System Calls brk(2)
brk, sbrk - change the amount of space allocated for the
calling process's data segment
int brk(void *endds);
void *sbrk(intptr_t incr);
The brk() and sbrk() functions are used to change dynami-
cally 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, how-
ever, the same memory space is reallocated to the same pro-
cess 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
The getrlimit(2) function may be used to determine the max-
imum permissible size of the data segment; it is not possi-
ble 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 allo-
cated 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:
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System Calls brk(2)
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
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)).
The behavior of brk() and sbrk() is unspecified if an appli-
cation also uses any other memory functions (such as
malloc(3C), mmap(2), free(3C)). The brk() and sbrk() func-
tions 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 attri-
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
| MT-Level | MT-Safe |
exec(2), getrlimit(2), mmap(2), shmop(2), ulimit(2),
end(3C), free(3C), malloc(3C)
The value of incr may be adjusted by the system before set-
ting 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.
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System Calls brk(2)
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().
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