Having some errors, learning c (Euclidean Algorithm) -

so have done programming in python before, , trying hands out old solutions/methods used there. having syntax/compiler errors , feel im missing something. dumb here code.

//euclidian algorithm in c //a=bq+r gcd(a,b)=gcd(b,r) //recursive solution //devin martin  #define _crt_secure_no_warnings    // avoid scanf warning or error #include <stdio.h> int main() {     int a, b = 0;     int gcd(a, b);      {         if (b == 0);             return a;         return gcd(b, % b);  //a%b = r     } } 

errors severity code description project file line

error lnk2019 unresolved external symbol _gcd referenced in function _main euclidianalgorithm d:\documents\visual studio 2015\projects\euclidianalgorithm\euclidianalg.obj 1

warning c4700 uninitialized local variable 'a' used euclidianalgorithm d:\documents\visual studio 2015\projects\euclidianalgorithm\euclidianalg.c 14

error lnk1120 1 unresolved externals euclidianalgorithm d:\documents\visual studio 2015\projects\euclidianalgorithm\debug\euclidianalgorithm.exe 1

you can't define function inside of function in c. also, function definition incorrect, need declare type of parameters inside of parameter list.

#include <stdio.h>  int gcd(int a, int b) {     if (b == 0)         return a;     return gcd(b, % b);  //a%b = r }  int main() {     printf("gcd(15,6)=%d\n",gcd(15,6)); }