CHMEM(1)

NAME
chmem - change memory allocation

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

EXAMPLES

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 3 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).