php man page for alloca

ALLOCA(3)						     Linux Programmer's Manual							 ALLOCA(3)

NAME
       alloca - allocate memory that is automatically freed

SYNOPSIS
       #include <alloca.h>

       void *alloca(size_t size);

DESCRIPTION
       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.

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.

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

       +----------+---------------+---------+
       |Interface | Attribute	  | Value   |
       +----------+---------------+---------+
       |alloca()  | Thread safety | MT-Safe |
       +----------+---------------+---------+
CONFORMING TO
       This function is not in POSIX.1.

       There  is  evidence  that  the alloca() function appeared in 32V, PWB, PWB.2, 3BSD, and 4BSD.  There is a man page for it in 4.3BSD.  Linux
       uses the GNU version.

NOTES
       The alloca() function is machine- and compiler-dependent.  For certain applications, its use can improve efficiency compared to the use	of
       malloc(3)  plus	free(3).  In certain cases, it can also simplify memory deallocation in applications that use longjmp(3) or siglongjmp(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  function	return	is
       jumped over by a call to longjmp(3) or siglongjmp(3).

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

   Notes on the GNU version
       Normally,  gcc(1)  translates  calls  to  alloca()  with  inlined code.	This is not done when either the -ansi, -std=c89, -std=c99, or the
       -std=c11 option is given and the header <alloca.h> is not included.  Otherwise, (without an -ansi or -std=c* option) the glibc  version	of
       <stdlib.h> includes <alloca.h> and that contains the lines:

	   #ifdef  __GNUC__
	   #define alloca(size)   __builtin_alloca (size)
	   #endif

       with messy consequences if one has a private version of this function.

       The  fact  that	the code is inlined means that it is impossible to take the address of this function, or to change its behavior by linking
       with a different library.

       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.

BUGS
       There  is no error indication if the stack frame cannot be extended.  (However, after a failed allocation, the program is likely to receive
       a SIGSEGV signal if it attempts to access the unallocated 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.

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

COLOPHON
       This  page is part of release 4.15 of the Linux man-pages project.  A description of the project, information about reporting bugs, and the
       latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.

GNU
								    2017-09-15								 ALLOCA(3)