PTHREAD...UARDSIZE(3POSIX)

💖 PTHREAD_ATTR_GETGUARDSIZE, PTHREAD_ATTR_SETGUARDSIZE 💖

okay bestie so this is a page from the POSIX Programmer's Manual, and we're diving into pthread_attr_getguardsize and pthread_attr_setguardsize — the get and set of the thread guardsize attribute. yes it's a mouthful, no we're not scared of it 🫶

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⚠️ PROLOG ⚠️

so real talk first: this manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ — so if you're on Linux, go peek at the corresponding Linux manual page for the actual Linux behavior. or the interface might not even be implemented on Linux at all. it's giving "check your own system, bestie" 💀

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✨ NAME ✨

pthread_attr_getguardsize, pthread_attr_setguardsize — get and set the thread guardsize attribute 🌸

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🎀 SYNOPSIS 🎀

#include <pthread.h>

int pthread_attr_getguardsize(const pthread_attr_t *restrict attr,
    size_t *restrict guardsize);
int pthread_attr_setguardsize(pthread_attr_t *attr,
    size_t guardsize);

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🦋 DESCRIPTION 🦋

okay so here's the whole vibe. 🫧

pthread_attr_getguardsize() shall get the guardsize attribute out of the attr object. That attribute gets returned in the guardsize parameter. simple as that, no cap.

pthread_attr_setguardsize() shall set the guardsize attribute in the attr object. The new value of this attribute shall be obtained from the guardsize parameter. Here's the tea:

• If guardsize is zero, a guard area shall not be provided for threads created with attr.
• If guardsize is greater than zero, a guard area of at least guardsize bytes shall be provided for each thread created with attr.

So what even is the guardsize attribute? It controls the size of the guard area for the created thread's stack, and it's basically the bodyguard for your stack pointer — it provides protection against overflow of the stack pointer. 🛡️ If a thread's stack is created with guard protection, the implementation allocates extra memory at the overflow end of the stack as a buffer against stack overflow of the stack pointer. And if an application overflows into this buffer, an error shall result — possibly in a SIGSEGV signal being delivered to the thread. 💀 so like, don't do that.

Now, a conforming implementation may round up the value contained in guardsize to a multiple of the configurable system variable {PAGESIZE} (see <sys/mman.h>). And if an implementation does round up the value of guardsize to a multiple of {PAGESIZE}, then a call to pthread_attr_getguardsize() specifying attr shall store in the guardsize parameter the guard size specified by the previous pthread_attr_setguardsize() function call. iykyk — you get back what you put in. ✨

The default value of the guardsize attribute is implementation-defined.

Here's the plot twist though 👀 — if the stackaddr attribute has been set (meaning the caller is allocating and managing its own thread stacks), the guardsize attribute shall be ignored and no protection shall be provided by the implementation. In that case it's your responsibility, bestie, to manage stack overflow along with stack allocation and management. the implementation is clocking out on that one. 🚪

💀 undefined behavior alert: the behavior is undefined if the value specified by the attr argument to pthread_attr_getguardsize() or pthread_attr_setguardsize() does not refer to an initialized thread attributes object. don't be that girl, initialize your attr.

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🔥 RETURN VALUE 🔥

If successful, the pthread_attr_getguardsize() and pthread_attr_setguardsize() functions shall return zero. Otherwise, an error number shall be returned to indicate the error. slay, or not slay, there is no in-between. 💅

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👑 ERRORS 👑

These functions shall fail if:

• EINVAL — The parameter guardsize is invalid.

And listen: these functions shall not return an error code of [EINTR]. periodt. 🎀

The following sections are informative.

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🌈 EXAMPLES 🌈

💜 Retrieving the guardsize Attribute

This example shows how to obtain the guardsize attribute of a thread attribute object. look at her go:

#include <pthread.h>

pthread_attr_t thread_attr;
size_t  guardsize;
int     rc;

/* code initializing thread_attr */
...

rc = pthread_attr_getguardsize (&thread_attr, &guardsize);
if (rc != 0)  {
    /* handle error */
    ...
}
else {
    if (guardsize > 0) {
    /* a guard area of at least guardsize bytes is provided */
    ...
    }
    else {
    /* no guard area provided */
    ...
    }
}

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🍓 APPLICATION USAGE 🍓

None. (yes really. nothing here. it's giving minimalism 💅)

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🧚 RATIONALE 🧚

okay so why does the guardsize attribute exist? the standard says it's provided to the application for two reasons:

1. Overflow protection can potentially result in wasted system resources. An application that creates a large number of threads, and which knows its threads never overflow their stack, can save system resources by turning off guard areas.
2. When threads allocate large data structures on the stack, large guard areas may be needed to detect stack overflow.

The default size of the guard area is left implementation-defined, since on systems supporting very large page sizes, the overhead might be substantial if at least one guard page is required by default. it's giving "we can't pick a number that works for everyone so we're not picking one at all" 😭

And one more thing: if an implementation detects that the value specified by the attr argument to pthread_attr_getguardsize() or pthread_attr_setguardsize() does not refer to an initialized thread attributes object, it is recommended that the function should fail and report an [EINVAL] error. (recommended, not required — note the difference, bestie 🫶)

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⭐ FUTURE DIRECTIONS ⭐

None. (the future is bright but this page isn't changing 💖)

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🔗 SEE ALSO 🔗

• The Base Definitions volume of POSIX.1‐2017, <pthread.h>, <sys_mman.h>

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📜 COPYRIGHT 📜

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .

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okay that's the whole page, you ate that 🫶✨

IEEE/The Open Group, 2017 — PTHREAD...UARDSIZE(3POSIX) — and that's a wrap, bestie 💖🎀🦋 no cap, you now understand thread guard areas fr fr.