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BRK(2)			 DragonFly System Calls Manual			BRK(2)


NAME

     brk, sbrk -- change data segment size (obsolete)


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.  brk() is no longer imple-
     mented by DragonFly and sbrk() has only limited functionality due to hav-
     ing to play nice with modern system calls such as mmap(2).

     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 break range is limited by the RLIMIT_DATA resource limit applied to
     the process.

     The brk() function sets the break to addr.  DragonFly no longer imple-
     ments this function.

     The sbrk() function raises the break by incr bytes, returning a pointer
     to the base of the new memory.

     In the traditional call, a negative incr lowers the break address by the
     specified number of bytes.  However, DragonFly no longer supports using
     this legacy function to lower the break address.  The reason is because
     the resource limit can be adjusted upward and downward at run-time and
     indirectly allow normal memory-mappings via mmap() to infiltrate the tra-
     ditional data area.  In addition, lowering the break address in this man-
     ner is not thread safe.  Any attempt to lower the break point will return
     (void *)-1 and set errno to EOPNOTSUPP.


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).

     The sbrk() function is thread-safe.  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 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 pointer if successful; other-
     wise the value (void *)-1 is returned and the global variable errno is
     set to indicate the error.


ERRORS

     brk() or sbrk() 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.

     [EOPNOTSUPP]	An attempt has been made to perform an action that is
			no longer supported by this function.


SEE ALSO

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


HISTORY

     A brk() function call 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).

DragonFly 5.5			 July 12, 1999			 DragonFly 5.5

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