sbrk(2)                        DG/UX 5.4R3.00                        sbrk(2)


NAME
       sbrk - change data segment space allocation

SYNOPSIS
       #include <unistd.h>

       void *sbrk(int increment);

   where:
       increment      The signed increment by which to change the data area
                      size

DESCRIPTION
       The sbrk() system call dynamically changes the amount of space
       allocated for the calling process's data segment; see exec(2).  The
       change is made by adding increment to the process's current break
       value and allocating or deallocating the appropriate amount of space.
       The break value is the address of the first byte beyond the end of
       the data segment.  The amount of allocated space increases as the
       break value increases.  If increment is positive, space is allocated,
       and any newly allocated pages will be initialized with zero bytes;
       that is, if these addresses are read before they are written, the
       contents will be zero.  If increment is negative, space is
       deallocated from the data segment.  The contents of the addresses
       from the new break value to the prior break value become undefined.

       There is a maximum possible break value for a process; this value may
       be obtained by calling the ulimit(2) function.  There is also a
       program-dependent minimum break value for a process; this minimum is
       greater than or equal to the address of the first byte in the data
       segment, and less than or equal to the program's initial break value.

       The sbrk() call will fail without making any change in the allocated
       space if an error occurs.

ACCESS CONTROL
       No access check is made.

RETURN VALUE
       Upon successful completion, sbrk() returns the previous break value.
       Otherwise, it returns the value (void *) -1, and sets errno to
       indicate an error.

DIAGNOSTICS
       Under the following conditions, sbrk() fails and sets errno to:

       ENOMEM         if the change would allocate more space than is
                      allowed by a system-imposed maximum (see ulimit(2)).

       ENOMEM         if the change would allocate more space than is
                      allowed by the current data resource limit (see
                      getrlimit(2)).

       ENOMEM         if the change would make the break value greater than



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sbrk(2)                        DG/UX 5.4R3.00                        sbrk(2)


                      or equal to the start address of an attached shared
                      memory segment (see shmat(2)).

       EFAULT         if the change would make the break value less than the
                      system-imposed minimum.

       EAGAIN         if the change would allocate more space than the
                      available physical memory and swap space.

       EAGAIN         if the MCLFUTURE memory locking option is in effect
                      for the calling process (see memcntl(2)), and the
                      system-imposed limit on space locked into physical
                      memory would be exceeded.

SEE ALSO
       exec(2), getrlimit(2), memcntl(2), ulimit(2).









































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