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Manual page for BRK(2)

brk, sbrk - change data segment size

SYNOPSIS

#include <sys/types.h>

int brk(addr) caddr_t addr;

caddr_t sbrk(incr) int incr;

DESCRIPTION

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).

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 using execve() the break is set at the highest location defined by the program and data storage areas.

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(), that is to say, ``etext + rlim.rlim_max.'' (See end.3 for the definition of etext().)

RETURN VALUES

brk() returns:

0: on success.
-1: on failure and sets errno to indicate the error.

sbrk() returns the old break value on success. On failure, it returns (caddr_t) -1 and sets errno to indicate the error.

ERRORS

brk() and sbrk() will fail and no additional memory will be allocated if one of the following occurs:

ENOMEM: The data segment size limit, as set by setrlimit() (see getrlimit.2 would be exceeded.
: The maximum possible size of a data segment (compiled into the system) would be exceeded.
: Insufficient space exists in the swap area to support the expansion.
: Out of address space; the new break value would extend into an area of the address space defined by some previously established mapping (see mmap.2

WARNINGS

Programs combining the brk() and sbrk() system calls and malloc() will not work. Many library routines use malloc() internally, so use brk() and sbrk() only when you know that malloc() definitely will not be used by any library routine.

BUGS

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().