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alloca(3) [osx man page]

ALLOCA(3)						   BSD Library Functions Manual 						 ALLOCA(3)

alloca -- memory allocator SYNOPSIS
#include <alloca.h> or #include <stdlib.h> void * alloca(size_t size); DESCRIPTION
The alloca() 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 32V AT&T UNIX. 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() BSD
September 5, 2006 BSD

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ALLOCA(3)						     Linux Programmer's Manual							 ALLOCA(3)

alloca - allocate memory that is automatically freed 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. CONFORMING TO
This function is not in POSIX.1-2001. There is evidence that the alloca() function appeared in 32V, PWB, PWB.2, 3BSD, and 4BSD. There is a man page for it in 4.3BSD. Linux uses the GNU version. NOTES
The alloca() function is machine- and compiler-dependent. For certain applications, its use can improve efficiency compared to the use of malloc(3) plus 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). Do not attempt to free(3) space allocated by alloca()! Notes on the GNU Version Normally, gcc(1) translates calls to alloca() with inlined code. This is not done when either the -ansi, -std=c89, -std=c99, or the -fno-builtin option is given (and the header <alloca.h> is not included). But beware! By default the glibc version of <stdlib.h> includes <alloca.h> and that contains the line: #define alloca(size) __builtin_alloca (size) with messy consequences if one has a private version of this function. The fact that the code is inlined means that it is impossible to take the address of this function, or to change its behavior by linking with a different library. The inlined code often consists of a single instruction adjusting the stack pointer, and does not check for stack overflow. Thus, there is no NULL error return. BUGS
There is no error indication if the stack frame cannot be extended. (However, after a failed allocation, the program is likely to receive a SIGSEGV signal if it attempts to access the unallocated 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) COLOPHON
This page is part of release 3.27 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at GNU
2008-01-24 ALLOCA(3)
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