Converting an integer to binary strings in c

Question

This is what I've tried, my logic is that my tmp shifting left 31 times will get compared to the user input integer I and a value of 1 or 0 will be inserted to index str[0] -> str[31] and the I null terminate str[32] with the \0.

However, I'm getting a segmentation fault.

P.S. I'm not allowed to change the parameters of this function and my professor set the size of str to be 33 in the main, which, I'm not allow to change either.

void int2bitstr(int I, char *str) {
        int tmp = 1 << 31;
        do{
                *str++ = !!(tmp & I) + '0';
        } while(tmp >>= 1);
        *str = '\0';
}

Building on what Eugene said, I think you would be surprised at your output if you print the value of tmp to your console with each iteration, especially in consideration of your loop-exit clause. — WhozCraig
– WhozCraig, Commented Sep 20, 2017 at 16:16
@HansPetterTaugbølKragset I got the program to work, I'm actually new to c and never heard of valgrind; thankyou for your suggestion, I'll keep it in mind! — Nam Vu
– Nam Vu, Commented Sep 20, 2017 at 18:58

ezod · Accepted Answer · 2017-09-20 16:15:59Z

1

Try making tmp an unsigned int. The behaviour of right-shifting a negative (signed) integer is implementation-defined, and in your case is likely shifting in 1s (the original MSB) thus causing the loop to exceed the length of str.

answered Sep 20, 2017 at 16:15

ezod

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2 Comments

chux Over a year ago

int tmp = 1 << 31; --> unsigned tmp = 1u << 31; (be sure to change to 1u)

Nam Vu Over a year ago

Thank you! The program works fine now, I can't believe I forget about this

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