If you are a programmer, you might have faced the error message “implicit declaration of function is invalid in C99”. It’s indeed a frustrating error and can be challenging to resolve if you don’t know what it means.
What is Implicit Declaration of Function?
Before we dive into the error, let’s first understand what implicit declaration of function means. In C programming, you must declare a function before calling it. However, if you don’t declare the function explicitly, C assumes the return type of the function to be “int”.
This means that C implicitly declares the function for you, assuming it returns an integer. However, this practice is no longer allowed in C99.
What is C99?
C99 is an ISO/IEC standard for the C programming language. It introduced several new features and improvements to the language, such as support for inline functions, variable-length arrays, and several new data types. C99 was released in 1999, hence the name.
Why is Implicit Declaration of Function Invalid in C99?
C99 requires you to declare a function explicitly before calling it. The reason behind this is to avoid potential bugs and improve the quality of the code. If you don’t declare the function, the compiler will not know the return type, the number of arguments, and their types. This can result in undefined behavior and crashes.
How to Fix the Error?
If you encounter the “implicit declaration of function is invalid in C99” error, the fix is relatively simple. You need to declare the function explicitly before calling it. You can do this by adding a function prototype or by including the header file that contains the function declaration.
For example, suppose you have a function called “calculate” that takes two integers as arguments and returns an integer. You can declare the function explicitly before calling it by adding the following code:
int calculate(int a, int b);
This code declares the function “calculate” explicitly, specifying the return type and the number and types of arguments.
Conclusion
Implicit declaration of function is invalid in C99, and you need to declare a function explicitly before calling it. This is to avoid potential bugs and crashes and to improve the quality of the code. If you encounter the error, declare the function explicitly by adding a function prototype or by including the header file that contains the function declaration.