NAME

       brk, sbrk — change data segment size

LIBRARY

       Standard C Library (libc, -lc)

SYNOPSIS

       #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.  They are deprecated and not present on  the  arm64  or  riscv  architectures.   The  mmap(2)
       interface should be used to allocate pages instead.

       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.   FreeBSD 11.0 introduced the arm64 and riscv
       architectures which do not support brk() or sbrk().

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).

       sbrk() is sometimes used to monitor heap use by calling with an argument of 0.  The result is unlikely to
       reflect actual utilization in combination with an mmap(2) based malloc.

       brk() and sbrk() are not thread-safe.

Debian                                            June 2, 2018                                            BRK(2)