C compiling - undefined reference to

Question

I am making a reliable data transfer protocol and have the function prototype  void tolayer5 int  char data       With the structs  struct msg     char data 20       struct pkt      int seqnum     int acknum     int checksum     char payload 20        And when I call the function in this format   tolayer5 A  packet payload     Where A is an int and packet payload is a struct pkt  I get the error  undefined reference to  tolayer5 int  char     Can you help me see what I m missing here   void tolayer5 int AorB  char data        int i    if  TRACE gt 2           printf  TOLAYER5  data received          for  i 0  i lt 20  i            printf   c  data i         printf   n              Thank you all for helping with the original issue     When trying to fix that one  however  I ran into an infinite loop that I think has something to do with me addressing characters in an array incorrectly  it s been awhile since I ve done C like this  Can you help me to find where I m creating an infinite loop   I have updated the above code to what I m now working with  Notice the main changes have been to my function   void tolayer5 int AorB  char data      And this line inside the function  printf   c  msgReceived data i    since now it s just   printf   c  data i

User · Accepted Answer

seems you need to link with the obj file that implements tolayer5()

Update: your function declaration doesn't match the implementation:

      void tolayer5(int AorB, struct msg msgReceived)
      void tolayer5(int, char data[])

So compiler would treat them as two different functions (you are using c++). and it cannot find the implementation for the one you called in main().

User · Answer

As stated by a few others  this is a linking error  The section of code where this function is being called doesn t know what this function is  It either needs to be declared in a header file an defined in its own source file  or defined or declared in the same source file  above where it s being called   Edit  In older versions of C  C89 C90  function declarations weren t actually required   So  you could just add the definition anywhere in the file in which you re using the function  even after the call and the compiler would infer the declaration  For example   int main       int a   func       int func        return 1      However  this isn t good practice today and most languages  C   for example  won t allow it  One way to get away with defining the function in the same source file in which you re using it  is to declare it at the beginning of the file  So  the previous example would look like this instead   int func     int main        int a   func       int func       return 1

User · Answer

Make sure your declare the tolayer5 function as a prototype  or define the full function definition  earlier in the file where you use it

[c] C compiling - "undefined reference to"?

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