Error with address for jmp in inline assembly - c

Question

What I do is
Get address of ExitProcess
Make space for opcode
Modify opcode in the space
Execute modified opcode by __asm__ ("jmp %%ecx"::"c"(opcode));
Here is my code:
#include <windows.h>
#include <stdio.h>
int main()
{
char addr[4];
*(int*)addr = GetProcAddress(GetModuleHandle("kernel32.dll"),"ExitProcess");
//push 0 == 0x6a 0x00
//call ExitProcess == 0xe8 0xd8 0x79 0xa2 0x75
char opcode[400] = {0x6a, 0x00, 0xe8,addr[0], addr[1],addr[2],addr[3]};
__asm__ ("jmp %%ecx" ::"c"(opcode));
//never be here
printf("never get here");
getchar();
return 0;
}
I expect program to exit normally, but the program terminates with a segmentation fault.
It seems that it jumps to somewhere, but not to the location I want it to jump.
How can I fix that?

Answer 1

Setting aside the odd thing you are trying to do...
Your problem is the opcode e8 is a relative jump. So you need to account for the address you are storing it at. Something like this maybe:
Update: Per taeyun, account for length of x.
#include <windows.h>
#include <stdio.h>
#pragma pack(1)
struct mfoo {
unsigned char x[3] = {0x6a, 0x00, 0xe8};
void *addr;
} foo;
int main()
{
unsigned char *a = (unsigned char *)GetProcAddress(GetModuleHandle("kernel32.dll"),"ExitProcess");
foo.addr = (void *)(a - sizeof(foo) - (unsigned char *)foo.x);
__asm__ ("jmp *%%ecx" ::"c"(&foo));
//never be here
printf("never get here");
getchar();
return 0;
}