brk / sbrk - glibc

Split from the chapter glibc: Virtual Memory Allocation And Paging into the per-function layout; see the method page and the glibc ledger for coverage status.

Declaration and gates

#include <unistd.h>

/* unistd.h:1063 gate: (__USE_XOPEN_EXTENDED && !__USE_XOPEN2K) || __USE_MISC */
int brk(void *addr);                 /* unistd.h:1070 */
void *sbrk(intptr_t delta);          /* unistd.h:1076 */

Semantics

The program break is the end of the heap’s data segment. brk(addr) sets the break to addr: zero on success, -1 with ENOMEM when the request would collide with another segment or exceed the data-size limit; an addr below the segment’s low end has no effect and is counted as success (brk.2; manual, Resizing the Data Segment). sbrk(delta) moves the break by delta and returns the previous break – i.e. the start of the freshly allocated region when delta > 0 – or (void *) -1 on error; sbrk(0) reports the current break without changing it.

glibc’s brk wrapper returns 0/-1, but the raw brk syscall returns the new break on success – a C library/kernel difference brk.2 spells out. The wrapper keeps the break in the __curbrk global. Raw syscall page: brk.

Gotchas and portability

• “Avoid using brk and sbrk” – malloc(3) is the supported interface; malloc itself uses these (or mmap), and mixing raw sbrk with malloc corrupts the allocator’s view of the heap (brk.2 NOTES).
• The sbrk argument type differs across systems (int, ssize_t, ptrdiff_t, intptr_t); glibc uses intptr_t (brk.2 NOTES).
• The break abstraction barely exists in modern mm implementations – malloc mostly mmaps – so portable code should not observe heap growth through sbrk(0).
• musl 1.2.5: both present (include/unistd.h lines 163-164, src/linux/brk.c); same semantics.

Errors

ENOMEM for both: the request would overlap another segment or exceed the process’s data storage limit (brk.2, manual).

References

• man2: brk.2 (covers sbrk; sbrk.2 is a .so link); man3 has no separate page
• glibc manual: memory.texi, node Resizing the Data Segment
• headers: /usr/include/unistd.h (lines 1063-1076)
• symbols: nm -D --with-symbol-versions libc.so.6