bsd/man/man2/sigaltstack.2

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  34 .\"     @(#)sigaltstack.2       8.1 (Berkeley) 6/4/93
  35 .\"
  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 <signal.h>
  44 .Ft int
  45 .Fo sigaltstack
  46 .Fa "const stack_t *restrict ss"
  47 .Fa "stack_t *restrict oss"
  48 .Fc
  49 .Sh DESCRIPTION
  50 .Fn Sigaltstack
  51 allows users to define an alternate stack on which signals
  52 are to be processed.
  53 If
  54 .Fa ss
  55 is non-zero,
  56 it specifies a pointer to and the size of a
  57 .Em "signal stack"
  58 on which to deliver signals,
  59 and tells the system if the process is currently executing
  60 on that stack.
  61 When a signal's action indicates its handler
  62 should execute on the signal stack (specified with a
  63 .Xr sigaction 2
  64 call), the system checks to see
  65 if the process is currently executing on that stack.
  66 If the process is not currently executing on the signal stack,
  67 the system arranges a switch to the signal stack for the
  68 duration of the signal handler's execution.
  69 .Pp
  70 If
  71 .Dv SA_DISABLE
  72 is set in
  73 .Fa ss_flags ,
  74 .Fa ss_sp
  75 and
  76 .Fa ss_size
  77 are ignored and the signal stack will be disabled.
  78 Trying to disable an active stack will cause
  79 .Nm
  80 to return -1 with
  81 .Va errno
  82 set to
  83 .Dv EINVAL .
  84 A disabled stack will cause all signals to be
  85 taken on the regular user stack.
  86 If the stack is later re-enabled then all signals that were specified
  87 to be processed on an alternate stack will resume doing so.
  88 .Pp
  89 If
  90 .Fa oss
  91 is non-zero, the current signal stack state is returned.
  92 The
  93 .Fa ss_flags
  94 field will contain the value
  95 .Dv SA_ONSTACK
  96 if the process is currently on a signal stack and
  97 .Dv SA_DISABLE
  98 if the signal stack is currently disabled.
  99 .Sh NOTES
 100 The value
 101 .Dv SIGSTKSZ
 102 is defined to be the number of bytes/chars that would be used to cover
 103 the usual case when allocating an alternate stack area.
 104 The following code fragment is typically used to allocate an alternate stack.
 105 .Bd -literal -offset indent
 106 if ((sigstk.ss_sp = malloc(SIGSTKSZ)) == NULL)
 107         /* error return */
 108 sigstk.ss_size = SIGSTKSZ;
 109 sigstk.ss_flags = 0;
 110 if (sigaltstack(&sigstk,0) < 0)
 111         perror("sigaltstack");
 112 .Ed
 113 An alternative approach is provided for programs with signal handlers
 114 that require a specific amount of stack space other than the default size.
 115 The value
 116 .Dv MINSIGSTKSZ
 117 is defined to be the number of bytes/chars that is required by
 118 the operating system to implement the alternate stack feature.
 119 In computing an alternate stack size,
 120 programs should add
 121 .Dv MINSIGSTKSZ
 122 to their stack requirements to allow for the operating system overhead.
 123 .Pp
 124 Signal stacks are automatically adjusted for the direction of stack
 125 growth and alignment requirements.
 126 Signal stacks may or may not be protected by the hardware and
 127 are not ``grown'' automatically as is done for the normal stack.
 128 If the stack overflows and this space is not protected
 129 unpredictable results may occur.
 130 .Sh RETURN VALUES
 131 Upon successful completion, a value of 0 is returned.
 132 Otherwise, a value of -1 is returned and
 133 .Va errno
 134 is set to indicate the error.
 135 .Sh ERRORS
 136 .Fn Sigaltstack
 137 will fail and the signal stack context will remain unchanged
 138 if one of the following occurs.
 139 .Bl -tag -width [ENOMEM]
 140 .\" ===========
 141 .It Bq Er EFAULT
 142 Either
 143 .Fa ss
 144 or
 145 .Fa oss
 146 points to memory that is not a valid part of the process
 147 address space.
 148 .\" ===========
 149 .It Bq Er EINVAL
 150 An attempt is made to disable an active stack.
 151 .\" ===========
 152 .It Bq Er EINVAL
 153 The
 154 .Fa ss
 155 argument is not a null pointer, and the ss_flags member
 156 pointed to by
 157 .Fa ss
 158 contains flags other than SS_DISABLE.
 159 .\" ===========
 160 .It Bq Er ENOMEM
 161 The size of the alternate stack area is less than or equal to
 162 .Dv MINSIGSTKSZ .
 163 .\" ===========
 164 .It Bq Er EPERM
 165 An attempt was made to modify an active stack.
 166 .El
 167 .Sh LEGACY SYNOPSIS
 168 .Fd #include <sys/types.h>
 169 .Fd #include <signal.h>
 170 .Pp
 171 The include file
 172 .In sys/types.h
 173 is necessary.
 174 .Pp
 175 .Bd -literal
 176 struct sigaltstack {
 177         char   *ss_sp;
 178         int     ss_size;
 179         int     ss_flags;
 180 };
 181 .Ed
 182 .Pp
 183 .Ft int
 184 .br
 185 .Fo sigaltstack
 186 .Fa "const struct sigaltstack *ss"
 187 .Fa "struct sigaltstack *oss"
 188 .Fc ;
 189 .Pp
 190 The variable types have changed.
 191 Specifically, the
 192 .Vt sigaltstack
 193 struct is no longer used.
 194 .Sh COMPATIBILITY
 195 Use of the (obsolete)
 196 .Vt sigaltstack
 197 struct will cause compiler diagnostics.
 198 Use
 199 .Vt stack_t ,
 200 defined in
 201 .In signal.h .
 202 .Sh SEE ALSO
 203 .Xr sigaction 2 ,
 204 .Xr setjmp 3 ,
 205 .Xr compat 5
 206 .Sh HISTORY
 207 The predecessor to
 208 .Nm sigaltstack ,
 209 the
 210 .Fn sigstack
 211 system call, appeared in
 212 .Bx 4.2 .