Strings are hard work in C.
#include <stdio.h>
int main()
{
int i;
char buf[12];
for (i = 0; i < 100; i++) {
snprintf(buf, 12, "pre_%d_suff", i); // puts string into buffer
printf("%s\n", buf); // outputs so you can see it
}
}
The 12 is enough bytes to store the text "pre_", the text "_suff", a string of up to two characters ("99") and the NULL terminator that goes on the end of C string buffers.
This will tell you how to use snprintf, but I suggest a good C book!