1SIGALTSTACK(3P)            POSIX Programmer's Manual           SIGALTSTACK(3P)
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PROLOG

6       This  manual  page is part of the POSIX Programmer's Manual.  The Linux
7       implementation of this interface may differ (consult the  corresponding
8       Linux  manual page for details of Linux behavior), or the interface may
9       not be implemented on Linux.
10

NAME

12       sigaltstack — set and get signal alternate stack context
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SYNOPSIS

15       #include <signal.h>
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17       int sigaltstack(const stack_t *restrict ss, stack_t *restrict oss);
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DESCRIPTION

20       The sigaltstack() function allows a process to define and  examine  the
21       state of an alternate stack for signal handlers for the current thread.
22       Signals that have been explicitly declared to execute on the  alternate
23       stack shall be delivered on the alternate stack.
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25       If  ss  is  not  a  null pointer, it points to a stack_t structure that
26       specifies the alternate signal stack that shall take effect upon return
27       from sigaltstack().  The ss_flags member specifies the new stack state.
28       If it is set to SS_DISABLE, the stack is disabled and ss_sp and ss_size
29       are  ignored.  Otherwise, the stack shall be enabled, and the ss_sp and
30       ss_size members specify the new address and size of the stack.
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32       The range of addresses starting  at  ss_sp  up  to  but  not  including
33       ss_sp+ss_size  is available to the implementation for use as the stack.
34       This function makes no assumptions regarding which  end  is  the  stack
35       base and in which direction the stack grows as items are pushed.
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37       If oss is not a null pointer, upon successful completion it shall point
38       to a stack_t structure that specifies the alternate signal  stack  that
39       was  in  effect  prior  to  the  call  to sigaltstack().  The ss_sp and
40       ss_size members specify  the  address  and  size  of  that  stack.  The
41       ss_flags member specifies the stack's state, and may contain one of the
42       following values:
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44       SS_ONSTACK  The process is currently executing on the alternate  signal
45                   stack.  Attempts to modify the alternate signal stack while
46                   the process is executing on it fail. This flag shall not be
47                   modified by processes.
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49       SS_DISABLE  The alternate signal stack is currently disabled.
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51       The  value  SIGSTKSZ is a system default specifying the number of bytes
52       that would be used to cover the usual case when manually allocating  an
53       alternate  stack area. The value MINSIGSTKSZ is defined to be the mini‐
54       mum stack size for a signal handler. In computing  an  alternate  stack
55       size,  a  program  should  add that amount to its stack requirements to
56       allow for the system implementation overhead. The constants SS_ONSTACK,
57       SS_DISABLE, SIGSTKSZ, and MINSIGSTKSZ are defined in <signal.h>.
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59       After  a  successful  call  to  one of the exec functions, there are no
60       alternate signal stacks in the new process image.
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62       In some implementations, a signal (whether or not indicated to  execute
63       on  the alternate stack) shall always execute on the alternate stack if
64       it is delivered while another signal is being caught using  the  alter‐
65       nate stack.
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67       Use  of  this function by library threads that are not bound to kernel-
68       scheduled entities results in undefined behavior.
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RETURN VALUE

71       Upon successful completion, sigaltstack() shall return 0; otherwise, it
72       shall return -1 and set errno to indicate the error.
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ERRORS

75       The sigaltstack() function shall fail if:
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77       EINVAL The  ss  argument is not a null pointer, and the ss_flags member
78              pointed to by ss contains flags other than SS_DISABLE.
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80       ENOMEM The size of the alternate stack area is less than MINSIGSTKSZ.
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82       EPERM  An attempt was made to modify an active stack.
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84       The following sections are informative.
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EXAMPLES

87   Allocating Memory for an Alternate Stack
88       The following example illustrates a method for allocating memory for an
89       alternate stack.
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92           #include <signal.h>
93           ...
94           if ((sigstk.ss_sp = malloc(SIGSTKSZ)) == NULL)
95               /* Error return. */
96           sigstk.ss_size = SIGSTKSZ;
97           sigstk.ss_flags = 0;
98           if (sigaltstack(&sigstk,(stack_t *)0) < 0)
99               perror("sigaltstack");
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APPLICATION USAGE

102       On  some  implementations,  stack  space  is  automatically extended as
103       needed. On those implementations, automatic extension is typically  not
104       available  for an alternate stack. If the stack overflows, the behavior
105       is undefined.
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RATIONALE

108       None.
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FUTURE DIRECTIONS

111       None.
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SEE ALSO

114       Section 2.4, Signal Concepts, exec, sigaction(), sigsetjmp()
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116       The Base Definitions volume of POSIX.1‐2017, <signal.h>
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119       Portions of this text are reprinted and reproduced in  electronic  form
120       from  IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
121       table Operating System Interface (POSIX), The Open Group Base  Specifi‐
122       cations  Issue  7, 2018 Edition, Copyright (C) 2018 by the Institute of
123       Electrical and Electronics Engineers, Inc and The Open Group.   In  the
124       event of any discrepancy between this version and the original IEEE and
125       The Open Group Standard, the original IEEE and The Open Group  Standard
126       is  the  referee document. The original Standard can be obtained online
127       at http://www.opengroup.org/unix/online.html .
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129       Any typographical or formatting errors that appear  in  this  page  are
130       most likely to have been introduced during the conversion of the source
131       files to man page format. To report such errors,  see  https://www.ker
132       nel.org/doc/man-pages/reporting_bugs.html .
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136IEEE/The Open Group                  2017                      SIGALTSTACK(3P)
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