1SIGALTSTACK(2)                System Calls Manual               SIGALTSTACK(2)
2
3
4

NAME

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

SYNOPSIS

9       #include <sys/types.h>
10       #include <signal.h>
11
12       struct sigaltstack {
13            caddr_t ss_base;
14            int  ss_size;
15            int     ss_flags;
16       };
17
18       int
19       sigaltstack(ss, oss)
20       struct sigaltstack *ss;
21       struct sigaltstack *oss;
22

DESCRIPTION

24       Sigaltstack  allows users to define an alternate stack on which signals
25       are to be processed.  If ss is non-zero, it specifies a pointer to  and
26       the  size  of a signal stack on which to deliver signals, and tells the
27       system if the process is currently executing on  that  stack.   When  a
28       signal's  action  indicates  its  handler  should execute on the signal
29       stack (specified with a sigaction(2) call), the system checks to see if
30       the  process  is  currently executing on that stack.  If the process is
31       not currently executing on the signal  stack,  the  system  arranges  a
32       switch  to  the  signal  stack for the duration of the signal handler's
33       execution.
34
35       If SA_DISABLE is set in ss_flags, ss_base and ss_size are  ignored  and
36       the  signal  stack will be disabled.  Trying to disable an active stack
37       will cause sigaltstack to return -1 with errno set to EINVAL.   A  dis‐
38       abled  stack  will  cause  all  signals to be taken on the regular user
39       stack.  If the stack is later re-enabled then  all  signals  that  were
40       specified to be processed on an alternate stack will resume doing so.
41
42       If  oss  is  non-zero, the current signal stack state is returned.  The
43       ss_flags field will contain the value SA_ONSTACK if the process is cur‐
44       rently  on  a  signal  stack and SA_DISABLE if the signal stack is cur‐
45       rently disabled.
46

NOTES

48       The value SIGSTKSZ is defined to be  the  number  of  bytes/chars  that
49       would  be  used  to  cover  the usual case when allocating an alternate
50       stack area.  The following code fragment is typically used to  allocate
51       an alternate stack.
52
53       if ((sigstk.ss_base = malloc(SIGSTKSZ)) == NULL)
54            /* error return */
55       sigstk.ss_size = SIGSTKSZ;
56       sigstk.ss_flags = 0;
57       if (sigaltstack(&sigstk,0) < 0)
58            perror("sigaltstack");
59
60       An  alternative  approach is provided for programs with signal handlers
61       that require a specific amount of stack space other  than  the  default
62       size.  The value MINSIGSTKSZ is defined to be the number of bytes/chars
63       that is required by the operating system  to  implement  the  alternate
64       stack  feature.   In computing an alternate stack size, programs should
65       add MINSIGSTKSZ to their stack requirements to allow for the  operating
66       system overhead.
67
68       Signal  stacks  are  automatically  adjusted for the direction of stack
69       growth and alignment requirements.  Signal stacks may  or  may  not  be
70       protected  by  the  hardware  and are not ``grown'' automatically as is
71       done for the normal stack.  If the stack overflows and  this  space  is
72       not protected unpredictable results may occur.
73

RETURN VALUES

75       Upon  successful  completion,  a  value of 0 is returned.  Otherwise, a
76       value of -1 is returned and errno is set to indicate the error.
77

ERRORS

79       Sigaltstack  will  fail  and  the  signal  stack  context  will  remain
80       unchanged if one of the following occurs.
81
82       EFAULT              Either  ss  or  oss  points to memory that is not a
83                           valid part of the process address space.
84
85       EINVAL              An attempt was made to disable an active stack.
86
87       ENOMEM              Size of alternate stack area is less than or  equal
88                           to MINSIGSTKSZ .
89

SEE ALSO

91       sigaction(2), setjmp(3)
92

HISTORY

94       The  predecessor  to sigaltstack, the sigstack system call, appeared in
95       4.2BSD.
96
97
98
994.4 Berkeley Distribution      September 3, 1997                SIGALTSTACK(2)
Impressum