

CHMEM(1)                  Minix Programmer's Manual                   CHMEM(1)


NAME
     chmem - change memory allocation

SYNOPSIS
     chmem [+] [-] [=] amount file

EXAMPLES

     chmem =50000 a.out  # Give a.out 50K of stack space

     chmem -4000 a.out   # Reduce the stack space by 4000 bytes

     chmem +1000 file1   # Increase each stack by 1000 bytes

DESCRIPTION

     When a program is loaded into memory, it is allocated enough  memory  for
     the text and data+bss segments, plus an area for the stack.  Data segment
     growth using malloc , brk , or sbrk eats up stack space from the low end.
     The  amount  of  stack  space  to allocate is derived from a field in the
     executable program's file header.  If the combined stack and data segment
     growth exceeds the stack space allocated, the program will be terminated.

     It is therefore important to set the amount of stack space carefully.  If
     too  little is provided, the program may crash.  If too much is provided,
     memory will be wasted, and fewer programs will be able to fit  in  memory
     and  run  simultaneously.   MINIX  does  not swap, so that when memory is
     full, subsequent attempts to fork will fail.  The compiler sets the stack
     space  to  the  largest  possible value (for the Intel CPUs, 64K - text -
     data).  For many programs, this value is  far  too  large.   Nonrecursive
     programs  that  do  not call brk , sbrk , or malloc , and do not have any
     local arrays usually do not need more than 8K of stack space.

     The chmem command changes the value of the header field  that  determines
     the  stack  allocation,  and thus indirectly the total memory required to
     run the program.  The = option sets the stack size to a  specific  value;
     the  +  and  -  options  increment and decrement the current value by the
     indicated amount.  The old and new stack sizes are printed.

SEE ALSO
     install(1), brk(2).











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