CODEWITHSUNDEEP
cembedded
user2988694
user2988694

Reputation: 89

storing a function at particular address in C

int (*FuncPtr)(int,int) = NULL;

int add(int a, int b)
{
return a+b;
}
int temp;

int main (void)
{
add = 0x100;
FuncPtr = add;
temp = (*FuncPtr)(10,20);
}

I am trying to copy a function to a particular address can kindly help us.

Upvotes: 1

Views: 456

Answers (2)

Alex
Alex

Reputation: 46

You can't copy a function to a particular address simply like this, but use gcc's attribute, you can do this in another way. first, the program should be like this:

#include <stdio.h>
int (*FuncPtr)(int, int) __attribute__((unused, section(".myown"))) = NULL;

int add(int a, int b)
{
        return a + b;
}

int temp;

int main (void)
{
        //add = 0x100;
        FuncPtr = add;
        temp = (*FuncPtr)(10,20);
}

then you must get the ld script, do this cmd in a shell:

ld --verbose > f.lds

the contents in the middle of "==================================================" is the script. add this just before the line "__bss_start = .;"

. = 0x90000000;
_myown_start = .;
  .myown           : { *(.myown) } = 0x90000000
  _myown_end = .;
  code_segment    : { *(code_segment) }

the address 0x90000000 is where you want to put the function pointer in, you can try other addresses, but that might not work. finally, compile your program like this:

gcc f.c -Wl,-Tf.lds

you can use objdump to see the result:

080483c2 <main>:
 80483c2:   55                      push   %ebp
 80483c3:   89 e5                   mov    %esp,%ebp
 80483c5:   83 e4 f0                and    $0xfffffff0,%esp
 80483c8:   83 ec 10                sub    $0x10,%esp
 80483cb:   c7 05 00 00 00 90 b4    movl   $0x80483b4,0x90000000
 80483d2:   83 04 08 
 80483d5:   a1 00 00 00 90          **mov    0x90000000,%eax**
 80483da:   c7 44 24 04 14 00 00    movl   $0x14,0x4(%esp)
 80483e1:   00 
 80483e2:   c7 04 24 0a 00 00 00    movl   $0xa,(%esp)
 80483e9:   ff d0                   call   *%eax
 80483eb:   a3 0c 00 00 90          mov    %eax,0x9000000c
 80483f0:   c9                      leave  
 80483f1:   c3                      ret

this is the example of putting a function pointer in a section, and you can use the attribute in the add function to put the whole function in the section like this:

int add(int a, int b) __attribute__((unused, section(".myown")));
int add(int a, int b)
{
        return a + b;
}

hope to be able to help you.

Upvotes: 3

suitianshi
suitianshi

Reputation: 3340

FuncPtr is a type not a variable, so you should do like this:

FuncPtr myFunc = add;
temp = (*myFunc)(1,1);   //actually this is the same as myFunc(1,1)

Upvotes: 0

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