sbrk(2) [osf1 man page]

brk(2)								System Calls Manual							    brk(2)

NAME

       brk, sbrk - Change space allocation

LIBRARY

       Standard C Library (libc.so, libc.a)

SYNOPSIS

       #include <unistd.h>

       int brk(      void *addr );

       void *sbrk(	intptr_t incr );

       The following function definitions do not conform to current standards and are supported only for backward compatibility:

       int brk(      char *addr );

       void *sbrk(	ssize_t incr );

       void *sbrk(	long incr );

STANDARDS

       Interfaces documented on this reference page conform to industry standards as follows:

       brk(), sbrk():  XPG4-UNIX

       Refer to the standards(5) reference page for more information about industry standards and associated tags.

PARAMETERS

       Points  to the effective address of the maximum available data.	Specifies the number of bytes to be added to the current break.  The value
       of incr may be positive or negative.

DESCRIPTION

       The brk() function sets the lowest data segment location not used by the program (called the break) to addr.

       In the alternate function sbrk(), incr more bytes are added to the program's data space, and a pointer to the start  of	the  new  area	is
       returned.

       When  a program begins execution with the execve() function, the break is set at the highest location defined by the program and data stor-
       age areas. Therefore, only programs with growing data areas should need to use sbrk().

       The current value of the program break is reliably returned by ``sbrk(0)''.  The getrlimit() function may be used to determine the  maximum
       permissible  size  of  the data segment. It is not possible to set the break beyond the value returned from a call to the getrlimit() func-
       tion.

       If the data segment was locked at the time of the brk() function, additional memory allocated to the data segment by  brk()  will  also	be
       locked.

NOTES

       Programmers  should  be aware that the concept of a current break is a historical remnant of earlier UNIX systems.  Many existing UNIX pro-
       grams were designed using this memory model, and these programs typically use the brk() or sbrk() functions to increase or  decrease  their
       available memory.

       The  use  of  the  mmap() function is now preferred because it can be used portably with all other memory allocation functions and with any
       function that uses other allocation functions.

RETURN VALUES

       Upon successful completion, the brk() function returns a value of 0 (zero), and the sbrk function returns the prior break value.  If either
       call fails, a value of -1 is returned and errno is set to indicate the error.

ERRORS

       If  the brk() or sbrk() function fails, no additional memory is allocated and errno may be set to the following value: Indicates an attempt
       to set a break value to less than the initial value at program startup.	The requested change would allocate more space than allowed by the
       limit as returned by the getrlimit() function.

RELATED INFORMATION

       Functions: exec(2), getrlimit(2), malloc(3), mmap(2), plock(2).

       Standards: standards(5) delim off

																	    brk(2)

Check Out this Related Man Page

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

NAME

     brk, sbrk -- change data segment size

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

     #include <unistd.h>

     int
     brk(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'').

     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 brk() function sets the break to addr.

     The sbrk() function raises the break by at least 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.

     sbrk() returns the prior address of the break.  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 the RLIMIT_DATA rlim_max value returned from a call to getrlimit(2), e.g.  ``etext + rlim.rlim_max''.  (see end(3) for the defi-
     nition of etext).

RETURN VALUES

     brk() returns 0 if successful; otherwise -1 with errno set to indicate why the allocation failed.

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

ERRORS

     brk() or sbrk() will fail and no additional memory will be allocated if one of the following are true:

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

     [ENOMEM]		The maximum possible size of a data segment (compiled into the system) was exceeded.

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

SEE ALSO

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

HISTORY

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

BUGS

     Note that mixing brk() and sbrk() with malloc(3), free(3), and similar functions may result in non-portable program behavior.  Caution is
     advised.

     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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sbrk(2) [osf1 man page]

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