| 1 | .TH CHMEM 1
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| 2 | .SH NAME
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| 3 | chmem \- change memory allocation
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| 4 | .SH SYNOPSIS
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| 5 | \fBchmem\fR [\fB+\fR]\fR [\fB\-\fR] [\fB=\fR] \fIamount file\fR
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| 6 | .br
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| 7 | .de FL
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| 8 | .TP
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| 9 | \\fB\\$1\\fR
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| 10 | \\$2
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| 11 | ..
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| 12 | .de EX
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| 13 | .TP 20
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| 14 | \\fB\\$1\\fR
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| 15 | # \\$2
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| 16 | ..
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| 17 | .SH EXAMPLES
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| 18 | .EX "chmem =50000 a.out" "Give \fIa.out\fP 50K of stack space"
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| 19 | .EX "chmem \-4000 a.out" "Reduce the stack space by 4000 bytes"
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| 20 | .EX "chmem +1000 file1" "Increase each stack by 1000 bytes"
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| 21 | .SH DESCRIPTION
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| 22 | .PP
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| 23 | When a program is loaded into memory, it is allocated enough memory
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| 24 | for the text and data+bss segments, plus
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| 25 | an area for the stack.
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| 26 | Data segment growth using
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| 27 | .I malloc ,
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| 28 | .I brk ,
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| 29 | or
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| 30 | .I sbrk
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| 31 | eats up stack space from the low end.
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| 32 | The amount of stack space to allocate is derived
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| 33 | from a field in the executable program's file header.
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| 34 | If the combined stack and data segment growth exceeds the stack space
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| 35 | allocated, the program will be terminated.
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| 36 | .PP
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| 37 | It is therefore important to set the amount of stack space carefully.
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| 38 | If too little is provided, the program may crash.
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| 39 | If too much is provided, memory will be wasted, and fewer programs will be able
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| 40 | to fit in memory and run simultaneously.
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| 41 | \s-1MINIX 3\s-1
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| 42 | does not swap, so that when memory is full, subsequent attempts to fork will
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| 43 | fail.
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| 44 | The compiler sets the stack space
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| 45 | to the largest possible value (for the Intel CPUs, 64K \- text \- data).
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| 46 | For many programs, this value is far too large.
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| 47 | Nonrecursive programs that do not call
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| 48 | .I brk ,
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| 49 | .I sbrk ,
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| 50 | or
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| 51 | .I malloc ,
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| 52 | and do not have any local arrays usually do not need more than 8K of stack
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| 53 | space.
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| 54 | .PP
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| 55 | The
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| 56 | .I chmem
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| 57 | command changes the value of the header field that determines the stack allocation, and
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| 58 | thus indirectly the total memory required to run the program.
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| 59 | The = option sets the stack size
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| 60 | to a specific value; the + and \- options increment and decrement the
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| 61 | current value by the indicated amount.
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| 62 | The old and new stack sizes are printed.
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| 63 | .SH "SEE ALSO"
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| 64 | .BR install (1),
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| 65 | .BR brk (2).
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