Estimating Remaining Stack Space in a C Program

Many programmers believe there is no reliable way to know how much stack space remains in a running C program. Because of this uncertainty, stack allocation techniques such as VLAs or alloca() are often considered unsafe unless the buffer size is extremely small.

On modern Unix systems (Linux, BSD, macOS), the runtime environment actually exposes enough information to estimate the remaining stack space with reasonable accuracy. By combining stack limits with the current stack pointer, it is possible to determine how much space remains before reaching the guard page.

I wrote a detailed explanation, including example code and a small utility function that estimates available stack space.

Full article (Medium, NO paywall):
How Much Stack Space Do You Have? Estimating Remaining Stack in C on Linux

The article discuss 3 techniques:

• getrlimit()
• pthread_getattr_np()
• GCC/CLANG constructor attribute.

Together these allow building a small helper API such as:

size_t stack_remaining(void)
{
    StackAddr sp = stack_marker_addr() ;
    if ( sp < stack_low_mark ) stack_low_mark = sp ;
    return sp - stack_base - safety_margin;    
}

This allows programs to inspect available stack space at runtime and make better decisions when choosing between stack allocation and heap allocation.

Have you ever needed to estimate stack usage in production code?