brk(2) System Calls Manual brk(2)
NAME
brk, sbrk - Change space allocation
LIBRARY
Standard C Library (libc.so, libc.a)
SYNOPSIS
#include <unistd.h>
int brk( void *addr );
void *sbrk( intptr_t incr );
The following function definitions 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 );
STANDARDS
Interfaces documented on this reference page conform to industry standards as follows:
brk(), sbrk(): XPG4-UNIX
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. Specifies 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 program'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 stor-
age areas. Therefore, only programs with growing data areas should need to use sbrk().
The current value of the program break is reliably returned by ``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, additional 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 pro-
grams 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
function 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 indicate the error.
ERRORS
If the brk() or sbrk() function fails, no additional memory is allocated and errno may be set to the following value: Indicates an attempt
to set a break value to less than the initial value at program startup. The requested change would allocate more space than allowed by the
limit as returned by the getrlimit() function.
RELATED INFORMATION
Functions: exec(2), getrlimit(2), malloc(3), mmap(2), plock(2).
Standards: standards(5) delim off
brk(2)