Program Execution

• We have seen how to start a new process. This chapter shows how to change the code the process is executing.
• You should read chapters 24-27
• We will look at the exec* family of functions.
• execve

  •        #include 
           int execve(const char *filename, char *const argv[],
                      char *const envp[]);
    	  

  • filename is the path to an executable program or script.
  • There is no path name searching, either local or absolute
  • argv[] is a null terminated array of c style strings.
  • envp[] is a null terminated array of c style strings
  • Both of these must be null terminated.
  • There is no return on success, the program changes.
• There are some actions that change based on exec
  • Files opened with FD_CLOEXEC are closed.
  • Exit handlers are removed.
  • Directory streams are closed
  • (And some other things we have not yet discussed)
• Library forms of exec.

  •        #include 
    
           extern char **environ;
    
           int execl(const char *path, const char *arg, ...);
           int execlp(const char *file, const char *arg, ...);
           int execle(const char *path, const char *arg,
                      ..., char * const envp[]);
           int execv(const char *path, char *const argv[]);
           int execvp(const char *file, char *const argv[]);
           int execvpe(const char *file, char *const argv[],
                       char *const envp[]);
    	

  • There is a nice chart of 568
    • Some forms need a path to the executable
      • Other forms will search the PATH environment variable.
      • Forms with a p in the name do this.
    • Some forms want argv as a list of arguments
      • Others will take char * argv[],
      • Forms with a v in the name do this.
      • Other forms use an l in the name
    • Some forms use the user's environment variables.
      • Others take an argument char * envp[]
      • Forms with an e do this.
  • execve - args a vector, environment as a vector, needs a search path
  • execle - args as arguments, searches PATH, uses programs environment.
  • execvpe - args as a vector, envp as a vector, uses PATH to search
• The system command

  •        #include <stdlib.h>
           int system(const char *command);
    	

  • Uses fork and execl

               execl("/bin/sh", "sh". "-c", command, (char *) 0);