#include <cstdio>
#include <unistd.h>
#include <sstream>
#include <vector>
#include <cerrno>
#include <cstring>
#include <sys/wait.h>

int main(int argc, char *argv[])
{
    fprintf(stderr,"main entered with pid: %d\n",getpid());
    if(argc==1) // parent has only binary name as argument (else logic to tell command lines apart)
    {
        pid_t pid = fork();
        if(pid) // parent
        {
            fprintf(stderr,"parent %d knows child at pid %d\n",(int)getpid(),(int)pid);
            fprintf(stderr,"parent's argv[0]: %s\n",argv[0]);
            waitpid(pid,nullptr,0); // wait for child to finish
        }
        else // child
        {
            pid = getppid();
            fprintf(stderr,"child %d knows parent at pid %d\n",(int)getpid(),(int)pid);
            std::ostringstream path_strm;
            path_strm << "/proc/" << pid << "/exe"; // path to parent's binary (symlink)
            // TODO: in advanced usage you may need to clean up certain
            // system resources here so that the child does not continue to inherit
            // them .. close open files, etc
            std::string path = path_strm.str();
            fprintf(stderr,"exec of %s\n", path.c_str());
            execvp(path.c_str(), std::vector<char *>({
                    // note: cast away const-ness
                    (char *)path.c_str(),
                    (char *)"arg1",
                    nullptr}).data()); // nullptr marks the end of the args
            // this line never reached unless there was an error, child starts at main() again
            // errno is a global used for error reporting by certain system/library calls
            fprintf(stderr,"execv error: %d (%s)\n", errno, strerror(errno));
        }
    }
    else // child after execv
    {
        fprintf(stderr,"child %d has argv:\n",(int)getpid());
        for(int i=0; i<argc; i++)
            fprintf(stderr,"argv[%d]: %s\n", i, argv[i]);
    }
    return 0;
}