Implicit function declaration error in c



How to fix GCC error: implicit declaration of function …

Problem:

While trying to compile your C/C++ program, you see an error message like

Solution:

implicit declaration of function means that you are trying to use a function that has not been declared. In our example above, StartBenchmark is the function that is implicitly declared.

This is how you call a function:

This is how you declare a function:

The following bullet points list the most common reasons and how to fix them:

  1. Missing #include : Check if the header file that contains the declaration of the function is #include d in each file where you call the function (especially the file that is listed in the error message), before the first call of the function (typically at the top of the file). Header files can be included via other headers,
  2. Function name typo: Often the function name of the declaration does not exactly match the function name that is being called. For example, startBenchmark() is declared while StartBenchmark() is being called. I recommend to fix this by copy-&-pasting the function name from the declaration to wherever you call it.
  3. Bad include guard: The include guard that is auto-generated by IDEs often looks like this:

Note that the include guard definition _EXAMPLE_FILE_NAME_H is not specific to the header filename that we are using (for example Benchmark.h ). Just the first of all header file names wil
Change the order of the #include statements: While this might seem like a bad hack, it often works just fine. Just move the #include statements of the header file containing the declarationto the top. For example, before the move:

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Implicit Declaration of Function in C

This tutorial discusses removing the warning of implicit function declaration by declaring the functions above the main function in C.

Implicit Declaration of Function in C

Sometimes, the compiler shows a warning of implicit declaration of the function in C language, which means that the function is not declared on top of the main() function or its header file is not included.

For example, the printf() function belongs to the stdio.h header file, and if we don’t include it before using it in a C source file, the compiler will show a warning that the function declaration is implicit.

In this case, we have to include the header file, which includes the function declaration or declare the function above the main() function. For example, let’s use the printf() function without including its header file.

See the code below.

In the above output, we can see that the compiler gave a warning saying that the declaration of the printf() function is implicit and we have to include the file, or we must provide a declaration of the printf() function in the source file.

We can also see that the value of a is printed, which means the printf() function worked perfectly even though we have not included its header file.

Now, let’s include the header file and repeat the above example. See the code below.

We can see above the warning of the implicit function declaration is not shown this time because we have included the header file for the printf() function.

The implicit function declaration warning will also be shown if we have created a function in a source file but have not declared it above the main() function.

The compiler warns when we try to call the function that the function declaration is implicit. For example, let’s create a function and call it from the main() function without declaring it above the main() function.

In the above output, we can see that the warning is shown for the function we created. We have to declare the function above the main() function to solve this problem.

For example, let’s declare the function above the main() function and repeat the above example. See the code below.

We can see that the warning is now gone. We can also declare the function in a header file and then include the header file in the source file, which is useful in the case of many functions because it will simplify the code.

Hello! I am Ammar Ali, a programmer here to learn from experience, people, and docs, and create interesting and useful programming content. I mostly create content about Python, Matlab, and Microcontrollers like Arduino and PIC.

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