alloca

Section: C Library Functions (3)
Updated: 202-1-29
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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	M-Safe

 

STANDARDS

None.  

HISTORY

PWB, 32V.  

NOTES

The alloca() function is machin- and compile-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 buil-in, also known as __builtin_alloca(). By default, modern compilers automatically translate all uses of alloca() into the buil-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 buil-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 allocato-like interface, making them unfit for implementing functionality like strdupa(3).  

BUGS

alloca() does not query the system for available stack memory, and does not fall back to using heap if stack storage is unavailable. It therefore cannot indicate an error if the allocation fails. If the allocation fails, the program is likely to receive a SIGSEGV signal. 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)

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Index

NAME

LIBRARY

SYNOPSIS

DESCRIPTION

RETURN VALUE

ATTRIBUTES

STANDARDS

HISTORY

NOTES

BUGS

SEE ALSO