sbrk(2) change data segment size

Other Alias



Lb libc


In sys/types.h In unistd.h Ft int Fn brk const void *addr Ft void * Fn sbrk intptr_t incr


Bf -symbolic The Fn brk and Fn sbrk functions are legacy interfaces from before the advent of modern virtual memory management. Ef

The Fn brk and Fn sbrk functions are used to change the amount of memory allocated in a process's data segment. They do this by moving the location of the ``break'' The break is the first address after the end of the process's uninitialized data segment (also known as the ``BSS )''

The Fn brk function sets the break to Fa addr .

The Fn sbrk function raises the break by Fa incr bytes, thus allocating at least Fa incr bytes of new memory in the data segment. If Fa incr is negative, the break is lowered by Fa incr bytes.


While the actual process data segment size maintained by the kernel will only grow or shrink in page sizes, these functions allow setting the break to unaligned values (i.e., it may point to any address inside the last page of the data segment).

The current value of the program break may be determined by calling Fn sbrk 0 . See also end(3).

The getrlimit(2) system call may be used to determine the maximum permissible size of the data segment. It will not be possible to set the break beyond ``etext + rlim.rlim_max '' where the rlim.rlim_max value is returned from a call to Fn getrlimit RLIMIT_DATA &rlim . (See end(3) for the definition of etext )


Rv -std brk

The Fn sbrk function returns the prior break value if successful; otherwise the value Po Vt void * Pc Ns -1 is returned and the global variable errno is set to indicate the error.


The Fn brk and Fn sbrk functions will fail if:

The requested break value was beyond the beginning of the data segment.
The data segment size limit, as set by setrlimit(2), was exceeded.
Insufficient space existed in the swap area to support the expansion of the data segment.


The Fn brk function appeared in AT&T System v7 .


Mixing Fn brk or Fn sbrk with malloc(3), free(3), or similar functions will result in non-portable program behavior.

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(2).