brk(2)

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 zero, in which case sbrk will return the process’ current break vlaue. Incr can also 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 a break value either greater than or equal to the starting address of an attached shared memory segment [see shmop(2)] or less than the starting address of data. [ENOMEM]

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

The process has its data region locked and the total amount of system memory available is temporarily insufficient [see plock(2)]. This may occur even though the space requested was less than the system-imposed maximum process size [see ulimit(2)]. [EAGAIN]

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.

As noted in the description, errno can be set to one of the following values depending on the error that occurs:

[EAGAIN]

[ENOMEM]

BUGS

brk and sbrk will not set the break value to an address below the value of end (see end(3C)).

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