SBRK(S)                  XENIX System V                   SBRK(S)



     Name
          sbrk, brk - Changes data segment space allocation.

     Syntax
          char *sbrk (incr)
          int incr;

          int brk (addr)
          char *addr;

     Description
          sbrk and brk are used to dynamically change the amount of
          space allocated for the data segment of the calling process;
          see exec(S).  The change is made by resetting the break
          value of the process.  The break value is the address of the
          first location beyond the end of the data segment.  The
          amount of allocated space increases as the break value
          increases.

          sbrk adds incr bytes to the break value and changes the
          allocated space accordingly.  incr can be negative, in which
          case the amount of allocated space is decreased.

          In 286 large model programs, if incr is greater than the
          number of unallocated bytes remaining in the current data
          segment, sbrk automatically allocates all the requested
          bytes in a new data segment.  This guarantees that the
          requested bytes will reside entirely in one segment.  If
          incr is negative and its absolute value is equal to the
          number of allocated bytes in the current data segment, the
          segment is automatically freed for other use.  If incr is
          negative and its absolute value is greater than the number
          of allocated bytes in the current segment, the segment is
          freed, and the additional bytes are removed from the
          previous data segment. (The previous data segment contains
          space allocated by the most recent sbrk that did not affect
          the current segment.)

          sbrk will fail without making any change in the allocated
          space if:

               A change would result in more space being allocated
               than is allowed by a system-imposed maximum (see
               ulimit(S)).  [ENOMEM]

               An attempt is made to remove more space than has
               actually been allocated.

               An attempt to remove space causes the new break value
               to be less than the original break value. The original
               break value is always taken to be break value when
               process execution began plus any shared data bytes that



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     SBRK(S)                  XENIX System V                   SBRK(S)



               have been allocated since that time.

          brk sets the the current break value to addr, and changes
          the allocated space accordingly. brk fails if the address
          references a data segment that does not exist, or if it
          references beyond the maximum possible size of the current
          data segment.

     Return Value
          Upon successful completion, sbrk returns a pointer to the
          beginning of the allocated space. brk returns 0 on
          successful completion.  Otherwise, a value of -1 is returned
          and errno is set to indicate the error.  In large model
          programs, if sbrk allocates a new data segment, the return
          value is the starting address of that new segment.

     See Also
          exec(S)

     Notes
          In 286 large model programs, the call ``sbrk(0)'' does not
          necessarily return the starting address of the next sbrk
          call.  In particular, if the next call causes an additional
          data segment to be allocated, the break values returned by
          these two calls will not be the same.  The return value from
          ``sbrk(0)'' should only be regarded as a marker for the
          original end of data.




























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