Error(s):

source_file.cpp: In function ‘void strcat(char*, char*)’:
source_file.cpp:11:45: warning: format ‘%d’ expects argument of type ‘int’, but argument 2 has type ‘long int’ [-Wformat=]
          while(*s!='\0')s++; printf("%d",s-p); i=s;
                                             ^
source_file.cpp:11:49: error: invalid conversion from ‘char*’ to ‘int’ [-fpermissive]
          while(*s!='\0')s++; printf("%d",s-p); i=s;
                                                 ^
source_file.cpp:12:35: warning: operation on ‘i’ may be undefined [-Wsequence-point]
          while((s[i++] = p[i++])!='\0' );//{*s = *p;s++;}
                                   ^