C Tutorial – Call by Value or Call by Reference

In this C language tutorial we will take a look at call by value and call by reference (also known as pass-by-value and pass-by-reference). These methods are different ways of passing (or calling) data to functions.

Call by Value

If data is passed by value, the data is copied from the variable used in for example main() to a variable used by the function. So if the data passed (that is stored in the function variable) is modified inside the function, the value is only changed in the variable used inside the function. Let’s take a look at a call by value example:

#include <stdio.h>

void call_by_value(int x) {
	printf("Inside call_by_value x = %d before adding 10.\n", x);
	x += 10;
	printf("Inside call_by_value x = %d after adding 10.\n", x);

int main() {
	int a=10;
	printf("a = %d before function call_by_value.\n", a);
	printf("a = %d after function call_by_value.\n", a);
	return 0;

The output of this call by value code example will look like this:

a = 10 before function call_by_value.
Inside call_by_value x = 10 before adding 10.
Inside call_by_value x = 20 after adding 10.
a = 10 after function call_by_value.

Ok, let’s take a look at what is happening in this call-by-value source code example. In the main() we create a integer that has the value of 10. We print some information at every stage, beginning by printing our variable a. Then function call_by_value is called and we input the variable a. This variable (a) is then copied to the function variable x. In the function we add 10 to x (and also call some print statements). Then when the next statement is called in main() the value of variable a is printed. We can see that the value of variable a isn’t changed by the call of the function call_by_value().

Call by Reference

If data is passed by reference, a pointer to the data is copied instead of the actual variable as is done in a call by value. Because a pointer is copied, if the value at that pointers address is changed in the function, the value is also changed in main(). Let’s take a look at a code example:

#include <stdio.h>

void call_by_reference(int *y) {
	printf("Inside call_by_reference y = %d before adding 10.\n", *y);
	(*y) += 10;
	printf("Inside call_by_reference y = %d after adding 10.\n", *y);

int main() {
	int b=10;
	printf("b = %d before function call_by_reference.\n", b);
	printf("b = %d after function call_by_reference.\n", b);
	return 0;

The output of this call by reference source code example will look like this:

b = 10 before function call_by_reference.
Inside call_by_reference y = 10 before adding 10.
Inside call_by_reference y = 20 after adding 10.
b = 20 after function call_by_reference.

Let’s explain what is happening in this source code example. We start with an integer b that has the value 10. The function call_by_reference() is called and the address of the variable b is passed to this function. Inside the function there is some before and after print statement done and there is 10 added to the value at the memory pointed by y. Therefore at the end of the function the value is 20. Then in main() we again print the variable b and as you can see the value is changed (as expected) to 20.

When to Use Call by Value and When to use Call by Reference?

One advantage of the call by reference method is that it is using pointers, so there is no doubling of the memory used by the variables (as with the copy of the call by value method). This is of course great, lowering the memory footprint is always a good thing. So why don’t we just make all the parameters call by reference?

There are two reasons why this is not a good idea and that you (the programmer) need to choose between call by value and call by reference. The reason are: side effects and privacy. Unwanted side effects are usually caused by inadvertently changes that are made to a call by reference parameter. Also in most cases you want the data to be private and that someone calling a function only be able to change if you want it. So it is better to use a call by value by default and only use call by reference if data changes are expected.

That is all for this C tutorial, where you (hopefully) have learned the difference between call-by-value and call-by-reference.

This entry was posted in C Tutorials. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed. Tweet This! Tweet This! or use to share this post with others.

There are currently 101 responses to “C Tutorial – Call by Value or Call by Reference”

Why not let us know what you think by adding your own comment!

  1. kaviya on June 20th, 2013:

    wow nice post.

  2. remo on June 27th, 2013:

    Okay, but what if i use “a=call_by_value(a)” in main and “return x” in last line of call_by_vale function?

  3. C Programming এর হাতে খড়ি পর্ব -৯ (Functions) | | বিডি পাঠশালা | on July 16th, 2013:

    […] C Tutorial – Call by Value or Call by Reference […]

  4. rajasekar on July 21st, 2013:

    thank u!!!!!!!!!!! its really simple

  5. swami on July 30th, 2013:

    Perfect yaaaarrr…

  6. Adil on August 3rd, 2013:

    A very good tutorial.. Simple examples to demonstrate different concepts.
    Thumbs up.

  7. parth on August 8th, 2013:

    good details in website…very help full..website…nice details

  8. deepak on August 12th, 2013:

    thanks sir for this tutorial, it is very helpful in completing my assignment.

  9. rama on August 18th, 2013:

    simple to understand the full concept…..
    thank u……

  10. Oviya on September 13th, 2013:

    Great job.
    Good example with explanation. Really nice.If its any example for array pointer in call by reference

  11. Oviya on September 13th, 2013:


  12. saurabh srivastav on September 13th, 2013:

    really gud one…:)

  13. abhishek on September 14th, 2013:

    thank you

  14. Rushikesh on September 15th, 2013:

    Thanks! Amazing 😀

  15. Annu on September 18th, 2013:

    It was really helpful

  16. vijayaraj v.s on September 19th, 2013:

    awesome explanation ……….

  17. siddu on September 20th, 2013:

    thanks…it s good tutorial…
    nice explanation..and very usefull…

  18. ganesh on September 24th, 2013:

    its realy good.now i was called to interview

  19. Kishan on October 4th, 2013:


  20. gadha on October 8th, 2013:

    it was really very helpful

  21. K.Revathy on October 9th, 2013:

    Thank you so………. much this Tutorial is very helpful…!

  22. suresh on October 19th, 2013:

    simple and very good tutorial and thank you very much. It was really helpful

  23. Bidyut on October 21st, 2013:

    Thanks. It is helpful for the new student.

  24. Sunil on October 22nd, 2013:

    Very well detailed.Keep it up.

  25. sujit kumar das on October 25th, 2013:

    thanxs it help me a lots of plz post this kind of tutorials a lot on the web

  26. ankur chavda on October 26th, 2013:

    very nice tutorial… it really cleared all my doubts regarding the above topic… the examples used were very helpful… great job!!

  27. subbu on November 12th, 2013:

    so helpful

  28. jaccob on November 20th, 2013:

    Thanks a lot….it was really helpfull for me…great job…

  29. vivek shinestar on November 25th, 2013:

    it is useful for us

  30. Siddharth Saxena on November 26th, 2013:

    Really nice tutorial….. very helpful and nicely explained….

  31. s karthik on December 4th, 2013:

    this is very good tutorial for c language thank u

  32. shrinidhi s uppoor on December 12th, 2013:

    it is really good … it helpped me a lot during exam time……

  33. Himanshu Mishra on December 14th, 2013:

    way in which examples are explained is very helpful for me.
    thank you.

  34. KIRUTHIKA on December 16th, 2013:

    so helpful to understand

  35. optimus on December 20th, 2013:

    Thanks! Helpful it is. 🙂

  36. sidhant on December 21st, 2013:

    this tutorial was quite helpful, at least it helped me to clear my 50%-60% confusions regarding call by value and call by reference. thank you

  37. HarishMaturi on January 1st, 2014:

    Nice Tutorial….Helpful…

  38. shyam on January 8th, 2014:

    Its very easy to understand…

  39. Danvir on January 9th, 2014:

    Nice explanation….

  40. DMV on January 15th, 2014:

    very very thnxxx
    nice tutorial

  41. Dani on January 18th, 2014:

    Simply thanx, helped a lot

  42. Shubham Choudhary on January 29th, 2014:

    thanx..thats really nice

  43. shree on February 11th, 2014:

    very nice…

  44. autkid on February 14th, 2014:


  45. Geetha on February 21st, 2014:

    its very useful & clear…thanks

  46. boviee on March 13th, 2014:

    tanx 4 d tutorial . . .
    simple and essesntial xplanation . . . . and useful 🙂

  47. avinash on March 13th, 2014:

    thanks a lot sir….

  48. Azizur Rahman on March 19th, 2014:

    Great.Explanation.Thanks a lot sir.

  49. Arun Sasidharan on March 28th, 2014:

    Thanks a lot. Useful for my exams.

  50. mohammed on April 4th, 2014:

    thanks alot that was great