BRK(2) System Calls BRK(2)


brk, sbrk - change the amount of space allocated for the calling
process's data segment


#include <unistd.h>

int brk(void *endds);

void *sbrk(intptr_t incr);


The brk() and sbrk() functions 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. Newly allocated
space is set to zero. If, however, the same memory space is reallocated
to the same process its contents are undefined.

When a program begins execution using execve() the break is set at the
highest location defined by the program and data storage areas.

The getrlimit(2) function may be used to determine the maximum
permissible size of the data segment; it is not possible to set the break
beyond the rlim_max value returned from a call to getrlimit(), that is to
say, "end + rlim.rlim_max." See end(3C).

The brk() function sets the break value to endds and changes the
allocated space accordingly.

The sbrk() function adds incr function bytes to the break value and
changes the allocated space accordingly. The incr function can be
negative, in which case the amount of allocated space is decreased.


Upon successful completion, brk() returns 0. Otherwise, it returns -1 and
sets errno to indicate the error.

Upon successful completion, sbrk() returns the prior break value.
Otherwise, it returns (void *)-1 and sets errno to indicate the error.


The brk() and sbrk() functions will fail and no additional memory will be
allocated if:

The data segment size limit as set by setrlimit() (see
getrlimit(2)) would be exceeded; the maximum possible size of a
data segment (compiled into the system) would be exceeded;
insufficient space exists in the swap area to support the
expansion; or the new break value would extend into an area of
the address space defined by some previously established
mapping (see mmap(2)).

Total amount of system memory available for private pages is
temporarily insufficient. This may occur even though the space
requested was less than the maximum data segment size (see


The behavior of brk() and sbrk() is unspecified if an application also
uses any other memory functions (such as malloc(3C), mmap(2), free(3C)).
The brk() and sbrk() functions have been used in specialized cases where
no other memory allocation function provided the same capability. The
use of mmap(2) is now preferred because it can be used portably with all
other memory allocation functions and with any function that uses other
allocation functions.

It is unspecified whether the pointer returned by sbrk() is aligned
suitably for any purpose.


See attributes(5) for descriptions of the following attributes:

|MT-Level | MT-Safe |


exec(2), getrlimit(2), mmap(2), shmop(2), ulimit(2), end(3C), free(3C),


The value of incr may be adjusted by the system before setting the new
break value. Upon successful completion, the implementation guarantees a
minimum of incr bytes will be added to the data segment if incr is a
positive value. If incr is a negative value, a maximum of incr bytes
will be removed from the data segment. This adjustment may not be
necessary for all machine architectures.

The value of the arguments to both brk() and sbrk() are rounded up for
alignment with eight-byte boundaries.


Setting the break may fail due to a temporary lack of swap space. It is
not possible to distinguish this from a failure caused by exceeding the
maximum size of the data segment without consulting getrlimit().

January 14, 1997 BRK(2)