How to write a a self replacing/updating binary?

Question

I am trying to write a C program which may lookup a url and incase a new version of it is avaiable it should be able to update itself.

The method i have tried:

1. Forkout a new process to Download the new binary say BINARY.tmp, code i am using to forkout the is:

   int
   forkout_cmd(char *cmdstr) {
     pid_t pid;
     char *cmd[4];
   
     cmd[0] = "/bin/bash";
     cmd[1] = "-c";
     cmd[2] = cmdstr;
     cmd[3] = NULL;
   
     pid = vfork();
     if( pid == -1 ) {
       logmsg("Forking for upgradation failed.");
       return -1;
     }else if( pid == 0 ){
       /* we are in child process */
       execvp(cmd[0], cmd);
       logmsg("execl failed while executing upgradation job.");
     }else{
       /* need not to wait for the child to complete. */
       wait(NULL);
     }
       return 0;
   }
   

2. The new process tries to overwrite the original BINARY

   for example you may consider the routine which forks out may be doing:

   forkout_cmd("wget -O BINARY.tmp https://someurl.com/BINARY_LATEST; /bin/mv -f BINARY.tmp BINARY");
   

But, the overwriting fails since the original binary is still in execution and hence busy on disk, can somebody provide me some suggestions here to overcome this problem.

Thanks in advance.

Answer 1

Rename the currently running binary to something else, write the new binary, run it, then delete the renamed binary later.