/in/ex2.c: In function 'main':
/in/ex2.c:6:16: error: implicit declaration of function 'malloc' [-Werror=implicit-function-declaration]
     s1=(char *)malloc(sizeof(char)*20);
                ^~~~~~
/in/ex2.c:6:16: error: incompatible implicit declaration of built-in function 'malloc' [-Werror]
/in/ex2.c:6:16: note: include '<stdlib.h>' or provide a declaration of 'malloc'
cc1: all warnings being treated as errors