yhoazk/C

C Sample codes

C Stuff

Special Macros

__LINE__ __COUNTER__

    printf("__LINE__ Macro %d\n", __LINE__);
    printf("__COUNTER__ Macro %d\n", __COUNTER__);
    printf("__COUNTER__ Macro %d\n", __COUNTER__);
    printf("__COUNTER__ Macro %d\n", __COUNTER__)

Gives

__LINE__ Macro 45
__COUNTER__ Macro 0
__COUNTER__ Macro 1
__COUNTER__ Macro 2

Variadic Macros

Stringize the macro input argument with #, this adds quotes to the input argument.

#define stringize(x) #x
 stringize(fff)

Produces:

 "fff"

Concatenate Macro

Change the entry point of an application:

When, for some reason, the entry point function has to be renamed, the linker is responsible to assign the entry point. For example:

#include <stdio.h>

int entry()
{
  return 0;
}

And the compile/link command is now:

gcc -o entry.exe entry.c -Wl, -eentry -nostartfiles

The -Wl,-eentry passes arguments to the linker, and the linker takes a -e argument to set the entry function.

The command -nostrartfiles omits the ctr*.o files containing _start but stills allows to use a libc.

Another way to acomplish this is to use the binutils, this provides the command objcopy to make an arbitrary function the new entry point.

This requires a relocatable compilation, which is done with the gcc flag -c

This way:

gcc -c program.c -o program.exe

Then redefine the the entry to be main:

objcopy --redefine-sym entry=main program.o

Now use gcc to compile the new object file:

gcc program.o -o program

If the program already has a function called main, performa a separated objcopy invocation:

objcopy --redefine-sym oldmain=main program.o