C Functions

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C Functions

A function is a group of statements that together perform a task. Every C program has at least one function, the function main(), and all the most trivial programs can define additional functions.

You can divide up your code into separate functions. How you divide up your code among different functions is up to you, but logically, the division is such that each function performs a specific task.

A function declaration tells the compiler about a function's name, return type, and parameters. A function definition provides the actual body of the function.

The C standard library provides numerous built-in functions that your program can call. For example, strcat() to concatenate two strings, memcpy() to copy one memory location to another location, and many more.

Functions are also known as methods, subroutines, or procedures.

Defining a Function

The general form of a function definition in C programming language is as follows:

return_type function_name( parameter list ) {
   body of the function
}

A function definition in C programming consists of a function header and a function body. Here are all the parts of a function:

Return Type: A function may return a value. The return_type is the data type of the value the function returns. Some functions perform the desired operations without returning a value. In this case, the return_type is the keyword void.
Function Name: This is the actual name of the function. The function name and the parameter list together constitute the function signature.
Parameters: A parameter is like a placeholder. When a function is invoked, you pass a value to the parameter. This value is referred to as actual parameter or argument. The parameter list refers to the type, order, and number of the parameters of a function. Parameters are optional; that is, a function may contain no parameters.
Function Body: The function body contains a collection of statements that define what the function does.

Example

Given below is the source code for a function called max(). This function takes two parameters num1 and num2 and returns the maximum value between the two:

/* function returning the max between two numbers */
int max(int num1, int num2) {
   /* local variable declaration */
   int result;
 
   if (num1 > num2)
      result = num1;
   else
      result = num2;
 
   return result; 
}

Function Declarations

A function declaration tells the compiler about a function name and how to call the function. The actual body of the function can be defined separately.

A function declaration has the following parts:

return_type function_name( parameter list );

For the above defined function max(), following is the function declaration:

int max(int num1, int num2);

Parameter names are not important in function declaration only their type is required, so following is also valid declaration:

int max(int, int);

Function declaration is required when you define a function in one source file and you call that function in another file. In such case, you should declare the function at the top of the file calling the function.

Calling a Function

While creating a C function, you give a definition of what the function has to do. To use a function, you will have to call that function to perform the defined task.

When a program calls a function, the program control is transferred to the called function. A called function performs a defined task and when its return statement is executed or when its function-ending closing brace is reached, it returns the program control back to the main program.

To call a function, you simply need to pass the required parameters along with the function name, and if the function returns a value, then you can store the returned value. For example:

Example

#include <stdio.h>
 
/* function declaration */
int max(int num1, int num2);
 
int main () {
   /* local variable definition */
   int a = 100;
   int b = 200;
   int ret;
 
   /* calling a function to get max value */
   ret = max(a, b);
 
   printf( "Max value is : %dn", ret );
 
   return 0;
}
 
/* function returning the max between two numbers */
int max(int num1, int num2) {
   /* local variable declaration */
   int result;
 
   if (num1 > num2)
      result = num1;
   else
      result = num2;
 
   return result; 
}

I kept max() function along with main() function and compiled the source code. While running the final executable, it would produce the following result:

Max value is : 200

Function Arguments

If a function is to use arguments, it must declare variables that accept the values of the arguments. These variables are called the formal parameters of the function.

Formal parameters behave like other local variables inside the function and are created upon entry into the function and destroyed upon exit.

While calling a function, there are two ways in which arguments can be passed to a function:

Call Type	Description
Call by value	This method copies the actual value of an argument into the formal parameter of the function. In this case, changes made to the parameter inside the function have no effect on the argument.
Call by reference	This method copies the address of an argument into the formal parameter. Inside the function, the address is used to access the actual argument used in the call. This means that changes made to the parameter affect the argument.

By default, C uses call by value to pass arguments. In general, it means the code within a function cannot alter the arguments used to call the function.

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347***568@qq.com 481 Based on whether a function can be called by other source files, functions are categorized into internal functions and external functions.

Internal Functions

If a function can only be called by other functions in the same file, it is called an internal function. When defining an internal function, add static before the function name and function type, i.e.,
```
static type name function_name (parameter list)
```
For example, the first line of the function:
```
static int max(int a, int b)
```
Internal functions are also called static functions. Using internal functions limits the function's scope to the file it resides in. Even if there are functions with the same name in different files, they do not interfere with each other, improving program reliability.

External Functions

If the keyword extern is added to the leftmost end of the function header when defining a function, then this function is an external function that can be called by other files.

For example, the function header can be:
```
extern int max (int a, int b)
```
C language specifies that if extern is omitted when defining a function, it is an external function by default.

In other files that need to call this function, a declaration of this function is required (do not forget, even when calling a function within the same file, use a function prototype for declaration). When declaring this function, add the keyword extern to indicate that the function is defined in another file as an external function.

Example

The following example implements inputting a string and then deleting a specified character through functions in multiple files:

file1.c (File 1)
```
#include <stdio.h>
static void delete_string(char str[], char ch);
int main()
{
    extern void enter(char str[]); // Function declaration
    extern void print(char str[]); // Function declaration
    char c, str;
    enter(str);
    scanf("%c", &c);
    delete_string(str, c);
    print(str);
    return 0;
}
static void delete_string(char str[], char ch) // Internal function
{
    int i, j;
    for (i = j = 0; str != ''; i++)
        if (str != ch)
            str[j++] = str;
    str = '';
}
```
file2.c (File 2)
```
#include <stdio.h>
void enter(char str) // Define external function enter
{
    fgets(str, 100, 
```

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C Functions

C Functions

-- Learning is not just about technology, but also about dreams!

C Tutorial

C Standard Library

Deep Dive

C Functions

Defining a Function

Example

Function Declarations

Calling a Function

Example

Function Arguments

15 Notes, Write a Note

Internal Functions

External Functions

Example

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