BRK(2) System Calls BRK(2)

NAME

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

SYNOPSIS

#include <unistd.h>

int brk(void *endds);


void *sbrk(intptr_t incr);

DESCRIPTION

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.

RETURN VALUES

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.

ERRORS

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

ENOMEM
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)).


EAGAIN
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 ulimit(2)).

USAGE

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.

ATTRIBUTES

See attributes(7) for descriptions of the following attributes:


+---------------+-----------------+
|ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+---------------+-----------------+
|MT-Level | MT-Safe |
+---------------+-----------------+

SEE ALSO

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

NOTES

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.

BUGS

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)