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Assign content to a function pointer

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Join Date: Jul 2004

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Reputation: Richard Wong is an unknown quantity at this point

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Richard Wong Richard Wong is offline

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Assign content to a function pointer

Jul 1st, 2004

Goal: I would like to assign the content of a known function to a function pointer in LINUX environment.
Problem: Get segmentation fault when running the function pointer after assignment.

Procedure:
1). declare a funtion pointer.
2). Allocate memory space to the function pointer using the function' malloc'
3). Copy the content of a known function to the function pointer using
'memcpy'. (the interface of the known function is the same as the function pointer).
4). check the memory content of the function pointer using 'memcmp'.
Result: two memory contents match.
5). Run the function pointer
Result: segmentation fault.

Thanks for your attention.

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Join Date: Apr 2004

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Reputation: Dave Sinkula has a brilliant future

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Dave Sinkula Dave Sinkula is offline

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long time no c

Re: Assign content to a function pointer

Jul 1st, 2004

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Originally Posted by Richard Wong

Goal: I would like to assign the content of a known function to a function pointer in LINUX environment.
Problem: Get segmentation fault when running the function pointer after assignment.

Procedure:
1). declare a funtion pointer.
2). Allocate memory space to the function pointer using the function' malloc'
3). Copy the content of a known function to the function pointer using
'memcpy'. (the interface of the known function is the same as the function pointer).
4). check the memory content of the function pointer using 'memcmp'.
Result: two memory contents match.
5). Run the function pointer
Result: segmentation fault.

Why not just this:

Declare function pointer.
Point to the function (assign the pointer).
Run the function pointer.

"One of the methods used by statists to destroy capitalism consists in establishing controls that tie a given industry hand and foot, making it unable to solve its problems, then declaring that freedom has failed and stronger controls are necessary." --Ayn Rand

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Join Date: Jun 2004

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Reputation: Chainsaw is an unknown quantity at this point

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Chainsaw

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Re: Assign content to a function pointer

Jul 1st, 2004

Right, the problem with allocating ram and MOVING code into the ram is that jumps and other address references 'fixed up' by the linker are no longer correct.

Here's what Dave was saying:

typedef int (*MyFunctionPtr)(int foo);

int SomeFunction( int foo )
{
return foo;
}

MyFunctionPtr f;

f = SomeFunction; // points to the function SomeFunction()

printf("%d\n",f(4)); // prints '4'