Xcode – Warning: Implicit declaration of function is invalid in C99

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The function has to be declared before it’s getting called. This could be done in various ways:

  • Write down the prototype in a header
    Use this if the function shall be callable from several source files. Just write your prototype
    int Fibonacci(int number);
    down in a .h file (e.g. myfunctions.h) and then #include "myfunctions.h" in the C code.

  • Move the function before it’s getting called the first time
    This means, write down the function
    int Fibonacci(int number){..}
    before your main() function

  • Explicitly declare the function before it’s getting called the first time
    This is the combination of the above flavors: type the prototype of the function in the C file before your main() function

As an additional note: if the function int Fibonacci(int number) shall only be used in the file where it’s implemented, it shall be declared static, so that it’s only visible in that translation unit.

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