Programming :: Deleting Unwanted Characters In File

Currently, I'm working on personal project. and I'm kinda stuck. What I want to do is that open a file, and edit that file (deleting unwanted characters). The problem arises after I deleted unwanted characters, the file still has the same length of the original one. Let's assume that we have a file with "1234" in it. I deleted "3" ( I overwrite "\0" ) so now when I check the file, it's 124. But when I check the length, the both have the same size as 4

Here is an example source code
int length, length2;
num = open("a.dat", 2)
length = lseek(num, 0, 2); // Initial length
lseek(num, 2, 0); // editing
write(num, "\0", 1);
length2 = lseek(num, 0, 2); // Final length
close(num);

When I print those values those are exactly the same. Length2 should be one less than length, but the both are 4. What's wrong in m code? Am I supposed to use different character rather than "\0"?