brk(2)brk(2)NAME
brk, sbrk - Change space allocation
SYNOPSIS
#include <unistd.h>
int brk(
void *addr ); void *sbrk(
intptr_t incr );
The following function prototypes do not conform to current standards
and are supported only for backward compatibility:
int brk(
char *addr ); void *sbrk(
ssize_t incr ); void *sbrk(
long incr );
LIBRARY
Standard C Library (libc)
STANDARDS
Interfaces documented on this reference page conform to industry stan‐
dards as follows:
brk(), sbrk(): XSH4.2
Refer to the standards(5) reference page for more information about
industry standards and associated tags.
PARAMETERS
Points to the effective address of the maximum available data. Speci‐
fies the number of bytes to be added to the current break. The value of
incr may be positive or negative.
DESCRIPTION
The brk() function sets the lowest data segment location not used by
the program (called the break) to addr.
In the alternate function sbrk(), incr more bytes are added to the pro‐
gram's data space, and a pointer to the start of the new area is
returned.
When a program begins execution with the execve() function, the break
is set at the highest location defined by the program and data storage
areas. Therefore, only programs with growing data areas should need to
use sbrk().
The current value of the program break is reliably returned by the call
sbrk(0). The getrlimit() function may be used to determine the maximum
permissible size of the data segment. It is not possible to set the
break beyond the value returned from a call to the getrlimit() func‐
tion.
If the data segment was locked at the time of the brk() function, addi‐
tional memory allocated to the data segment by brk() will also be
locked.
NOTES
Programmers should be aware that the concept of a current break is a
historical remnant of earlier UNIX systems. Many existing UNIX programs
were designed using this memory model, and these programs typically use
the brk() or sbrk() functions to increase or decrease their available
memory.
The use of the mmap() function is now preferred because it can be used
portably with all other memory allocation functions and with any func‐
tion that uses other allocation functions.
RETURN VALUES
Upon successful completion, the brk() function returns a value of 0
(zero), and the sbrk() function returns the prior break value. If
either call fails, a value of -1 is returned and errno is set to indi‐
cate the error.
ERRORS
If the brk() or sbrk() function fails, no additional memory is allo‐
cated and errno is set to one of the following values: [Tru64
UNIX] Indicates an attempt to set a break value to less than the ini‐
tial value at program startup. The requested change would allocate
more space than allowed by the limit as returned by the getrlimit()
function.
SEE ALSO
Functions: exec(2), getrlimit(2), mmap(2), plock(2), malloc(3)
Standards: standards(5)brk(2)
