BRK(2)                    FreeBSD System Calls Manual                   BRK(2)

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.

FreeBSD 13.1-RELEASE-p6          June 2, 2018          FreeBSD 13.1-RELEASE-p6