Thread: Why the compiler is showing strange output for the strncpy function?

Please check the following program:

Code:

#include<stdio.h>
#include<string.h>


int main(void){
    char nameB[]="Jerry";
    char nameA[]="Tom";


    printf("\n nameA: %s",nameA);
    printf("\n nameB: %s",nameB);


    strncpy(nameA,nameB,4);


    printf("\n nameA: %s",nameA);
    printf("\n nameB: %s",nameB);
    printf("\n");
    return 0;




}

Output is following:

Code:

nameA: Tom
nameB: Jerry
nameA: JerrJerry
nameB: Jerry

Any idea why this strange result is coming? I think buffer overflow is a reason for it because the string nameA contains only 3 characters but the strncpy function copied 4 characters in it.

If it is the case, then I expect a random output from the memory/other errors but here the output for nameA is JerrJerry which suggests the first 4 characters of nameB is copied successfully and nameB is printed again. Any clue?

The output changes when I change the order of 5th and 6th line in the same program.

Code:

#include<stdio.h>
#include<string.h>


int main(void){
    char nameA[]="Tom";
    char nameB[]="Jerry";




    printf("\n nameA: %s",nameA);
    printf("\n nameB: %s",nameB);


    strncpy(nameA,nameB,4);


    printf("\n nameA: %s",nameA);
    printf("\n nameB: %s",nameB);
    printf("\n");
    return 0;




}

Logically, there should be no reason for a different type of result but the following output comes:

Code:

nameA: Tom
nameB: Jerry
nameA: JerrH "
nameB: Jerry

Now, it looks like a random result is generated for NameA for buffer overflow. But here also, first 4 characters are copied which should not happen.

What is the reason of strange result of case 1 & 2 and why for the same program, different output is coming? I am using Codeblock with MinGW compiler.

Any help will be highly appreciated!