error: implicit declaration of function 'kill' is invalid in C99

The error message "implicit declaration of function 'kill' is invalid in C99" typically occurs when you try to use the kill function in C99 without including the appropriate header file. The kill function is defined in the <sys/types.h> and <signal.h> header files.

To fix this error, you need to include the appropriate header files at the beginning of your C++ program. Here's an example of how you can fix the error:

#include <sys/types.h>
#include <signal.h>

// Rest of your code...

int main() {
    // Your code here...
    return 0;
}

By including these header files, you are providing the necessary declarations and definitions for the kill function, allowing the compiler to recognize it when you use it in your code.

I hope this explanation helps! Let me know if you have any further questions.