alloca - allocate memory that is automatically freed
The alloca() function returns a pointer to the beginning of the allocated space. If the allocation causes stack overflow, program behavior is undefined.
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.
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.
The space allocated by alloca() is
automatically deallocated if the pointer that refers to it simply goes
out of scope.
Normally, gcc(1) translates calls to
alloca() with inlined code. This is not done when
-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
<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.
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.
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