It returns NULL to indicate errors? Not any platform I've used lately. The Linux man page is in fact explicit that it never returns NULL: RETURN VALUE

It returns NULL to indicate errors? Not any platform I've used lately. The Linux man page is in fact explicit that it never returns NULL: RETURN VALUE The alloca() function returns a pointer to the beginning of the allocated space. If the allocation causes stack overflow, program behavior is undefined. NOTES ... The inlined code often consists of a single instruction adjusting the stack pointer, and does not check for stack overflow. Thus, there is no NULL error return. As a practical matter it is difficult for an implementation to have alloca return an error value even if they wanted to: it would be both two slow and in some cases "impossible" to know ahead of time in the same way that malloc is unlikely to return null on some systems.