CODEWITHSUNDEEP
c
Beginner
Beginner

Reputation: 286

linking two C Programs

I'm combining two C programs using a header file.

first program 

#include<explore.h>

int main()
{
    int a =10;
    explore();
}

explore program

#include<stdio.h> 

int explore()
{
    // I want to use the value of a. Can I use it? How can sue it?
}

I want to use the value of a in explore() in explore program file.

Can I use it? How can use it, is possible?

Upvotes: 0

Views: 4086

Answers (5)

Elias Van Ootegem
Elias Van Ootegem

Reputation: 76395

Ok, first off. With the code as it stands now, you won't be able to use the int a anywhere, except in the main function, since it's a local variable to that function.
Either pass it as an argument, or declare it as a global (and an extern for that matter). Either way, I'd opt to pass it as an argument, if a will be changed by the function explore, you can do 1 of 2 things:

int explore( int val)
{
    //do stuff
    return new_val;
}
//call:
a = explore(a);//assigns return int to a

Or, if the returned int signals a status of some kind, pass a pointer to a. This may, if you're new to pointers, seem like a sure way to over-complicate things, but it's very common, and very useful (added note on why this is useful to the bottom):

int explore(int *val)
{
    *val += 123;//add 123 to val's value
    //this is NOT the same as val += 123, because that's shifting the pointer!
    return 0;
}
//call:
int return_val = explore(&a);//pass &[address-of]a

Now, linking two source files is easy, but you say you're using a header file, all well and good, but you don't seem to be including it anywhere, nor are you showing what it looks like. I suspect you're having trouble compiling your code... if so, show us what you've tried (how you're compiling the code), and what errors you're getting.
If you need a basic example of how to manually link 2 source files: Here's a previous answer of mine that shows, step by step, how to link 2 files

Why pointers?:
Many reasons, really. Here's just a few:

  • limit memory usage: If you're passing big structs by value all over the place (ie, copying the same data over and over), your code will be slow, and your stack might end up cluttered with the same value. (recursion induced stack overflow)
  • C can allocate heap memory, which can only be accessed through pointers, you can't turn a pointer variable into a non-pointer.
  • Return values are often ways to notify you of an error that may have occurred.

The last point is crucial. If you're passing a to explore for some complex computation, changing the value of a along the way. If, half way down the function, something goes pear-shaped, how do you notify the caller that the value of a is no longer reliable? Simple:

int complex_function( int *val)
{
    _Bool negative = *val > 0 ? false : true;
    //do all sorts of magic stuff
    //val wasn't negative, but is now, possible maxed int?
    if (negative == false && *val < 0) return -1;//notify using -1
    return 0;//usually, 0 means everything went well ok
}
//call like so:
if ((complex_function(&a)) != 0) exit( EXIT_FAILURE);//it's all gone wrong
//else, continue

Now that's where pointers are very useful. It doesn't require you to mess about with all too many temp variables, creating copies of values all over the place, and compare those to get to the same conclusion.

Upvotes: 1

parrowdice
parrowdice

Reputation: 1942

First of all, they are not two programs. It's a single program. You are writing a function. If you want to pass the value of a to that function, you need to pass it as a parameter:

#include<explore.h>
int main()
{
  int a =10;
  explore(a);
}

#include<stdio.h> 
int explore(int a)
{
  // you can now use a here
}

Upvotes: 0

Dayal rai
Dayal rai

Reputation: 6606

Change to int explore(int some_var) and then you are free to use some_var inside this function. In main you need to call as explore(a).

Upvotes: 0

Gianluca Ghettini
Gianluca Ghettini

Reputation: 11618

just pass the value (or pointer) of a to your explore function

pass by value (a copy of a)

#include<stdio.h> 
int explore(int value)
{

}

pass by reference (the pointer pointing at a)

#include<stdio.h> 
int explore(int *value)
{

}

Upvotes: 0

JeffRSon
JeffRSon

Reputation: 11166

Call explore with a as parameter.

Nevertheless, you don't really seem to have two "programs". You have two source files in one program. So another way to use a in both parts could be to define a in main.c and declare it as "extern" in explore.h. Keep in mind, though, that a is somewhat "global" to your program then.

Upvotes: 0

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