replace existing process with the new one

[memus]

Like in topic. How to run process and then quit existing one ?
I was trying to play around with QProcess by when I killed process that was running I was also killing the process that had been running before
This is example what i want to do but in c:

pid_t pid;
if ((pid = fork()) < 0)
  system("...."); //the new one
else if (pid != 0)
  exit (0);
 
setsid();

pid_t pid;
if ((pid = fork()) < 0)
  system("...."); //the new one
else if (pid != 0)
  exit (0);

setsid();

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Any ideas ?