Chapter 27, Program Execution, exec

• This is a avery short chapter.
• There is one main command, execve.
• There are half a dozen library versions (exec*)
• Shells are discussed briefly
• There is a cheater version (system)
• execve
  • int execve(const char *pathname, char *const argv[], char *const envp[]);
  • Needs unistd.h
  • path is the path to the executable or script which must be:
    • Absolute : ie start with /
    • Relative to cwd.
    • NOTE The PATH environment variable, or other such devices are NOT used in this call.
  • argv is a null terminated array of arguments.
    • By convention argv[0] contains the program name.
    • But this is not required.
    • And argv[argc] = NULL (or 0)
  • envp is a null terminated array of environment variables.
    • conventionally key=value.
  • On success, this program does not return
    • All most memory segments are replaced with new values.
      • text, data, stack, ...
    • Signal handlers are removed.
    • Shared memory is removed
    • exit handlers are removed.
    • And many other such items are removed.
  • Supprisingly, most file descritpors remain open.
    • If 0,1,2 are closed, the "system may open an unspecified file for each of these file descriptors."
• Take a look at myshell.cpp
• There are a number of library functions to support exec

  •        #include <unistd.h>
    
           extern char **environ;
    
           int execl(const char *pathname, const char *arg, ...
                           /*, (char *) NULL */);
           int execlp(const char *file, const char *arg, ...
                           /*, (char *) NULL */);
           int execle(const char *pathname, const char *arg, ...
                           /*, (char *) NULL, char *const envp[] */);
           int execv(const char *pathname, char *const argv[]);
           int execvp(const char *file, char *const argv[]);
           int execvpe(const char *file, char *const argv[], char *const envp[]);
           

  • Endings with p will search the values listed in the PATH environment variable.
  • Endings with l provide for arguments one per parameter, with the last one null.
  • Endings with e provide for calls passing an envornmnet variable array
    • Otherwise the environment sent on is the envrionment of the program.