man2/sigaltstack.2

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  27 .TH SIGALTSTACK 2 2016-03-15 "Linux" "Linux Programmer's Manual"
  28 .SH NAME
  29 sigaltstack \- set and/or get signal stack context
  30 .SH SYNOPSIS
  31 .B #include <signal.h>
  32 .sp
  33 .BI "int sigaltstack(const stack_t *" ss ", stack_t *" oss );
  34 .sp
  35 .in -4n
  36 Feature Test Macro Requirements for glibc (see
  37 .BR feature_test_macros (7)):
  38 .in
  39 .sp
  40 .BR sigaltstack ():
  41 .ad l
  42 .RS 4
  43 .PD 0
  44 _XOPEN_SOURCE\ >=\ 500
  45 .\"    || _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED
  46     || /* Since glibc 2.12: */ _POSIX_C_SOURCE\ >=\ 200809L
  47     || /* Glibc versions <= 2.19: */ _BSD_SOURCE
  48 .PD
  49 .RE
  50 .ad
  51 .SH DESCRIPTION
  52 .BR sigaltstack ()
  53 allows a process to define a new alternate
  54 signal stack and/or retrieve the state of an existing
  55 alternate signal stack.
  56 An alternate signal stack is used during the
  57 execution of a signal handler if the establishment of that handler (see
  58 .BR sigaction (2))
  59 requested it.
  60
  61 The normal sequence of events for using an alternate signal stack
  62 is the following:
  63 .TP 3
  64 1.
  65 Allocate an area of memory to be used for the alternate
  66 signal stack.
  67 .TP
  68 2.
  69 Use
  70 .BR sigaltstack ()
  71 to inform the system of the existence and
  72 location of the alternate signal stack.
  73 .TP
  74 3.
  75 When establishing a signal handler using
  76 .BR sigaction (2),
  77 inform the system that the signal handler should be executed
  78 on the alternate signal stack by
  79 specifying the \fBSA_ONSTACK\fP flag.
  80 .P
  81 The \fIss\fP argument is used to specify a new
  82 alternate signal stack, while the \fIoss\fP argument
  83 is used to retrieve information about the currently
  84 established signal stack.
  85 If we are interested in performing just one
  86 of these tasks, then the other argument can be specified as NULL.
  87 Each of these arguments is a structure of the following type:
  88 .sp
  89 .in +4n
  90 .nf
  91 typedef struct {
  92     void  *ss_sp;     /* Base address of stack */
  93     int    ss_flags;  /* Flags */
  94     size_t ss_size;   /* Number of bytes in stack */
  95 } stack_t;
  96 .fi
  97 .in
  98
  99 To establish a new alternate signal stack,
 100 \fIss.ss_flags\fP is set to zero, and \fIss.ss_sp\fP and
 101 \fIss.ss_size\fP specify the starting address and size of
 102 the stack.
 103 The constant \fBSIGSTKSZ\fP is defined to be large enough
 104 to cover the usual size requirements for an alternate signal stack,
 105 and the constant \fBMINSIGSTKSZ\fP defines the minimum
 106 size required to execute a signal handler.
 107
 108 When a signal handler is invoked on the alternate stack,
 109 the kernel automatically aligns the address given in \fIss.ss_sp\fP
 110 to a suitable address boundary for the underlying hardware architecture.
 111
 112 To disable an existing stack, specify \fIss.ss_flags\fP
 113 as \fBSS_DISABLE\fP.
 114 In this case, the remaining fields
 115 in \fIss\fP are ignored.
 116
 117 If \fIoss\fP is not NULL, then it is used to return information about
 118 the alternate signal stack which was in effect prior to the
 119 call to
 120 .BR sigaltstack ().
 121 The \fIoss.ss_sp\fP and \fIoss.ss_size\fP fields return the starting
 122 address and size of that stack.
 123 The \fIoss.ss_flags\fP may return either of the following values:
 124 .TP
 125 .B SS_ONSTACK
 126 The process is currently executing on the alternate signal stack.
 127 (Note that it is not possible
 128 to change the alternate signal stack if the process is
 129 currently executing on it.)
 130 .TP
 131 .B SS_DISABLE
 132 The alternate signal stack is currently disabled.
 133 .SH RETURN VALUE
 134 .BR sigaltstack ()
 135 returns 0 on success, or \-1 on failure with
 136 \fIerrno\fP set to indicate the error.
 137 .SH ERRORS
 138 .TP
 139 .B EFAULT
 140 Either \fIss\fP or \fIoss\fP is not NULL and points to an area
 141 outside of the process's address space.
 142 .TP
 143 .B EINVAL
 144 \fIss\fP is not NULL and the \fIss_flags\fP field contains
 145 a nonzero value other than
 146 .BR SS_DISABLE .
 147 .TP
 148 .B ENOMEM
 149 The specified size of the new alternate signal stack
 150 .I ss.ss_size
 151 was less than
 152 .BR MINSTKSZ .
 153 .TP
 154 .B EPERM
 155 An attempt was made to change the alternate signal stack while
 156 it was active (i.e., the process was already executing
 157 on the current alternate signal stack).
 158 .SH ATTRIBUTES
 159 For an explanation of the terms used in this section, see
 160 .BR attributes (7).
 161 .TS
 162 allbox;
 163 lb lb lb
 164 l l l.
 165 Interface       Attribute       Value
 166 T{
 167 .BR sigaltstack ()
 168 T}      Thread safety   MT-Safe
 169 .TE
 170
 171 .SH CONFORMING TO
 172 POSIX.1-2001, POSIX.1-2009, SUSv2, SVr4.
 173 .SH NOTES
 174 The most common usage of an alternate signal stack is to handle the
 175 .B SIGSEGV
 176 signal that is generated if the space available for the
 177 normal process stack is exhausted: in this case, a signal handler for
 178 .B SIGSEGV
 179 cannot be invoked on the process stack; if we wish to handle it,
 180 we must use an alternate signal stack.
 181 .P
 182 Establishing an alternate signal stack is useful if a process
 183 expects that it may exhaust its standard stack.
 184 This may occur, for example, because the stack grows so large
 185 that it encounters the upwardly growing heap, or it reaches a
 186 limit established by a call to \fBsetrlimit(RLIMIT_STACK, &rlim)\fP.
 187 If the standard stack is exhausted, the kernel sends
 188 the process a \fBSIGSEGV\fP signal.
 189 In these circumstances the only way to catch this signal is
 190 on an alternate signal stack.
 191 .P
 192 On most hardware architectures supported by Linux, stacks grow
 193 downward.
 194 .BR sigaltstack ()
 195 automatically takes account
 196 of the direction of stack growth.
 197 .P
 198 Functions called from a signal handler executing on an alternate
 199 signal stack will also use the alternate signal stack.
 200 (This also applies to any handlers invoked for other signals while
 201 the process is executing on the alternate signal stack.)
 202 Unlike the standard stack, the system does not
 203 automatically extend the alternate signal stack.
 204 Exceeding the allocated size of the alternate signal stack will
 205 lead to unpredictable results.
 206 .P
 207 A successful call to
 208 .BR execve (2)
 209 removes any existing alternate
 210 signal stack.
 211 A child process created via
 212 .BR fork (2)
 213 inherits a copy of its parent's alternate signal stack settings.
 214 .P
 215 .BR sigaltstack ()
 216 supersedes the older
 217 .BR sigstack ()
 218 call.
 219 For backward compatibility, glibc also provides
 220 .BR sigstack ().
 221 All new applications should be written using
 222 .BR sigaltstack ().
 223 .SS History
 224 4.2BSD had a
 225 .BR sigstack ()
 226 system call.
 227 It used a slightly
 228 different struct, and had the major disadvantage that the caller
 229 had to know the direction of stack growth.
 230 .SH EXAMPLE
 231 The following code segment demonstrates the use of
 232 .BR sigaltstack ():
 233
 234 .in +4n
 235 .nf
 236 stack_t ss;
 237
 238 ss.ss_sp = malloc(SIGSTKSZ);
 239 if (ss.ss_sp == NULL)
 240     /* Handle error */;
 241 ss.ss_size = SIGSTKSZ;
 242 ss.ss_flags = 0;
 243 if (sigaltstack(&ss, NULL) == \-1)
 244     /* Handle error */;
 245 .fi
 246 .in
 247 .SH SEE ALSO
 248 .BR execve (2),
 249 .BR setrlimit (2),
 250 .BR sigaction (2),
 251 .BR siglongjmp (3),
 252 .BR sigsetjmp (3),
 253 .BR signal (7)