A running program cannot change its source code, supposed that you mean something like #define PARAMETER 23
. You need variables instead of constants.
One primitive solution is this:
Invent a global variable per parameter, declare all of them in an extra header file and define all of them in an extra source file for better maintenance.
In the new header file undefine all parameter macros and redefine them to use the variable instead of the literals.
In the using source files, include the original header file, after that include your new header file.
Initialize the variables initially, and change parameters as you wish during run time. (Initialization could be done in the new source file.)
This solution avoids heavy editing the using source files and leaves the original header file intact.
Example:
/* original.h */
#define PARAMETER 23
int f(void); /* returns PARAMETER */
/* new.h */
#if defined(PARAMETER)
#undef PARAMETER
#define PARAMETER parameter
#endif
extern int parameter;
/* new.c */
#include "new.h" /* ensures that declarations and definitions match */
int parameter = 23;
/* original.c */
#include "original.h"
#include "new.h"
int f(void) {
return PARAMETER;
}
/* main.c */
#include <stdio.h>
#include "original.h"
#include "new.h"
int main(void) {
PARAMETER = 42;
printf("%d\n", f());
}
If you like to change the original source code, feel free to get rid of all this preprocessor stuff, and directly use variables instead of constants. But then you should re-think your design and provide parameters as arguments to existing or new functions. Global variables should be avoided, reasons are left as an exercise to you.