brk, sbrk

Purpose

     Changes data segment space allocation.

Syntax

     int brk (endds)                           char *sbrk (incr)
     char *endds;                              int incr;
     Description

     The  brk and  sbrk  system calls  dynamically change  the
     amount of space allocated  for the calling process's data
     segment.   (For  information  about  data  segments,  see
     "exec:  execl, execv, execle, execve, execlp, execvp.")

     The change is made by resetting the process's break value
     and  allocating the  appropriate  amount  of space.   The
     break value is  the address of the  first location beyond
     the current end of the data segment.  The amount of allo-
     cated space increases as  the break value increases.  The
     newly  allocated space  is initialized  to 0.   The break
     value can  be automatically rounded  up to a  size appro-
     priate for the memory management architecture.

     The brk system call sets the  break value to the value of
     the  endds  parameter  and changes  the  allocated  space
     accordingly.

     The sbrk system  call adds to the break  value the number
     of bytes contained in the  incr parameter and changes the
     allocated space accordingly.  The incr parameter can be a
     negative number,  in which  case the amount  of allocated
     space is decreased.

     Return Value

     Upon successful completion, the brk system call returns a
     value  of 0,  and the  sbrk system  call returns  the old
     break value.  If the brk or the sbrk system calls fail, a
     value of -1 is returned and  errno is set to indicate the
     error.

     Diagnostics

     The brk and the sbrk  system calls fail and the allocated
     space remains unchanged  if one or more  of the following
     are true:

     ENOMEM     The requested change  will allocate more space
                than is  allowed by a  system-imposed maximum.
                (For information on the system-imposed maximum
                on memory space, see "ulimit.")

     ENOMEM     The requested change will  set the break value
                to a value greater than  or equal to the start
                address of any attached shared memory segment.
                (For information on  shared memory operations,
                see "shmat,"  "shmdt," and "shmget.")

     Related Information

     In  this book:   "exec:   execl,  execv, execle,  execve,
     execlp, execvp," "shmat,"  "shmdt," and  "ulimit."