2005-05-02 15:01:42 +02:00
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.TH CHMEM 1
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.SH NAME
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chmem \- change memory allocation
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.SH SYNOPSIS
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\fBchmem\fR [\fB+\fR]\fR [\fB\-\fR] [\fB=\fR] \fIamount file\fR
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.br
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.de FL
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.TP
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\\fB\\$1\\fR
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\\$2
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..
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.de EX
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.TP 20
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\\fB\\$1\\fR
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# \\$2
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..
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.SH EXAMPLES
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.EX "chmem =50000 a.out" "Give \fIa.out\fP 50K of stack space"
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.EX "chmem \-4000 a.out" "Reduce the stack space by 4000 bytes"
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.EX "chmem +1000 file1" "Increase each stack by 1000 bytes"
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.SH DESCRIPTION
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.PP
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When a program is loaded into memory, it is allocated enough memory
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for the text and data+bss segments, plus
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an area for the stack.
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Data segment growth using
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.I malloc ,
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.I brk ,
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or
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.I sbrk
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eats up stack space from the low end.
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The amount of stack space to allocate is derived
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from a field in the executable program's file header.
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If the combined stack and data segment growth exceeds the stack space
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allocated, the program will be terminated.
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.PP
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It is therefore important to set the amount of stack space carefully.
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If too little is provided, the program may crash.
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If too much is provided, memory will be wasted, and fewer programs will be able
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to fit in memory and run simultaneously.
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2005-08-22 14:56:02 +02:00
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\s-1MINIX 3\s-1
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2005-05-02 15:01:42 +02:00
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does not swap, so that when memory is full, subsequent attempts to fork will
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fail.
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The compiler sets the stack space
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to the largest possible value (for the Intel CPUs, 64K \- text \- data).
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For many programs, this value is far too large.
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Nonrecursive programs that do not call
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.I brk ,
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.I sbrk ,
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or
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.I malloc ,
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and do not have any local arrays usually do not need more than 8K of stack
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space.
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.PP
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The
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.I chmem
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command changes the value of the header field that determines the stack allocation, and
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thus indirectly the total memory required to run the program.
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The = option sets the stack size
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to a specific value; the + and \- options increment and decrement the
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current value by the indicated amount.
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The old and new stack sizes are printed.
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.SH "SEE ALSO"
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.BR install (1),
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.BR brk (2).
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