alloca(3)                                             Library Functions Manual                                            alloca(3)

       alloca - allocate memory that is automatically freed

       Standard C library (libc, -lc)

       #include <alloca.h>

       void *alloca(size_t size);

       The alloca() function allocates size bytes of space in the stack frame of the caller.  This temporary space is automatically
       freed when the function that called alloca() returns to its caller.

       The alloca() function returns a pointer to the beginning of the allocated space.  If the allocation causes  stack  overflow,
       program behavior is undefined.

       For an explanation of the terms used in this section, see attributes(7).

       │Interface                                                                                        │ Attribute     │ Value   │
       │alloca()                                                                                         │ Thread safety │ MT-Safe │

       This function is not in POSIX.1.

       alloca() originates from PWB and 32V, and appears in all their derivatives.

       The  alloca()  function is machine- and compiler-dependent.  Because it allocates from the stack, it's faster than malloc(3)
       and free(3).  In certain cases, it can also simplify memory  deallocation  in  applications  that  use  longjmp(3)  or  sig‐
       longjmp(3).  Otherwise, its use is discouraged.

       Because  the space allocated by alloca() is allocated within the stack frame, that space is automatically freed if the func‐
       tion return is jumped over by a call to longjmp(3) or siglongjmp(3).

       The space allocated by alloca() is not automatically deallocated if the pointer that refers to it simply goes out of scope.

       Do not attempt to free(3) space allocated by alloca()!

       By necessity, alloca() is a compiler built-in, also known as __builtin_alloca().  By default, modern compilers automatically
       translate  all  uses  of  alloca()  into  the  built-in, but this is forbidden if standards conformance is requested (-ansi,
       -std=c*), in which case <alloca.h> is required, lest a symbol dependency be emitted.

       The fact that alloca() is a built-in means it is impossible to take its address or to change its behavior by linking with  a
       different library.

       Variable length arrays (VLAs) are part of the C99 standard, optional since C11, and can be used for a similar purpose.  How‐
       ever, they do not port to standard C++, and, being variables, live in their block scope and don't have an allocator-like in‐
       terface, making them unfit for implementing functionality like strdupa(3).

       Due  to the nature of the stack, it is impossible to check if the allocation would overflow the space available, and, hence,
       neither is indicating an error.  (However, the program is likely to receive a SIGSEGV signal if it attempts  to  access  un‐
       available space.)

       On many systems alloca() cannot be used inside the list of arguments of a function call, because the stack space reserved by
       alloca() would appear on the stack in the middle of the space for the function arguments.

       brk(2), longjmp(3), malloc(3)

Linux man-pages 6.03                                         2022-12-15                                                   alloca(3)