/in/ex1.c:12:5: error: unknown type name 'fileExt'
     fileExt *next;
     ^~~~~~~
/in/ex1.c: In function 'main':
/in/ex1.c:24:15: error: conversion to 'int' from 'size_t {aka long unsigned int}' may alter its value [-Werror=conversion]
         len = strlen(argv[i]);
               ^~~~~~
/in/ex1.c:30:27: error: implicit declaration of function 'towlower' [-Werror=implicit-function-declaration]
             temp[index] = towlower(argv[i][j+1]);
                           ^~~~~~~~
/in/ex1.c:30:27: error: incompatible implicit declaration of built-in function 'towlower' [-Werror]
/in/ex1.c:30:27: note: include '<wctype.h>' or provide a declaration of 'towlower'
/in/ex1.c:30:36: error: conversion to 'unsigned int' from 'char' may change the sign of the result [-Werror=sign-conversion]
             temp[index] = towlower(argv[i][j+1]);
                                    ^~~~
/in/ex1.c:30:27: error: conversion to 'char' from 'unsigned int' may alter its value [-Werror=conversion]
             temp[index] = towlower(argv[i][j+1]);
                           ^~~~~~~~
/in/ex1.c:20:10: error: variable 'temp' set but not used [-Werror=unused-but-set-variable]
     char temp[21] = {'\0'};
          ^~~~
/in/ex1.c:19:14: error: unused variable 'head' [-Werror=unused-variable]
     fileExt *head = NULL;
              ^~~~
/in/ex1.c: In function 'isIn':
/in/ex1.c:43:20: error: assignment from incompatible pointer type [-Werror=incompatible-pointer-types]
         head->next = new;
                    ^
/in/ex1.c:45:1: error: control reaches end of non-void function [-Werror=return-type]
 }
 ^
cc1: all warnings being treated as errors