Allocates memory on the stack.
| #include <malloc.h> | Required only for function declarations |
void *_alloca( size_t size);
| size | Bytes to be allocated from stack |
The _alloca routine allocates size bytes from the program's stack. The allocated space is automatically freed when the calling function is exited.
Observe the following restrictions when using _alloca:
When you compile with optimization on (either by default or by using one of the /O options), the stack pointer may not be restored properly in functions that have no local variables and that also reference the _alloca function. (This restriction does not apply to DOS32X.) The following program demonstrates the problem:
/* Compile with CL /AM /Ox /Fc */
#include <malloc.h>
void main( void )
{
func( 10 );
}
void func( register int i )
{
_alloca( i );
}
To ensure that the stack pointer is properly restored, make sure that any function referencing _alloca declares at least one local variable.
The pointer value returned by _alloca should never be passed as an argument to free.
The _alloca function should never be used in an expression that is an argument to a function.
The _alloca routine returns a void pointer to the allocated space, which is guaranteed to be suitably aligned for storage of any type of object. To get a pointer to a type other than char, use a type cast on the return value. The return value is NULL if the space cannot be allocated.
Standards:UNIX
16-Bit:DOS
32-Bit:DOS32X
Use _alloca for compatibility with ANSI naming conventions of non-ANSI functions. Use alloca and link with OLDNAMES.LIB for UNIX compatibility.
calloc functions, malloc functions, realloc functions
/* ALLOCA.C: This program checks the stack space available before
* and after using the _alloca function to allocate space on the stack.
*/
#include <malloc.h>
#include <stdio.h>
void main( void )
{
char *buffer;
printf( "Bytes available on stack: %u\n", _stackavail() );
/* Allocate memory for string. */
buffer = _alloca( 120 * sizeof( char ) );
printf( "The _alloca function just allocated" );
printf( " memory from the program stack.\n" );
printf( "Enter a string: " );
gets( buffer );
printf( "\"%s\" was stored in the program stack.\n", buffer );
printf( "Bytes available on stack: %u\n", _stackavail() );
}
Bytes available on stack: 1744
The _alloca function just allocated memory from the program stack.
Enter a string: Store this on the stack.
"Store this on the stack." was stored in the program stack.
Bytes available on stack: 1614