ALLOCA(3)              FreeBSD Library Functions Manual              ALLOCA(3)

NAME

     alloca - memory allocator

SYNOPSIS

     #include <stdlib.h>

     void *
     alloca(size_t size);

DESCRIPTION

     The alloca() function or macro allocates size bytes of space in the stack
     frame of the caller.  This temporary space is automatically freed on
     return.

RETURN VALUES

     alloca() returns a pointer to the beginning of the allocated space.

SEE ALSO

     brk(2), calloc(3), getpagesize(3), malloc(3), realloc(3)

HISTORY

     alloca() appeared in Version 7 AT&T UNIX/32V.

BUGS

     alloca() is machine and compiler dependent; its use is discouraged.

     alloca() is slightly unsafe because it cannot ensure that the pointer
     returned points to a valid and usable block of memory.  The allocation
     made may exceed the bounds of the stack, or even go further into other
     objects in memory, and alloca() cannot determine such an error.  Avoid
     alloca() with large unbounded allocations.

     The use of C99 variable-length arrays and alloca() in the same function
     will cause the lifetime of alloca()'s storage to be limited to the block
     containing the alloca().  For example, in the following snippet, p's
     lifetime does not extend outside of the block, whereas it would've if vla
     hadn't been defined or had been defined as a fixed-length array:

           char *p;
           {
                   const int n = 100;
                   int vla[n];
                   p = alloca(32);
                   strcpy(p, "Hello, world!");
                   printf("Inside: %s\n", p); /* Valid. */
           }
           printf("Outside: %s\n", p); /* Undefined. */

FreeBSD 15.1-STABLE-HBSD       February 19, 2026                     ALLOCA(3)