alloca

alloca(3)                   Library Functions Manual                   alloca(3)

NAME
       alloca - allocate memory that is automatically freed

LIBRARY
       Standard C library (libc, -lc)

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 │
       └─────────────────────────────────────────────┴───────────────┴─────────┘

STANDARDS
       None.

HISTORY
       PWB, 32V.

NOTES
       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 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).

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

       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.  However, they do not
       port to standard C++, and, being variables, live in their block scope and
       don't have an allocator-like interface, making them unfit for
       implementing functionality like strdupa(3).

BUGS
       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 unavailable 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)

Linux man-pages 6.15               2025-05-17                          alloca(3)