man3/pthread_attr_setguardsize.3

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  26 .TH PTHREAD_ATTR_SETGUARDSIZE 3 2015-08-08 "Linux" "Linux Programmer's Manual"
  27 .SH NAME
  28 pthread_attr_setguardsize, pthread_attr_getguardsize \- set/get guard size
  29 attribute in thread attributes object
  30 .SH SYNOPSIS
  31 .nf
  32 .B #include <pthread.h>
  33
  34 .BI "int pthread_attr_setguardsize(pthread_attr_t *" attr \
  35 ", size_t " guardsize );
  36 .BI "int pthread_attr_getguardsize(const pthread_attr_t *" attr \
  37 ", size_t *" guardsize );
  38 .sp
  39 Compile and link with \fI\-pthread\fP.
  40 .fi
  41 .SH DESCRIPTION
  42 The
  43 .BR pthread_attr_setguardsize ()
  44 function sets the guard size attribute of the
  45 thread attributes object referred to by
  46 .I attr
  47 to the value specified in
  48 .IR guardsize .
  49
  50 If
  51 .I guardsize
  52 is greater than 0,
  53 then for each new thread created using
  54 .I attr
  55 the system allocates an additional region of at least
  56 .I guardsize
  57 bytes at the end of the thread's stack to act as the guard area
  58 for the stack (but see BUGS).
  59
  60 If
  61 .I guardsize
  62 is 0, then new threads created with
  63 .I attr
  64 will not have a guard area.
  65
  66 The default guard size is the same as the system page size.
  67
  68 If the stack address attribute has been set in
  69 .I attr
  70 (using
  71 .BR pthread_attr_setstack (3)
  72 or
  73 .BR pthread_attr_setstackaddr (3)),
  74 meaning that the caller is allocating the thread's stack,
  75 then the guard size attribute is ignored
  76 (i.e., no guard area is created by the system):
  77 it is the application's responsibility to handle stack overflow
  78 (perhaps by using
  79 .BR mprotect (2)
  80 to manually define a guard area at the end of the stack
  81 that it has allocated).
  82
  83 The
  84 .BR pthread_attr_getguardsize ()
  85 function returns the guard size attribute of the
  86 thread attributes object referred to by
  87 .I attr
  88 in the buffer pointed to by
  89 .IR guardsize .
  90 .SH RETURN VALUE
  91 On success, these functions return 0;
  92 on error, they return a nonzero error number.
  93 .SH ERRORS
  94 POSIX.1 documents an
  95 .B EINVAL
  96 error if
  97 .I attr
  98 or
  99 .I guardsize
 100 is invalid.
 101 On Linux these functions always succeed
 102 (but portable and future-proof applications should nevertheless
 103 handle a possible error return).
 104 .SH VERSIONS
 105 These functions are provided by glibc since version 2.1.
 106 .SH ATTRIBUTES
 107 For an explanation of the terms used in this section, see
 108 .BR attributes (7).
 109 .TS
 110 allbox;
 111 lbw28 lb lb
 112 l l l.
 113 Interface       Attribute       Value
 114 T{
 115 .BR pthread_attr_setguardsize (),
 116 .BR pthread_attr_getguardsize ()
 117 T}      Thread safety   MT-Safe
 118 .TE
 119 .SH CONFORMING TO
 120 POSIX.1-2001, POSIX.1-2008.
 121 .SH NOTES
 122 A guard area consists of virtual memory pages that are protected
 123 to prevent read and write access.
 124 If a thread overflows its stack into the guard area,
 125 then, on most hard architectures, it receives a
 126 .B SIGSEGV
 127 signal, thus notifying it of the overflow.
 128 Guard areas start on page boundaries,
 129 and the guard size is internally rounded up to
 130 the system page size when creating a thread.
 131 (Nevertheless,
 132 .BR pthread_attr_getguardsize ()
 133 returns the guard size that was set by
 134 .BR pthread_attr_setguardsize ().)
 135
 136 Setting a guard size of 0 may be useful to save memory
 137 in an application that creates many threads
 138 and knows that stack overflow can never occur.
 139
 140 Choosing a guard size larger than the default size
 141 may be necessary for detecting stack overflows
 142 if a thread allocates large data structures on the stack.
 143 .SH BUGS
 144 As at glibc 2.8, the NPTL threading implementation includes
 145 the guard area within the stack size allocation,
 146 rather than allocating extra space at the end of the stack,
 147 as POSIX.1 requires.
 148 (This can result in an
 149 .B EINVAL
 150 error from
 151 .BR pthread_create (3)
 152 if the guard size value is too large,
 153 leaving no space for the actual stack.)
 154
 155 The obsolete LinuxThreads implementation did the right thing,
 156 allocating extra space at the end of the stack for the guard area.
 157 .\" glibc includes the guardsize within the allocated stack size,
 158 .\" which looks pretty clearly to be in violation of POSIX.
 159 .\"
 160 .\" Filed bug, 22 Oct 2008:
 161 .\" http://sources.redhat.com/bugzilla/show_bug.cgi?id=6973
 162 .\"
 163 .\" Older reports:
 164 .\" https//bugzilla.redhat.com/show_bug.cgi?id=435337
 165 .\" Reportedly, LinuxThreads did the right thing, allocating
 166 .\" extra space at the end of the stack:
 167 .\" http://sourceware.org/ml/libc-alpha/2008-05/msg00086.html
 168 .SH EXAMPLE
 169 See
 170 .BR pthread_getattr_np (3).
 171 .SH SEE ALSO
 172 .BR mmap (2),
 173 .BR mprotect (2),
 174 .BR pthread_attr_init (3),
 175 .BR pthread_attr_setstack (3),
 176 .BR pthread_attr_setstacksize (3),
 177 .BR pthread_create (3),
 178 .BR pthreads (7)