1SIGALTSTACK(2)             Linux Programmer's Manual            SIGALTSTACK(2)
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NAME

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

9       #include <signal.h>
10
11       int sigaltstack(const stack_t *ss, stack_t *oss);
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13   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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15       sigaltstack(): _BSD_SOURCE || _XOPEN_SOURCE >= 500
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DESCRIPTION

18       sigaltstack()  allows  a process to define a new alternate signal stack
19       and/or retrieve the state of an existing alternate  signal  stack.   An
20       alternate signal stack is used during the execution of a signal handler
21       if the establishment of that handler (see sigaction(2)) requested it.
22
23       The normal sequence of events for using an alternate  signal  stack  is
24       the following:
25
26       1. Allocate  an  area  of  memory  to  be used for the alternate signal
27          stack.
28
29       2. Use sigaltstack() to inform the system of the existence and location
30          of the alternate signal stack.
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32       3. When  establishing  a  signal handler using sigaction(2), inform the
33          system that the signal handler should be executed on  the  alternate
34          signal stack by specifying the SA_ONSTACK flag.
35
36       The  ss argument is used to specify a new alternate signal stack, while
37       the oss argument is used to retrieve information  about  the  currently
38       established  signal stack.  If we are interested in performing just one
39       of these tasks then the other argument can be specified as NULL.   Each
40       of these arguments is a structure of the following type:
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42           typedef struct {
43               void  *ss_sp;     /* Base address of stack */
44               int    ss_flags;  /* Flags */
45               size_t ss_size;   /* Number of bytes in stack */
46           } stack_t;
47
48       To  establish a new alternate signal stack, ss.ss_flags is set to zero,
49       and ss.ss_sp and ss.ss_size specify the starting address  and  size  of
50       the  stack.   The  constant  SIGSTKSZ  is defined to be large enough to
51       cover the usual size requirements for an alternate  signal  stack,  and
52       the constant MINSIGSTKSZ defines the minimum size required to execute a
53       signal handler.
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55       When a signal handler is invoked on the  alternate  stack,  the  kernel
56       automatically  aligns  the  address  given  in  ss.ss_sp  to a suitable
57       address boundary for the underlying hardware architecture.
58
59       To disable an existing stack, specify ss.ss_flags  as  SS_DISABLE.   In
60       this case, the remaining fields in ss are ignored.
61
62       If  oss  is  not  NULL, then it is used to return information about the
63       alternate signal stack which was in effect prior to the call to sigalt‐
64       stack().   The  oss.ss_sp  and  oss.ss_size  fields return the starting
65       address and size of that stack.  The oss.ss_flags may return either  of
66       the following values:
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68       SS_ONSTACK
69              The  process  is  currently  executing  on  the alternate signal
70              stack.  (Note that it is not possible to  change  the  alternate
71              signal stack if the process is currently executing on it.)
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73       SS_DISABLE
74              The alternate signal stack is currently disabled.
75

RETURN VALUE

77       sigaltstack()  returns 0 on success, or -1 on failure with errno set to
78       indicate the error.
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ERRORS

81       EFAULT Either ss or oss is not NULL and points to an  area  outside  of
82              the process's address space.
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84       EINVAL ss  is  not NULL and the ss_flags field contains a nonzero value
85              other than SS_DISABLE.
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87       ENOMEM The  specified  size  of  the   new   alternate   signal   stack
88              (ss.ss_size) was less than MINSTKSZ.
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90       EPERM  An  attempt  was made to change the alternate signal stack while
91              it was active (i.e., the process was already  executing  on  the
92              current alternate signal stack).
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CONFORMING TO

95       SUSv2, SVr4, POSIX.1-2001.
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NOTES

98       The  most  common  usage  of an alternate signal stack is to handle the
99       SIGSEGV signal that is generated if the space available for the  normal
100       process  stack is exhausted: in this case, a signal handler for SIGSEGV
101       cannot be invoked on the process stack; if we wish  to  handle  it,  we
102       must use an alternate signal stack.
103
104       Establishing  an  alternate signal stack is useful if a process expects
105       that it may exhaust its standard stack.  This may occur,  for  example,
106       because  the stack grows so large that it encounters the upwardly grow‐
107       ing heap, or it  reaches  a  limit  established  by  a  call  to  setr‐
108       limit(RLIMIT_STACK,  &rlim).   If  the standard stack is exhausted, the
109       kernel sends the process a SIGSEGV signal.  In these circumstances  the
110       only way to catch this signal is on an alternate signal stack.
111
112       On  most  hardware  architectures supported by Linux, stacks grow down‐
113       wards.  sigaltstack() automatically takes account of the  direction  of
114       stack growth.
115
116       Functions called from a signal handler executing on an alternate signal
117       stack will also use the alternate signal stack.  (This also applies  to
118       any  handlers  invoked for other signals while the process is executing
119       on the alternate signal stack.)  Unlike the standard stack, the  system
120       does  not  automatically  extend the alternate signal stack.  Exceeding
121       the allocated size of the alternate signal stack will  lead  to  unpre‐
122       dictable results.
123
124       A  successful  call  to execve(2) removes any existing alternate signal
125       stack.  A child process created via fork() inherits a copy of its  par‐
126       ent's alternate signal stack settings.
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128       sigaltstack() supersedes the older sigstack() call.  For backwards com‐
129       patibility, glibc  also  provides  sigstack().   All  new  applications
130       should be written using sigaltstack().
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132   History
133       4.2BSD  had  a  sigstack()  system  call.  It used a slightly different
134       struct, and had the major disadvantage that the caller had to know  the
135       direction of stack growth.
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EXAMPLE

138       The following code segment demonstrates the use of sigaltstack():
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140           stack_t ss;
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142           ss.ss_sp = malloc(SIGSTKSZ);
143           if (ss.ss_sp == NULL)
144               /* Handle error */;
145           ss.ss_size = SIGSTKSZ;
146           ss.ss_flags = 0;
147           if (sigaltstack(&ss, NULL) == -1)
148               /* Handle error */;
149

SEE ALSO

151       execve(2),  setrlimit(2),  sigaction(2),  siglongjmp(3),  sigsetjmp(3),
152       signal(7)
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COLOPHON

155       This page is part of release 3.25 of the Linux  man-pages  project.   A
156       description  of  the project, and information about reporting bugs, can
157       be found at http://www.kernel.org/doc/man-pages/.
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161Linux                             2008-10-04                    SIGALTSTACK(2)
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