Copyright (c) 1980 Regents of the University of California.
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@(#)brk.2 6.1 (Berkeley) 05/09/85

C 4 brk, sbrk - change data segment size

#include <sys/types.h>

caddr_t brk(addr)
caddr_t addr;

caddr_t sbrk(incr)
int incr;

Brk sets the system's idea of the lowest data segment location not used by the program (called the break) to addr (rounded up to the next multiple of the system's page size). Locations greater than addr and below the stack pointer are not in the address space and will thus cause a memory violation if accessed.

In the alternate function sbrk , incr more bytes are added to the program's data space and a pointer to the start of the new area is returned.

When a program begins execution via execve the break is set at the highest location defined by the program and data storage areas. Ordinarily, therefore, only programs with growing data areas need to use sbrk .

The getrlimit (2) system call may be used to determine the maximum permissible size of the data segment; it will not be possible to set the break beyond the rlim_max value returned from a call to getrlimit , e.g. \*(lqetext + rlp\(->rlim_max.\*(rq (See end (3) for the definition of etext .)

Zero is returned if the brk could be set; -1 if the program requests more memory than the system limit. Sbrk returns -1 if the break could not be set. Sbrk will fail and no additional memory will be allocated if one of the following are true:

15 [ENOMEM] The limit, as set by setrlimit (2), was exceeded.

15 [ENOMEM] The maximum possible size of a data segment (compiled into the system) was exceeded.

15 [ENOMEM] Insufficient space existed in the swap area to support the expansion.

execve(2), getrlimit(2), malloc(3), end(3) Setting the break may fail due to a temporary lack of swap space. It is not possible to distinguish this from a failure caused by exceeding the maximum size of the data segment without consulting getrlimit .