alloca(3)

alloca3

Memory allocator

Usage

#include <stdlib.h>

void *

alloca(size_t size);

Overview

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.

Releases

87 releases since FreeBSD 2.0.5-RELEASE; the text changed 43 times.

  1. 2.0.5
  2. 2.1.5–3.4
  3. 3.5–3.5.1
  4. 4.0
  5. 4.1–4.3
  6. 4.4
  7. 4.5–4.11
  8. 5.0–5.1
  9. 5.2–6.1
  10. 6.2–6.3
  11. 6.4
  12. 7.0
  13. 7.1
  14. 7.2
  15. 7.3
  16. 7.4
  17. 8.0
  18. 8.1
  19. 8.2
  20. 8.3
  21. 8.4
  22. 9.0
  23. 9.1
  24. 9.2
  25. 9.3
  26. 10.0
  27. 10.1
  28. 10.2
  29. 10.3
  30. 10.4
  31. 11.0
  32. 11.1
  33. 11.2
  34. 11.3
  35. 11.4
  36. 12.0
  37. 12.1
  38. 12.2
  39. 12.3
  40. 12.4
  41. 13.0–13.2
  42. 13.3–14.5
  43. 15.0
  44. 15.1–16.0

History

alloca() appeared in AT&T UNIX v32.

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(Ns, '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. */

From base 16.0-CURRENT-20261027-61c70dc16a79 in FreeBSD 16.0-CURRENT (2026-10-27). Parsed by carta from the page's mdoc source.