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Read-Only Author
Ajit Pachore Posted
14-Aug-2004 16:44 GMT Toolset
C51 |  Global Variable. Ajit Pachore In my project I have two files. as follows
File A
int x;
main()
{
x=2;
}
File B
extern int x;
{
x=4;
}
When I compile both files independatly, there is no error. When I link both modules, there is no error. But when I execute this program, I am unable to assign x = 4. Always the value of x remains 2. But in file A I can assign any value to x. Why I can not assign any value to x in File B? I am trying this with Keil compiler. |
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Read-Only Author
Dan Henry Posted
14-Aug-2004 17:00 GMT Toolset
C51 | RE: Global Variable. Dan Henry File B has no function to execute. File A's main() simple assigns the value 2 to 'x' and exits. Try:
File A
extern void change_x(void);
int x;
main()
{
x = 2;
change_x();
}
File B
extern int x;
void change_x(void)
{
x = 4;
}
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Read-Only Author
Ajit Pachore Posted
14-Aug-2004 17:09 GMT Toolset
C51 | RE: Global Variable. Ajit Pachore what i mean by my question is
File A
int x;
main()
{
x= 2;
change_x();
}
File B
extern int x;
change_x()
{
x=4;
}
The expected answer is 4. But I am getting 2. |
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Read-Only Author
Dan Henry Posted
14-Aug-2004 17:38 GMT Toolset
C51 | RE: Global Variable. Dan Henry "The expected answer is 4. But I am getting 2."
And I am getting 4. What are you using for a debugger? Try using uVision's simulator, add 'x' to a watch window and step through the program. |
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Read-Only Author
Hans-Bernhard Broeker Posted
18-Aug-2004 09:21 GMT Toolset
C51 | RE: Global Variable. Hans-Bernhard Broeker Actually there can be no "expected answer" from your displayed code at all, because it doesn't output anything, anywhere. For what it's worth, the linker would be in its full right to just optimize 'x' away, since you never use the value for anything. |
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Read-Only Author
Andrew Neil Posted
18-Aug-2004 09:24 GMT Toolset
C51 | RE: Global Variable. Andrew Neil "the linker would be in its full right to just optimize 'x' away"
To be precise, the Compiler would perform this optimisation... |
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Read-Only Author
Hans-Bernhard Broeker Posted
18-Aug-2004 10:08 GMT Toolset
C51 | RE: Global Variable. Hans-Bernhard Broeker the Compiler would perform this optimisation
Not on a global variable like this it wouldn't. Or if it did, that'd be a bug.
The compiler simply doesn't have the information needed to justify eliminating any object of external linkage --- it doesn't know what other translation units might be doing with it. |
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Read-Only Author
Andrew Neil Posted
18-Aug-2004 10:20 GMT Toolset
C51 | RE: Global Variable. Andrew Neil "Not on a global variable like this it wouldn't."
Oh yeah...!
Obviously the scope of my reply was too limited (didn't read the whole thread).
Sorry. |
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Read-Only Author
Keil Support, Intl. Posted
25-Aug-2004 13:48 GMT Toolset
C51 | RE: Global Variable. Keil Support, Intl. The end of your program is undefined. You should have a while (1) loop in the 'main' function to avoid that your program jumps to nowhere. |
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