In C language, a function is a block of code that performs a specific task. It can be called from other parts of a program to execute the task that it is designed to perform. Functions are used to modularize a program and make it easier to understand, maintain, and debug.

A function in C has the following syntax:

return_type function_name(parameter_list) {
// function body
// statements that define the task of the function
}

The return_type is the data type of the value that the function returns. If the function does not return a value, the return_type is void.

The function_name is the name of the function. It can be any valid identifier that follows the rules for naming variables in C.

The parameter_list is a list of parameters that the function takes as input. Each parameter is a variable that is used to pass a value to the function. The parameters are separated by commas, and the data type of each parameter must be specified. If the function does not take any parameters, the parameter_list is empty.

The function body is a block of code that defines the task of the function. It is enclosed in curly braces ‘{}’ and contains one or more statements that perform the task of the function.

Here is an example of a simple function in C that calculates the average of two numbers:

#include <stdio.h>

// function to calculate the average of two numbers
float average(float a, float b) {
// calculate and return the average
return (a + b) / 2;
}

int main() {
// call the function and print the result
printf("The average of 2 and 4 is %.2f\n", average(2, 4));

return 0;
}

In this example, the function average() calculates the average of two numbers ‘a’ and ‘b’ and returns the result as a float. The main function calls the average() function and passes the values 2 and 4 as arguments. The function calculates the average and returns it, and the main function prints the result using the printf function. The output of the program will be:

The average of 2 and 4 is 3.00

Functions are a powerful feature of C language that allow you to modularize a program, reuse code, and make it easier to understand, maintain, and debug. They are an essential part of any C program and are used extensively in most C programs.