NAME

       brk, sbrk — change data segment size

LIBRARY

       Standard C Library (libc, -lc)

SYNOPSIS

       #include <sys/types.h>
       #include <unistd.h>

       int
       brk(const void *addr);

       void *
       sbrk(intptr_t incr);

DESCRIPTION

       The  brk()  and  sbrk()  functions  are legacy interfaces from before the advent of modern virtual memory
       management.

       The brk() and 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 brk() function sets the break to addr.

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

NOTES

       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 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 getrlimit(RLIMIT_DATA, &rlim).  (See end(3) for the definition of etext).

RETURN VALUES

       The  brk()  function returns the value 0 if successful; otherwise the value -1 is returned and the global
       variable errno is set to indicate the error.

       The sbrk() function returns the prior break value if  successful;  otherwise  the  value  (void  *)-1  is
       returned and the global variable errno is set to indicate the error.

ERRORS

       The brk() and sbrk() functions will fail if:

       [EINVAL]           The requested break value was beyond the beginning of the data segment.

       [ENOMEM]           The data segment size limit, as set by setrlimit(2), was exceeded.

       [ENOMEM]           Insufficient  space  existed  in  the  swap  area to support the expansion of the data
                          segment.

SEE ALSO

       execve(2), getrlimit(2), mmap(2), end(3), free(3), malloc(3)

HISTORY

       The brk() function appeared in Version 7 AT&T UNIX.

BUGS

       Mixing brk() or 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).

Debian                                            July 12, 1999                                           BRK(2)