foo.c: In function 'main':
foo.c:14:13: warning: array subscript has type 'char' [-Wchar-subscripts]
             while(temp&&s[i]!=s[temp])
             ^
foo.c:18:13: warning: array subscript has type 'char' [-Wchar-subscripts]
             if(s[i]==s[temp])
             ^
foo.c:28:13: error: 'for' loop initial declarations are only allowed in C99 mode
             for(int j=i+1;j<len;j++)
             ^
foo.c:28:13: note: use option -std=c99 or -std=gnu99 to compile your code
foo.c:40:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^