brk man page on FreeBSD

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BRK(2)			    BSD System Calls Manual			BRK(2)

     brk, sbrk — change data segment size

     Standard C Library (libc, -lc)

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

     brk(const void *addr);

     void *
     sbrk(intptr_t incr);

     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 loca‐
     tion 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.

     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 permis‐
     sible 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 defi‐
     nition of etext).

     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

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

     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.

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

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

     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).

BSD				 July 12, 1999				   BSD

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