Getting conflicting types for function in C why

Question

I m using the below code   char dest 5   char src 5     test    printf  String   s n   do something dest  src     char  do something char  dest  const char  src        return dest      The implementation of do something is not important here  When I try to compile the above I get these two exception      error  conflicting types for  do something   at the printf call    error  previous implicit declaration of  do something  was here  at the prototype line    Why

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In  classic  C language  C89 90  when you call an undeclared function  C assumes that it returns an int and also attempts to derive the types of its parameters from the types of the actual arguments  no  it doesn t assume that it has no parameters  as someone suggested before    In your specific example the compiler would look at do something dest  src  call and implicitly derive a declaration for do something  The latter would look as follows  int do something char    char      However  later in the code you explicitly declare do something as  char  do something char    const char      As you can see  these declarations are different from each other  This is what the compiler doesn t like

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Watch again   char dest 5   char src 5     test    printf  String   s n   do something dest  src      Focus on this line   printf  String   s n   do something dest  src      You can clearly see that the do something function is not declared   If you look a little further   printf  String   s n   do something dest  src     char  do something char  dest  const char  src    return dest      you will see that you declare the function after you use it   You will need to modify this part with this code   char  do something char  dest  const char  src    return dest     printf  String   s n   do something dest  src      Cheers

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When you don t give a prototype for the function before using it  C assumes that it takes any number of parameters and returns an int   So when you first try to use do something  that s the type of function the compiler is looking for   Doing this should produce a warning about an  implicit function declaration    So in your case  when you actually do declare the function later on  C doesn t allow function overloading  so it gets pissy because to it you ve declared two functions with different prototypes but with the same name   Short answer  declare the function before trying to use it

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This often happens when you modify a c function definition and forget to update the corresponding header definition

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C Commandment  3   K amp R  3 Thou shalt always prototype your functions or else the C compiler will extract vengence     http   www ee ryerson ca 8080  elf hack God vs K R html

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You are trying to call do something before you declare it   You need to add a function prototype before your printf line   char  do something char   const char      Or you need to move the function definition above the printf line   You can t use a function before it is declared

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Make sure that types in the function declaration are declared first       start of the header file           struct intr frame         must be first         void kill  struct intr frame               end of the header file

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You didn t declare it before you used it   You need something like  char  do something char    const char       before the printf   Example    include  lt stdio h gt  char  do something char    const char     char dest 5   char src 5     test   int main      printf  String   s n   do something dest  src     return 0     char  do something char  dest  const char  src    return dest      Alternatively  you can put the whole do something function before the printf

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A C Function-Declaration Backgrounder  In C  function declarations don t work like they do in other languages   The C compiler itself doesn t search backward and forward in the file to find the function s declaration from the place you call it  and it doesn t scan the file multiple times to figure out the relationships either   The compiler only scans forward in the file exactly once  from top to bottom   Connecting function calls to function declarations is part of the linker s job  and is only done after the file is compiled down to raw assembly instructions   This means that as the compiler scans forward through the file  the very first time the compiler encounters the name of a function  one of two things have to be the case   It either is seeing the function declaration itself  in which case the compiler knows exactly what the function is and what types it takes as arguments and what types it returns     or it s a call to the function  and the compiler has to guess how the function will eventually be declared    There s a third option  where the name is used in a function prototype  but we ll ignore that for now  since if you re seeing this problem in the first place  you re probably not using prototypes    History Lesson  In the earliest days of C  the fact that the compiler had to guess types wasn t really an issue   All of the types were more-or-less the same     pretty much everything was either an int or a pointer  and they were the same size    In fact  in B  the language that preceded C  there were no types at all  everything was just an int or pointer and its type was determined solely by how you used it    So the compiler could safely guess the behavior of any function just based on the number of parameters that were passed   If you passed two parameters  the compiler would push two things onto the call stack  and presumably the callee would have two arguments declared  and that would all line up   If you passed only one parameter but the function expected two  it would still sort-of work  and the second argument would just be ignored garbage   If you passed three parameters and the function expected two  it would also still sort-of work  and the third parameter would be ignored and stomped on by the function s local variables    Some old C code still expects these mismatched-argument rules will work  too    But having the compiler let you pass anything to anything isn t really a good way to design a programming language   It worked well in the early days because the early C programmers were mostly wizards  and they knew not to pass the wrong type to functions  and even if they did get the types wrong  there were always tools like lint that could do deeper double-checking of your C code and warn you about such things   Fast-forward to today  and we re not quite in the same boat   C has grown up  and a lot of people are programming in it who aren t wizards  and to accommodate them  and to accommodate everyone else who regularly used lint anyway   the compilers have taken on many of the abilities that were previously part of lint     especially the part where they check your code to ensure it s type-safe   Early C compilers would let you write int foo    hello   and it would just blithely assign the pointer to the integer  and it was up to you to make sure you weren t doing anything stupid   Modern C compilers complain loudly when you get your types wrong  and that s a good thing   Type Conflicts  So what s all this got to do with the mysterious conflicting-type error on the line of the function declaration   As I said above  C compilers still have to either know or guess what a name means the first time they see that name as they scan forward through the file   They can know what it means it if it s an actual function declaration itself  or a function  prototype   more on that shortly   but if it s just a call to the function  they have to guess   And  sadly  the guess is often wrong   When the compiler saw your call to do something    it looked at how it was invoked  and it concluded that do something   would eventually be declared like this   int do something char arg1    char arg2                  Why did it conclude that   Because that s how you called it    Some C compilers may conclude that it was int do something int arg1  int arg2   or simply int do something       both of which are even farther from what you want  but the important point is that regardless of how the compiler guesses the types  it guesses them differently from what your actual function uses    Later on  as the compiler scans forward in the file  it sees your actual declaration of char  do something char    char      That function declaration isn t even close to the declaration that the compiler guessed  which means that the line where the compiler compiled the call was compiled wrong  and the program is just not going to work   So it rightly prints an error telling you that your code isn t going to work as written   You might be wondering   Why does it assume I m returning an int    Well  it assumes that type because there s no information to the contrary   printf   can take in any type in its variable arguments  so without a better answer  int is as good a guess as any    Many early C compilers always assumed int for every unspecified type  and assumed you meant     for the arguments for every function declared f       not void     which is why many modern code standards recommend always putting void in for the arguments if there really aren t supposed to be any    The Fix  There are two common fixes for the function-declaration error   The first solution  which is recommended by many other answers here  is to put a prototype in the source code above the place where the function is first called   A prototype looks just like the function s declaration  but it has a semicolon where the body should be   char  do something char  dest  const char  src     By putting the prototype first  the compiler then knows what the function will eventually look like  so it doesn t have to guess   By convention  programmers often put prototypes at the top of the file  just under the  include statements  to ensure that they ll always be defined before any potential usages of them   The other solution  which also shows up in some real-world code  is to simply reorder your functions so that the function declarations are always before anything that calls them   You could move the entire char  do something char  dest  const char  src          function above the first call to it  and the compiler then would know exactly what the function looks like and wouldn t have to guess   In practice  most people use function prototypes  because you can also take function prototypes and move them into header   h  files so that code in other  c files can call those functions   But either solution works  and many codebases use both   C99 and C11  It is useful to note that the rules are slightly different in the newer versions of the C standard   In the earlier versions  C89 and K amp R   the compiler really would guess the types at function-call time  and K amp R-era compilers often wouldn t even warn you if they were wrong    C99 and C11 both require that the function declaration prototype must precede the first call  and it s an error if it doesn t   But many modern C compilers     mainly for backward compatibility with earlier code     will only warn about a missing prototype and not consider it an error

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You have to declare the function before you use it  If the function name appears before its declaration  C compiler will follow certain rules and makes the declaration itself  If it is wrong  you will get that error   You have two options   1  define it before you use it  or  2  use forward declaration without implementation  For example   char  do something char  dest  const char  src     Note the semicolon at the end

[c] Getting "conflicting types for function" in C, why?

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