brk(2)

Purpose

brk() sets the location of the program break: the first address after the process’s uninitialized data segment (BSS). Raising the break allocates memory; lowering it releases memory. It is the oldest Linux allocation primitive (since 1.0, inherited from 4.3BSD) and what classic malloc(3) arena implementations are built on. The man page says to avoid brk()/sbrk() in new code and use malloc(3). Domain index: memory hub.

Synopsis

#include <unistd.h>
int brk(void *addr);
void *sbrk(intptr_t increment);   /* libc only; no syscall */

Arguments and behavior

addr is the requested new break; it succeeds only when the value is reasonable, memory is available, and the process stays within its maximum data size (RLIMIT_DATA, setrlimit(2)). sbrk(increment) moves the break relatively; sbrk(0) reports the current break. On Linux sbrk() is a pure library function that calls the brk() syscall and keeps bookkeeping so it can return the old break.

Return value and errors

The raw syscall returns the new break on success and the current (unchanged) break on failure – there is no -1 value. The glibc wrapper compares the result against addr and produces the documented 0 on success / -1 with errno convention. Only ENOMEM is documented (limit exceeded, out of memory). sbrk() returns the previous break, or (void *) -1 with errno = ENOMEM on failure.

Flags and constants

None; one pointer argument, no flags. Conforming to 4.3BSD; SUSv1, marked LEGACY in SUSv2, removed in POSIX.1-2001.

Security and notes

• RLIMIT_DATA caps how far the break may grow (man page “maximum data size” check).
• Modern glibc malloc uses brk() only for the main arena and mmap(2) elsewhere, so a failed brk() does not mean the process is out of memory.
• Portability trap: code checking brk() == -1 on the raw syscall (or via syscall(2)) is wrong – only the glibc wrapper yields -1.
• The sbrk() argument type varies across systems (int, ssize_t, ptrdiff_t, intptr_t); glibc uses intptr_t.

References

• Cached man page: source-cache/man-pages-sparse/man2/brk.2 (RETURN VALUE, “C library/kernel differences”, CONFORMING TO).
• Numbers cross-checked against cached syscall_64.tbl (12), syscall_32.tbl (45), syscall_generic.tbl (214), syscall_arm.tbl (45); hub: overview.