exec*

Objectives

1. Look at exec

Notes

• int execve(const char *pathname, char *const _Nullable argv[], char *const _Nullable envp[]);
  • Needs unistd.h
  • pathname must point to an executable file: binary or script.
    • If it is a script it must start with #!interpreter
  • Argv as discussed, but argv[0] should be the name of the program executed.
  • envp as discussed.
  • The man page for this is great and you should read it.
• execl, execlp, execle, execv, execvp, execvpe
  • Library functions to make exec easier
  • If tehre is an l:
    • argvs are passed as individual parameters.
    • The last arg must be NULL, (0), or nullptr.
  • If there is a v:
    • Args are passed as a vector.
    • The first should be the name of the program.
    • The last must contain a NULL, 0, or nullptr
  • If threre is an e:
    • A new environment is passed
    • Null terminated list of pointers.
    • Otherwise the current environment is used.
  • If there is a p: the file name is given not the full path, and the environment variable PATH is searched for the executable.
    • This is ignored if the name contains a /
    • The current environment, not the environment passed.
    • It will continue if it finds a non-executable version
    • If it doesn't know the interpreter it will use /bin/sh
• int system(const char * command)
  • Will spawn a child process (shell) and run the command.
  • The man page says execl("/bin/sh", "sh", "-c", command, (char *) NULL);
  • If the command is NULL, return value indicates if there is a shell available.
    • 0 no shell
    • not zero, shell
  • Otherwise no return, or -1 on error.
  • This is inefficient.
  • But a quick way to run a process