT_FAILURE);
238           }
239

SEE ALSO

241       execve(2),  setrlimit(2),  sigaction(2),  siglongjmp(3),  sigsetjmp(3),
242       signal(7)
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245
246Linux man-pages 6.05              2023-07-20                    sigaltstack(2)
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