Exec family - A quick note
unix
Published: 2018-11-11

This is just for personal future reference:

There is a family of exec() library functions, all of which have slightly different characteristics:

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[]);
  • If p in the name: accepts a program name and it searches for the program in $PATH env var
  • If l or v in the name: the program name must be given in full(path).
  • If e in the name: function accepts additional array of env vars.

Usage Notes

execl("/bin/echo","echo", "Hello World", NULL);

When a program is invoked from the shell, the shell sets the first element of the argument list argv[0]) to the name of the program, the second element of the argument list (argv [1]) to the first command-line argument, and so on. When you use an exec function in your programs, you, too, should pass the name of the function as the first element of the argument list.

This is the reason we are passing "echo" as the second argument in execl()

Additional information:

All of these are user space frontends to execve() system call.

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

execve() executes the program pointed to by filename. This causes the program that is currently being run by the calling process to be replaced with a new program, with newly initialized stack, heap, and (initialized and uninitialized) data segments.

  • see man 2 execve for the systemcall. Also see What does the “ve” in “execve” mean?
  • see man 3 exec for library function info on exec family
  • see man 1 exec for info on the exec shell utility (which i did not cover in this blog, but just added for fun)

Extra resources