OSF

NAME

brk, sbrk − Changes data segment size

LIBRARY

Standard C Library (libc.a)

SYNOPSIS

int brk(
char ∗addr ); int sbrk(
int incr );

PARAMETERS

addrPoints to the effective address of the maximum available data. 

incrSpecifies 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, 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 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 “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() function.  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 programs were designed using this memory model, and these programs typically use the brk() or sbrk() functions to increase or decrease their available memory.  OSF/1 provides a more flexible memory model and allows the use of discontiguous memory areas (see, for example, the mmap() function).  Therefore, references to areas above the break may be legitimate memory references which will not produce memory violations. 

RETURN VALUES

Upon successful completion, the brk() function returns a value of 0 (zero), and the sbrk function returns the old 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:

[ENOMEM]The requested change would allocate more space than allowed by the limit as returned by the getrlimit() function.  If the brk() function cannot allocate the requested memory, the following message is printed:

cmd: could not sbrk, return = n

Where cmd is the name of the command currently executing, and n is the internal kernel error code returned from the memory allocation routine, vm_allocate().  Note that this may occur if the requested break value would cause the data segment to collide with previously allocated memory (for example, memory obtained via the mmap() or vm_allocate() call). 

RELATED INFORMATION

Functions: exec(2), getrlimit(2), malloc(3), plock(2), mmap(2)