Padding zeros in C - c

Question

I have to add zero to the shorter bit string, but if this doesn't work for
str1 = 0011
str2 = 0101111100011
but it works if
str1 = 0101111100011
str2 = 0011
void addLeadingZero(char* str1 , char* str2) {
int len1 = strlen(str1);
int len2 = strlen(str2);
int diff = (len1 > len2) ? len1 : len2;
while (len1 < diff) {
for (int i = len1; i >= 0; i--) {
str1[i + 1] = str1[i]; // expend the shorter array length.
//until there is no difference in lengths.
}
str1[0] = '0'; //add zeros to it.
len1++;
}// loop stops when max diff is reached.
while (len2 < diff) { //same concept as above.
for (int i = len2; i >= 0; i--) {
str2[i + 1] = str2[i];
}
str2[0] = '0';
len2++;
}
}
output :
0000000000011
011
expected output
0000000000011
0101111100011

Answer 1

Your diff does not show the difference.
You can use memmove and memset for this task:
void addLeadingZero(char* str1 , char* str2)
{
size_t len1 = strlen(str1);
size_t len2 = strlen(str2);
char *wrk = len1 > len2 ? str2 : str1;
size_t diff = len1 > len2 ? (len1 - len2) : (len2 - len1);
if(diff)
{
memmove(wrk + diff, wrk, len1 > len2 ? len2 + 1 : len1 + 1);
memset(wrk, '0', diff);
}
}
Remember that str1 & str2 have to be modifiable and large enough to accommodate the padded string.

Answer 2

In order to pad a string, the containing buffer must have enough space for the padding and the original string, and it must be modifiable. Otherwise, Undefined Behaviour will be the result.
String literals are not modifiable, and should not be passed to such a function.
Arrays which have their size determined by the initialization (e.g., char foo[] = "bar";) will not contain any additional room for padding.
Dynamically allocated buffers could be resized with realloc to create space, or a new buffer could be allocated for the result.
For a different method, using standard library functions:
Use memmove to move the existing string towards the end of the buffer.
Use memset to fill the gap.
A simple example where the buffers are explicitly sized to have enough room:
#include <stdio.h>
#include <string.h>
void pad(char *string, size_t length, size_t new_length)
{
size_t diff = new_length - length;
memmove(string + diff, string, length + 1);
memset(string, '0', diff);
}
int main(void)
{
char a[64] = "123";
char b[64] = "456789";
size_t a_len = strlen(a);
size_t b_len = strlen(b);
if (a_len > b_len)
pad(b, b_len, a_len);
else if (b_len > a_len)
pad(a, a_len, b_len);
puts(a);
puts(b);
}
Output:
000123
456789