User defined functions in C

January 19, 2023

C User-defined functions

User-defined functions in C are functions that are created by the user to perform a specific task. These functions can be called from anywhere within the program, just like built-in functions. In C, you can define your own functions to perform specific tasks.

Use-defined functions in C

C functions

  • Function definitions can be placed anywhere in a C program, but it is common to define all functions at the beginning of the program before the main function. This allows the program to use the functions before they are defined.
  • To perform input and output operations on a file in C, you will need to use the file handling functions provided by the standard library. Here is an example program that demonstrates how to open a file for reading, read the contents of the file, and then close the file:

Run

#include <stdio.h>
int main() {
// Declare a file pointer
FILE *fp;

// Open the file for reading
fp = fopen("input.txt", "r");

// Check if the file was successfully opened
if (fp == NULL) {
printf("Error opening file\n");
return 1;
}

// Declare a character array to store the contents of the file
char buffer[100];

// Read the contents of the file into the buffer
fgets(buffer, 100, fp);

// Print the contents of the buffer
printf("%s", buffer);

// Close the file
fclose(fp);

return 0;
}

Output

Error opening file
C tutorial desk gif

Type conversion in C

Function in C

Call by reference

Call by value

Call by reference v/s call by value

User-Defined Functions

This program will open the file input.txt for reading, read the first 100 characters from the file into the buffer array, and then print the contents of the buffer array to the console.

To write to a file in C, you can use the fputs function. Here is an example program that demonstrates how to open a file for writing, write some text to the file, and then close the file:

Run
#include <stdio.h>
int addNumbers (int x, int y);	// function prototype

int
main ()
{
  int l1, l2, sum;

  printf ("Enters two numbers: ");
  scanf ("%d %d", &l1, &l2);

  sum = addNumbers (l1, l2);	// function call
  printf ("sum = %d", sum);

  return 0;
}

int
addNumbers (int x, int y)	// function definition 
{
  int result;
  result = x + y;
  return result;		// return statement
}

Output

Enters two numbers: sum = 40

This program will open the file output.txt for writing, write the string “Hello, World!” to the file, and then close the file. If the file does not exist, it will be created. If the file already exists, its contents will be overwritten.

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