Michael Malcolm’s (later of Kaleidescape) class with his custom compiler which definitely had function prototypes (I still have the class notes documenting the language) and he definitely required them in our code (marks would be taken off if they weren’t present). I started using C in the summer of 1984 in Prof. This is interesting as it appears I got access to more cutting edge development tools than was out in the mainstream while I was at the University of Waterloo. With this option added to the compiler settings it gets flagged as an error: This warning is enabled by -Wall (as a warning, not an error). The form -Wno-error-implicit-function-declaration is not supported. Werror-implicit-function-declaration Give a warning (or error) whenever a function is used before being declared. The other good news is that every C compiler usually has an option to turn this warning into an error (what I recommend if you are not paying attention to warnings). The good news is that compiling such a thing with in C++ will produce an error, another good reason to compile plain C code with C++ mode. Unfortunately in C this is not an error but a warning (for legacy reasons, to be able to compile old non-compliant code). Such an ‘implicit declaration’ is really an oversight or error by the programmer, because the C compiler needs to know about the types of the parameters and return value to correctly allocate them on the stack. It might be only a warning if the linker does not find that symbol ( calcMatrix in this case) somewhere, so if you do not pay attention to warnings, your application might crash or behave in wrong way. Still, it is clearly wrong and bad, bad, bad. It might ‘work’ if the types by chance match the actual implementation or will be less of a runtime problem if parameters and return values are passed in registers. If that parameter is passed on the stack it will result in a stack space mismatch, so overall very bad. Which of course is wrong in many ways: not only the parameter 3 is not converted to a double, it means that the wrong size ( int vs. So the compiler has to assume according to the rules: Without knowing the interface, the compiler has to assume (implicitly) the interface from the actual parameters used, and that the function returns an int. For the case it missed a declaration like this:ĮrrorCode_t calcMatrix(matrix_t *m, double weight) Įither because I did not provide a ‘forward declaration’ of it or I missed to include the header file with that declaration, or that declaration is missing in the header file. So what does the gcc warning ‘implicit declaration of function’ mean (other compiler report a ‘implicit parameter declaration’)? Basically it means that the compiler has found a call to function for which he does not have a prototype.
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