Project ForgeProgramming in C
Programming Fundamentals
GATE 2018
Hard
Consider the following C program:
void fun1(char *s1, char *s2){
char *tmp;
tmp = s1; s1 = s2; s2 = tmp;
}
void fun2(char **s1, char **s2){
char *tmp;
tmp = *s1; *s1 = *s2; *s2 = tmp;
}
int main(){
char *str1 = "Hi", *str2 = "Bye";
fun1(str1, str2); printf("%s %s ", str1, str2);
fun2(&str1, &str2); printf("%s %s", str1, str2);
return 0;
}The output of the program above is
- Hi Bye Bye Hi
- BHi Bye Hi Bye
- CBye Hi Hi Bye
- DBye Hi Bye Hi
Explanation
fun1 receives copies of the pointers (pass-by-value) and swaps only its LOCAL copies — str1, str2 in main are unaffected, so the first printf shows 'Hi Bye '. fun2 receives the ADDRESSES of str1, str2 and swaps through the double-pointer dereference, which DOES change the actual variables in main — so after fun2, str1='Bye', str2='Hi', and the second printf shows 'Bye Hi'. Total output: 'Hi Bye Bye Hi'.
Source: GATE CS 2018 (2018)
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