How to send a variable between two C programs

Question

I have a C program, xyz.c which does some computations on a certain variable, say transferme. I want to transfer this variable/ pass this variable to another C program, say jkl.c.

I have been trying to do the following:

Fork the xyz.c and use execvp() to pass this transferme to an executable of jkl.c [Turns out execvp will accept only paths to a binary executable or a shell script.]

What is happening is,

1. execvp() does not transfer variables, it needs const char * to be the type of the argument.
2. passing the path to the binary executable does not work. [I tried a simple forking and execvp, without trying to pass the variable.]

Could anyone please:

1. Tell me how to use execvp() here in this case? where the binary executable is in the home folder, say the path is /home/user/jkl.o
2. Tell me how to pass this variable to the other program?

I tried looking into pipe, but I got more confused.

EDIT: I meant jkl.out

Answer 1

You have serveral ways to transfer data to another program. (pipin', files, and way more. I can edit my answer later and give a more standard guide about sending data between two C programs if it's necessary) Let's look execl() for your case.

We have to agree first that the program jkl.o will be started by your main program, and will receive as its start parameter a string containing the argument.

Let's look at the man of execl() :

The const char *arg and subsequent ellipses in the execl(), execlp(), and execle() functions can be thought of as arg0, arg1, ..., argn. Together they describe a list of one or more pointers to null-terminated strings that represent the argument list available to the executed program. The first argument, by convention, should point to the filename associated with the file being executed. The list of arguments must be terminated by a NULL pointer, and, since these are variadic functions, this pointer must be cast (char *) NULL.

OK. So you first have to give execl() your path, and then, in each row an argument. You pointed out that you need to fork, it's true. So let's do that, we fork, and then we send a string containing your transferme. I'm going to assume that this variable is a regular string.

 child_pid = fork() 
/* fork() == 0 for child process */

   if(child_pid == 0)
   {  
      /*executing  jkl.o */
      execl("/home/user/a.out", "a.out" , transferme);  /* with a.out being your second program, you can change that to whatever you'd want*/
   }
/* parent stuff*/

This should do the trick.

---

[update from comment:]

I forgot that argv[0] (ie the second argument of execl must be the name of the program itself.), so try

execl("/home/user/d.out", "d.out", transferme, (char*) NULL);