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| .TH PTHREAD_ATTR_SETGUARDSIZE 3 2021-03-22 "Linux" "Linux Programmer's Manual" |
| .SH NAME |
| pthread_attr_setguardsize, pthread_attr_getguardsize \- set/get guard size |
| attribute in thread attributes object |
| .SH SYNOPSIS |
| .nf |
| .B #include <pthread.h> |
| .PP |
| .BI "int pthread_attr_setguardsize(pthread_attr_t *" attr \ |
| ", size_t " guardsize ); |
| .BI "int pthread_attr_getguardsize(const pthread_attr_t *restrict " attr , |
| .BI " size_t *restrict " guardsize ); |
| .PP |
| Compile and link with \fI\-pthread\fP. |
| .fi |
| .SH DESCRIPTION |
| The |
| .BR pthread_attr_setguardsize () |
| function sets the guard size attribute of the |
| thread attributes object referred to by |
| .I attr |
| to the value specified in |
| .IR guardsize . |
| .PP |
| If |
| .I guardsize |
| is greater than 0, |
| then for each new thread created using |
| .I attr |
| the system allocates an additional region of at least |
| .I guardsize |
| bytes at the end of the thread's stack to act as the guard area |
| for the stack (but see BUGS). |
| .PP |
| If |
| .I guardsize |
| is 0, then new threads created with |
| .I attr |
| will not have a guard area. |
| .PP |
| The default guard size is the same as the system page size. |
| .PP |
| If the stack address attribute has been set in |
| .I attr |
| (using |
| .BR pthread_attr_setstack (3) |
| or |
| .BR pthread_attr_setstackaddr (3)), |
| meaning that the caller is allocating the thread's stack, |
| then the guard size attribute is ignored |
| (i.e., no guard area is created by the system): |
| it is the application's responsibility to handle stack overflow |
| (perhaps by using |
| .BR mprotect (2) |
| to manually define a guard area at the end of the stack |
| that it has allocated). |
| .PP |
| The |
| .BR pthread_attr_getguardsize () |
| function returns the guard size attribute of the |
| thread attributes object referred to by |
| .I attr |
| in the buffer pointed to by |
| .IR guardsize . |
| .SH RETURN VALUE |
| On success, these functions return 0; |
| on error, they return a nonzero error number. |
| .SH ERRORS |
| POSIX.1 documents an |
| .B EINVAL |
| error if |
| .I attr |
| or |
| .I guardsize |
| is invalid. |
| On Linux these functions always succeed |
| (but portable and future-proof applications should nevertheless |
| handle a possible error return). |
| .SH VERSIONS |
| These functions are provided by glibc since version 2.1. |
| .SH ATTRIBUTES |
| For an explanation of the terms used in this section, see |
| .BR attributes (7). |
| .ad l |
| .nh |
| .TS |
| allbox; |
| lbx lb lb |
| l l l. |
| Interface Attribute Value |
| T{ |
| .BR pthread_attr_setguardsize (), |
| .BR pthread_attr_getguardsize () |
| T} Thread safety MT-Safe |
| .TE |
| .hy |
| .ad |
| .sp 1 |
| .SH CONFORMING TO |
| POSIX.1-2001, POSIX.1-2008. |
| .SH NOTES |
| A guard area consists of virtual memory pages that are protected |
| to prevent read and write access. |
| If a thread overflows its stack into the guard area, |
| then, on most hard architectures, it receives a |
| .B SIGSEGV |
| signal, thus notifying it of the overflow. |
| Guard areas start on page boundaries, |
| and the guard size is internally rounded up to |
| the system page size when creating a thread. |
| (Nevertheless, |
| .BR pthread_attr_getguardsize () |
| returns the guard size that was set by |
| .BR pthread_attr_setguardsize ().) |
| .PP |
| Setting a guard size of 0 may be useful to save memory |
| in an application that creates many threads |
| and knows that stack overflow can never occur. |
| .PP |
| Choosing a guard size larger than the default size |
| may be necessary for detecting stack overflows |
| if a thread allocates large data structures on the stack. |
| .SH BUGS |
| As at glibc 2.8, the NPTL threading implementation includes |
| the guard area within the stack size allocation, |
| rather than allocating extra space at the end of the stack, |
| as POSIX.1 requires. |
| (This can result in an |
| .B EINVAL |
| error from |
| .BR pthread_create (3) |
| if the guard size value is too large, |
| leaving no space for the actual stack.) |
| .PP |
| The obsolete LinuxThreads implementation did the right thing, |
| allocating extra space at the end of the stack for the guard area. |
| .\" glibc includes the guardsize within the allocated stack size, |
| .\" which looks pretty clearly to be in violation of POSIX. |
| .\" |
| .\" Filed bug, 22 Oct 2008: |
| .\" http://sources.redhat.com/bugzilla/show_bug.cgi?id=6973 |
| .\" |
| .\" Older reports: |
| .\" https//bugzilla.redhat.com/show_bug.cgi?id=435337 |
| .\" Reportedly, LinuxThreads did the right thing, allocating |
| .\" extra space at the end of the stack: |
| .\" http://sourceware.org/ml/libc-alpha/2008-05/msg00086.html |
| .SH EXAMPLES |
| See |
| .BR pthread_getattr_np (3). |
| .SH SEE ALSO |
| .BR mmap (2), |
| .BR mprotect (2), |
| .BR pthread_attr_init (3), |
| .BR pthread_attr_setstack (3), |
| .BR pthread_attr_setstacksize (3), |
| .BR pthread_create (3), |
| .BR pthreads (7) |