bsd/man/man2/sigaltstack.2

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  34 .\"     @(#)sigaltstack.2       8.1 (Berkeley) 6/4/93
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  36 .Dd June 4, 1993
  37 .Dt SIGALTSTACK 2
  38 .Os BSD 4.2
  39 .Sh NAME
  40 .Nm sigaltstack
  41 .Nd set and/or get signal stack context
  42 .Sh SYNOPSIS
  43 .Fd #include <sys/types.h>
  44 .Fd #include <signal.h>
  45 .Bd -literal
  46 struct sigaltstack {
  47         char   *ss_sp;
  48         int     ss_size;
  49         int     ss_flags;
  50 };
  51 .Ed
  52 .Ft int
  53 .Fn sigaltstack "const struct sigaltstack *ss" "struct sigaltstack *oss"
  54 .Sh DESCRIPTION
  55 .Fn Sigaltstack
  56 allows users to define an alternate stack on which signals
  57 are to be processed.
  58 If
  59 .Fa ss
  60 is non-zero,
  61 it specifies a pointer to and the size of a
  62 .Em "signal stack"
  63 on which to deliver signals,
  64 and tells the system if the process is currently executing
  65 on that stack.
  66 When a signal's action indicates its handler
  67 should execute on the signal stack (specified with a
  68 .Xr sigaction 2
  69 call), the system checks to see
  70 if the process is currently executing on that stack.
  71 If the process is not currently executing on the signal stack,
  72 the system arranges a switch to the signal stack for the
  73 duration of the signal handler's execution.
  74 .Pp
  75 If
  76 .Dv SA_DISABLE
  77 is set in
  78 .Fa ss_flags ,
  79 .Fa ss_sp
  80 and
  81 .Fa ss_size
  82 are ignored and the signal stack will be disabled.
  83 Trying to disable an active stack will cause
  84 .Nm
  85 to return -1 with
  86 .Va errno
  87 set to
  88 .Dv EINVAL .
  89 A disabled stack will cause all signals to be
  90 taken on the regular user stack.
  91 If the stack is later re-enabled then all signals that were specified
  92 to be processed on an alternate stack will resume doing so.
  93 .Pp
  94 If
  95 .Fa oss
  96 is non-zero, the current signal stack state is returned.
  97 The
  98 .Fa ss_flags
  99 field will contain the value
 100 .Dv SA_ONSTACK
 101 if the process is currently on a signal stack and
 102 .Dv SA_DISABLE
 103 if the signal stack is currently disabled.
 104 .Sh NOTES
 105 The value
 106 .Dv SIGSTKSZ
 107 is defined to be the number of bytes/chars that would be used to cover
 108 the usual case when allocating an alternate stack area.
 109 The following code fragment is typically used to allocate an alternate stack.
 110 .Bd -literal -offset indent
 111 if ((sigstk.ss_sp = malloc(SIGSTKSZ)) == NULL)
 112         /* error return */
 113 sigstk.ss_size = SIGSTKSZ;
 114 sigstk.ss_flags = 0;
 115 if (sigaltstack(&sigstk,0) < 0)
 116         perror("sigaltstack");
 117 .Ed
 118 An alternative approach is provided for programs with signal handlers
 119 that require a specific amount of stack space other than the default size.
 120 The value
 121 .Dv MINSIGSTKSZ
 122 is defined to be the number of bytes/chars that is required by
 123 the operating system to implement the alternate stack feature.
 124 In computing an alternate stack size,
 125 programs should add
 126 .Dv MINSIGSTKSZ
 127 to their stack requirements to allow for the operating system overhead.
 128 .Pp
 129 Signal stacks are automatically adjusted for the direction of stack
 130 growth and alignment requirements.
 131 Signal stacks may or may not be protected by the hardware and
 132 are not ``grown'' automatically as is done for the normal stack.
 133 If the stack overflows and this space is not protected
 134 unpredictable results may occur.
 135 .Sh RETURN VALUES
 136 Upon successful completion, a value of 0 is returned.
 137 Otherwise, a value of -1 is returned and
 138 .Va errno
 139 is set to indicate the error.
 140 .Sh ERRORS
 141 .Fn Sigstack
 142 will fail and the signal stack context will remain unchanged
 143 if one of the following occurs.
 144 .Bl -tag -width [ENOMEM]
 145 .It Bq Er EFAULT
 146 Either
 147 .Fa ss
 148 or
 149 .Fa oss
 150 points to memory that is not a valid part of the process
 151 address space.
 152 .It Bq Er EINVAL
 153 An attempt was made to disable an active stack.
 154 .It Bq Er ENOMEM
 155 Size of alternate stack area is less than or equal to
 156 .Dv MINSIGSTKSZ .
 157 .El
 158 .Sh SEE ALSO
 159 .Xr sigaction 2 ,
 160 .Xr setjmp 3
 161 .Sh HISTORY
 162 The predecessor to
 163 .Nm sigaltstack ,
 164 the
 165 .Fn sigstack
 166 system call, appeared in
 167 .Bx 4.2 .