NAME

brk, sbrk − change data segment space allocation

SYNOPSIS

int brk (endds)
char *endds;

char *sbrk (incr)
int incr;

DESCRIPTION

Brk and sbrk are used to change dynamically the amount of space allocated for the calling process’s data segment; see exec(2). The change is made by resetting the process’s break value and allocating the appropriate amount of space. The break value is the address of the first location beyond the end of the data segment. The amount of allocated space increases as the break value increases. The newly allocated space is set to zero.

Brk sets the break value to endds and changes the allocated space accordingly. 

Sbrk adds incr bytes to the break value and changes the allocated space accordingly.  Incr can be negative, in which case the amount of allocated space is decreased. 

Brk and sbrk will fail without making any change in the allocated space if one or more of the following are true:

Such a change would result in more space being allocated than is allowed by a system-imposed maximum (see ulimit(2)). ­[ENOMEM]

Such a change would result in the break value being greater than or equal to the start address of any attached shared memory segment (see shmop(2)).

RETURN VALUE

Upon successful completion, brk returns a value of 0 and sbrk returns the old break value.  Otherwise, a value of −1 is returned and errno is set to indicate the error. 

SEE ALSO

exec(2). 

ASSEMBLER

moveq#17,D0
movlendds,A0
trap#0

Carry bit cleared if the brk could be set; brk fails if the program requests more memory than the system limit or, on memory management CPUs, if too many segmentation registers would be required to implement the break. 

Version 3.6  —  December 20, 1987