Rajesh Rajesh - 1 year ago 112
C Question

while loop is not breaking

I am not able to find out the reason for the misbehavior of the below code. This is a simple code to accept characters until either * is entered or until array size is reached and then print the characters read from the keyboard. It looks like the reading part is fine. Also if I enter * before array size is reached everything is OK. But if I do not enter * and wait until array size is reached in reading portion, I have the trouble. While printing it prints the characters read, but after that some garbage is printed. Ran through debugger, but while loop is not breaking when index is 3 or more.

int main()
char myStr [3];
unsigned int index=0;
printf("Enter Single characters. Enter * to stop\n");
scanf(" %c",&myStr[index]);
} while ((myStr[index-1]!='*')&&((index)<(sizeof(myStr)/sizeof(myStr[0]))));


while ((myStr[index]!='*')&&(index<(sizeof(myStr)/sizeof(myStr[0]))))


alk alk
Answer Source

The code runs into undefined behaviour on the printf loop's last iteration here

    while ((myStr[index]!='*')&&(index<(sizeof(myStr)/sizeof(myStr[0]))))

as it in fact is doing

    while ((myStr[3] ....

with myStr[3] accessing myStr out-of-bounds.

To fix this do:

    while ((index < (sizeof(myStr)/sizeof(myStr[0]))) && (myStr[index] != '*'))

Boolean short-circuiting will take care of myStr[3] not being executed.

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