I'm trying to understand this kind of program in C, but I can't. Exactly, I can't figure out how *s is changed, and why the compiler shows 210012.
#include <stdio.h> #include <stdlib.h> #include <string.h> void WhatIamDoing(char *s) { char ch; if (*s) { ch = *s; s++; WhatIamDoing(s); putchar(ch); } } int main() { char s[20] = "012" ; WhatIamDoing(s) ; printf( "%s", s ) ; } I think it is easy to think this way. In void function char *s is a pointer that pointed to a char variable or char array. In your case it pointed to the char array s[20]="012". In WhatIamDoing function s is pointed to '0' character and it's assign to char ch variable. Then s++ now 's' is pointed to character '1'. Again you are calling the function WhatIamDoing(s), it also happens the same (this is like a recursive function) and in the last WhatIamDoing(s), char ch is assign as '2'. After all the characters are done, 
(finally when it comes to null character) if condition is false. In last function by command putchar you print '2' then '1' then '0'. It means after you run the WhatIamDoing function you print the char array in reverse order. In main function you again print the s string. So then you get the the "210021". hope you get the idea.
putchar prints characters of string in reversed order
landk? Recommend posting output exactly.