Programming Forums > Application Development > C

aloksave · Sep 30th, 2005, 12:16 PM

Quote:

Originally Posted by Ringo

What development environment are you talking about dude ?

Anyway, i may have been incorrect - not completely, but a bit:

A compiler can allocate variables to the stack or to a register (if it's only used locally) - that is the compilers choice.

I am used to programming in an environment where memory is expensive (not so true now but where physical size is an issue (ie cell phones) its still holds) and efficient code is paramount - embedded software or thereabouts. Anyway, i wouldn't really be very happy if a compiler decided to put a temp variable on the stack - thats what registers are for. HOWEVER, i have no experience in other environments - so your words may be correct for your circumstances.

Just be careful - "All variables created in C are created on the stack". It's nothing to do with C - its to do with the compiler.

As i said before, sorry for hogging your thread and sorry for not providing an answer to your original question.

Hey Ringo thanks for the correction, yes i messed out in that post..
as you pointed out compiler will decide whether to allocate the variable on a stack or register( this is for optimization?? right...i mean fetching from registers is much faster than from memory so is it? )..we can declare variables as having register storage type which suggests the compiler to store variable in a register since it will be used more frequently,but its mere suggestion compiler decides on it..
but i do not understand how compiler will decide on whether to store in stack or in register..i mean what are criterions...can you elaborate on that..I will be very grateful if you do.
Thanks in advance

Ringo · Sep 30th, 2005, 4:19 PM

Here's a list ripped from: http://www.ece.utexas.edu/~valvano/assmbly/local.htm

Reasons why we place local variables on the stack include:

- dynamic allocation/release allows for reuse of memory
- limited scope of access provides for data protection
- only the program that created the local variable can access it
- since an interrupt will save registers and create its own stack frame, the code can be made reentrant.
- since absolute addressing is not used, the code is relocatable
- the number of variables is only limited by the size of the stack allocation
that can be much larger than using local variables in registers

Draw your own conclusions from that!

jim mcnamara · Sep 30th, 2005, 4:26 PM

The C standard from the getgo was to allocate auto variables in user stack space.
COBOL, FORTRAN and other languages didn't do that when C was originally designed.
It's part of the reason that native C code is faster and less memory intense than FORTRAN and COBOL were.

I learned C in 1978 on a Digitial PDP-8 with a 16K core. You had to mind your p's and q's memory-wise, so functions like alloca() really helped in memory management.

alloca - like malloc but on the stack of the calling functions, freed when the function reutrns.

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Sep 30th, 2005, 4:19 PM	#12
Ringo Newbie Join Date: Sep 2005 Location: Cambridge, UK Posts: 13 Rep Power: 0	Here's a list ripped from: http://www.ece.utexas.edu/~valvano/assmbly/local.htm Reasons why we place local variables on the stack include: - dynamic allocation/release allows for reuse of memory - limited scope of access provides for data protection - only the program that created the local variable can access it - since an interrupt will save registers and create its own stack frame, the code can be made reentrant. - since absolute addressing is not used, the code is relocatable - the number of variables is only limited by the size of the stack allocation that can be much larger than using local variables in registers Draw your own conclusions from that!

Sep 30th, 2005, 4:26 PM	#13
jim mcnamara Hobbyist Programmer Join Date: Jun 2005 Location: New Mexico Posts: 228 Rep Power: 4	The C standard from the getgo was to allocate auto variables in user stack space. COBOL, FORTRAN and other languages didn't do that when C was originally designed. It's part of the reason that native C code is faster and less memory intense than FORTRAN and COBOL were. I learned C in 1978 on a Digitial PDP-8 with a 16K core. You had to mind your p's and q's memory-wise, so functions like alloca() really helped in memory management. alloca - like malloc but on the stack of the calling functions, freed when the function reutrns.

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Programming Forums > Application Development > C

stack query