Implicit function declarations in C

Question

What is meant by the term  implicit declaration of a function   A call to a standard library function without including the appropriate header file produces a warning as in the case of   int main      printf  How is this not an error       return 0      Shouldn t using a function without declaring it be an error  Please explain in detail  I searched this site and found similar questions  but could not find a definitive answer  Most answers said something about including the header file to get rid of the warning  but I want to know how this is not an error

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To complete the picture  since -Werror might considered too  invasive  for gcc  and llvm  a more precise solution is to transform just this warning in an error  using the option   -Werror implicit-function-declaration   See Make one gcc warning an error   Regarding general use of -Werror  Of course  having warningless code is recommendable  but in some stage of development it might slow down the prototyping

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It should be considered an error  But C is an ancient language  so it s only a warning  Compiling with -Werror  gcc  fixes this problem   When C doesn t find a declaration  it assumes this implicit declaration  int f     which means the function can receive whatever you give it  and returns an integer  If this happens to be close enough  and in case of printf  it is   then things can work  In some cases  e g  the function actually returns a pointer  and pointers are larger than ints   it may cause real trouble   Note that this was fixed in newer C standards  C99  C11   In these standards  this is an error  However  gcc doesn t implement these standards by default  so you still get the warning

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C is a very low-level language  so it permits you to create almost any legal object   o  file that you can conceive of   You should think of C as basically dressed-up assembly language   In particular  C does not require functions to be declared before they are used   If you call a function without declaring it  the use of the function becomes it s  implicit  declaration   In a simple test I just ran  this is only a warning in the case of built-in library functions like printf  at least in GCC   but for random functions  it will compile just fine   Of course  when you try to link  and it can t find foo  then you will get an error   In the case of library functions like printf  some compilers contain built-in declarations for them so they can do some basic type checking  so when the implicit declaration  from the use  doesn t match the built-in declaration  you ll get a warning

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Because of historical reasons going back to the very first version of C  functions are assumed to have an implicit definition of int function int arg1  int arg2  int arg3  etc    Edit  no  I was wrong about int for the arguments  Instead it passes whatever type the argument is  So it could be an int or a double or a char   Without a prototype the compiler will pass whatever size the argument is and the function being called had better use the correct argument type to receive it   For more details look up K amp R C

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An implicitly declared function is one that has neither a prototype nor a definition  but is called somewhere in the code  Because of that  the compiler cannot verify that this is the intended usage of the function  whether the count and the type of the arguments match   Resolving the references to it is done after compilation  at link-time  as with all other global symbols   so technically it is not a problem to skip the prototype   It is assumed that the programmer knows what he is doing and this is the premise under which the formal contract of providing a prototype is omitted   Nasty bugs can happen if calling the function with arguments of a wrong type or count  The most likely manifestation of this is a corruption of the stack   Nowadays this feature might seem as an obscure oddity  but in the old days it was a way to reduce the number of header files included  hence faster compilation

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Implicit declarations are not valid in C   C99 removed this feature  present in C89    gcc chooses to only issue a warning by default with -std c99 but a compiler has the right to refuse to translate such a program

[c] Implicit function declarations in C

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