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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ATTRIBUTES | STANDARDS | HISTORY | NOTES | BUGS | SEE ALSO | COLOPHON | |
alloca(3) Library Functions Manual alloca(3)alloca - allocate memory that is automatically freed
Standard C library (libc, -lc)
#include <alloca.h>void *alloca(size_t size);
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.
The alloca() function returns a pointer to the beginning of the allocated space. If the allocation causes stack overflow, program behavior is undefined.
For an explanation of the terms used in this section, see attributes(7). ┌──────────────────────────────────────┬───────────────┬─────────┐ │ Interface │ Attribute │ Value │ ├──────────────────────────────────────┼───────────────┼─────────┤ │ alloca() │ Thread safety │ MT-Safe │ └──────────────────────────────────────┴───────────────┴─────────┘
None.
PWB, 32V.
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).
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.
brk(2), longjmp(3), malloc(3)
This page is part of the man-pages (Linux kernel and C library user-space interface documentation) project. Information about the project can be found at ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report for this manual page, see ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩. This page was obtained from the tarball man-pages-6.10.tar.gz fetched from ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on 2025-02-02. If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up- to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which is not part of the original manual page), send a mail to man-pages@man7.org Linux man-pages 6.10 2024-11-19 alloca(3)Pages that refer to this page: malloc(3), strdup(3)
HTML rendering created 2025-02-02 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. | |
