man3/pthread_attr_setguardsize.3

   1 '\" t
   2 .\" Copyright (c) 2008 Linux Foundation, written by Michael Kerrisk
   3 .\"     <mtk.manpages@gmail.com>
   4 .\"
   5 .\" SPDX-License-Identifier: Linux-man-pages-copyleft
   6 .\"
   7 .TH pthread_attr_setguardsize 3 (date) "Linux man-pages (unreleased)"
   8 .SH NAME
   9 pthread_attr_setguardsize, pthread_attr_getguardsize \- set/get guard size
  10 attribute in thread attributes object
  11 .SH LIBRARY
  12 POSIX threads library
  13 .RI ( libpthread ", " \-lpthread )
  14 .SH SYNOPSIS
  15 .nf
  16 .B #include <pthread.h>
  17 .PP
  18 .BI "int pthread_attr_setguardsize(pthread_attr_t *" attr \
  19 ", size_t " guardsize );
  20 .BI "int pthread_attr_getguardsize(const pthread_attr_t *restrict " attr ,
  21 .BI "                              size_t *restrict " guardsize );
  22 .fi
  23 .SH DESCRIPTION
  24 The
  25 .BR pthread_attr_setguardsize ()
  26 function sets the guard size attribute of the
  27 thread attributes object referred to by
  28 .I attr
  29 to the value specified in
  30 .IR guardsize .
  31 .PP
  32 If
  33 .I guardsize
  34 is greater than 0,
  35 then for each new thread created using
  36 .I attr
  37 the system allocates an additional region of at least
  38 .I guardsize
  39 bytes at the end of the thread's stack to act as the guard area
  40 for the stack (but see BUGS).
  41 .PP
  42 If
  43 .I guardsize
  44 is 0, then new threads created with
  45 .I attr
  46 will not have a guard area.
  47 .PP
  48 The default guard size is the same as the system page size.
  49 .PP
  50 If the stack address attribute has been set in
  51 .I attr
  52 (using
  53 .BR pthread_attr_setstack (3)
  54 or
  55 .BR pthread_attr_setstackaddr (3)),
  56 meaning that the caller is allocating the thread's stack,
  57 then the guard size attribute is ignored
  58 (i.e., no guard area is created by the system):
  59 it is the application's responsibility to handle stack overflow
  60 (perhaps by using
  61 .BR mprotect (2)
  62 to manually define a guard area at the end of the stack
  63 that it has allocated).
  64 .PP
  65 The
  66 .BR pthread_attr_getguardsize ()
  67 function returns the guard size attribute of the
  68 thread attributes object referred to by
  69 .I attr
  70 in the buffer pointed to by
  71 .IR guardsize .
  72 .SH RETURN VALUE
  73 On success, these functions return 0;
  74 on error, they return a nonzero error number.
  75 .SH ERRORS
  76 POSIX.1 documents an
  77 .B EINVAL
  78 error if
  79 .I attr
  80 or
  81 .I guardsize
  82 is invalid.
  83 On Linux these functions always succeed
  84 (but portable and future-proof applications should nevertheless
  85 handle a possible error return).
  86 .SH ATTRIBUTES
  87 For an explanation of the terms used in this section, see
  88 .BR attributes (7).
  89 .TS
  90 allbox;
  91 lbx lb lb
  92 l l l.
  93 Interface       Attribute       Value
  94 T{
  95 .na
  96 .nh
  97 .BR pthread_attr_setguardsize (),
  98 .BR pthread_attr_getguardsize ()
  99 T}      Thread safety   MT-Safe
 100 .TE
 101 .sp 1
 102 .SH STANDARDS
 103 POSIX.1-2008.
 104 .SH HISTORY
 105 glibc 2.1.
 106 POSIX.1-2001.
 107 .SH NOTES
 108 A guard area consists of virtual memory pages that are protected
 109 to prevent read and write access.
 110 If a thread overflows its stack into the guard area,
 111 then, on most hard architectures, it receives a
 112 .B SIGSEGV
 113 signal, thus notifying it of the overflow.
 114 Guard areas start on page boundaries,
 115 and the guard size is internally rounded up to
 116 the system page size when creating a thread.
 117 (Nevertheless,
 118 .BR pthread_attr_getguardsize ()
 119 returns the guard size that was set by
 120 .BR pthread_attr_setguardsize ().)
 121 .PP
 122 Setting a guard size of 0 may be useful to save memory
 123 in an application that creates many threads
 124 and knows that stack overflow can never occur.
 125 .PP
 126 Choosing a guard size larger than the default size
 127 may be necessary for detecting stack overflows
 128 if a thread allocates large data structures on the stack.
 129 .SH BUGS
 130 As at glibc 2.8, the NPTL threading implementation includes
 131 the guard area within the stack size allocation,
 132 rather than allocating extra space at the end of the stack,
 133 as POSIX.1 requires.
 134 (This can result in an
 135 .B EINVAL
 136 error from
 137 .BR pthread_create (3)
 138 if the guard size value is too large,
 139 leaving no space for the actual stack.)
 140 .PP
 141 The obsolete LinuxThreads implementation did the right thing,
 142 allocating extra space at the end of the stack for the guard area.
 143 .\" glibc includes the guardsize within the allocated stack size,
 144 .\" which looks pretty clearly to be in violation of POSIX.
 145 .\"
 146 .\" Filed bug, 22 Oct 2008:
 147 .\" https://www.sourceware.org/bugzilla/show_bug.cgi?id=6973
 148 .\"
 149 .\" Older reports:
 150 .\" https//bugzilla.redhat.com/show_bug.cgi?id=435337
 151 .\" Reportedly, LinuxThreads did the right thing, allocating
 152 .\" extra space at the end of the stack:
 153 .\" http://sourceware.org/ml/libc-alpha/2008-05/msg00086.html
 154 .SH EXAMPLES
 155 See
 156 .BR pthread_getattr_np (3).
 157 .SH SEE ALSO
 158 .BR mmap (2),
 159 .BR mprotect (2),
 160 .BR pthread_attr_init (3),
 161 .BR pthread_attr_setstack (3),
 162 .BR pthread_attr_setstacksize (3),
 163 .BR pthread_create (3),
 164 .BR pthreads (7)