Exec

• This is a family of functions
  • One system call
  • A bunch of library calls.
  • exec*
    • A v means the arguments are vector
    • A e means that the environment is specified (not the default environment)
    • A l means arguments are in long form
    • A p means the path is searched.
      • Unless the path begins with a /
  • execve is an exec where
    • the arguments are given in vector form,
    • the environment is specified (not inherited from the parent),
    • the path is not searched.
  • execve
    • This is the system call
    • int execve(const char *filename, char *const argv[], char *const envp[]);
      • The filename is the file to execute.
        • It must be either an absolute or relative path.
          • The environment variable PATH will not be searched.
          • And no ~ expansion.
        • It must be marked as executable.
          • This can be a binary executable (from a compiler)
          • Or a script for an interpreter.
        • If it is a binary, the appropriate loader is called.
        • If it is a script
          • The first line must be in the form

          • #!/path/to/interpreter [optional arguments]
            

      • argv is a null terminated list of arguments.
        • argv[0] by convention is the name of the program.
        • By convention it does not include the path.
        • The function basename might be helpful here
          • char *dirname(char *path);
          • Needs libgen.h
          • This might modify the string, so send in a copy
      • envp is a null terminated list of environment variables.
    • The function returns -1 on failure
    • It will never return on success.
  • You should read the book/manpage for things that happen on exec
    • For the most part, the general environment (not environment variables) is cleaned up.
    • But files are left open (unless otherwise specified)
  • look at
    • blabber.C
    • blabber.sh
    • runner.C
    • runner blabber a b c
    • runner blabber.sh a b c
    • runner /usr/bin/echo a b c
    • runner `pwd`/blabber a b c
  • There are a bunch of library equivalents
    • See man 3 exec.