c - convert string into signed int

Question

I want to convert a string into a signed int. Following is the requirement. I have stored hex value as a string in buffer. Now I want to convert that value into signed int.

buf = "fb869e" Convert this into signed int. So o/p should be -293218. but when I'm trying to convert using strtol I'm getting 16483998. So what I should I do?

Answer 1

The hexadecimal number 0xfb869e is not negative. The inbuilt number conversion functions will not convert it to a negative value, since its value is positive.

What you are saying is that this is the unsigned hexadecimal equivalent of a 24-bit 2s complement negative number, and you want that number. The way to get that is to convert it to the positive number, then use calculations to convert it to the 24-bit 2s complement equivalent:

char *buf = "fb869e";
long n;

n = strtol(buf, NULL, 16);
if (n > 0x7fffffL)
    n -= 0x1000000L;