/in/foo.c: In function 'main':
/in/foo.c:13:3: warning: implicit declaration of function 'strcpy' [-Wimplicit-function-declaration]
   strcpy(temp, B);
   ^~~~~~
/in/foo.c:13:3: warning: incompatible implicit declaration of built-in function 'strcpy'
/in/foo.c:13:3: note: include '<string.h>' or provide a declaration of 'strcpy'
/in/foo.c:13:16: error: 'B' undeclared (first use in this function)
   strcpy(temp, B);
                ^
/in/foo.c:13:16: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:14:9: warning: implicit declaration of function 'strcat' [-Wimplicit-function-declaration]
         strcat(temp, A);
         ^~~~~~
/in/foo.c:14:9: warning: incompatible implicit declaration of built-in function 'strcat'
/in/foo.c:14:9: note: include '<string.h>' or provide a declaration of 'strcat'
/in/foo.c:14:22: error: 'A' undeclared (first use in this function)
         strcat(temp, A);
                      ^