How to truncate C char*?

If you don't need to copy the string somewhere else and can change it

/* make sure strlen(name) >= 3 */
namelen = strlen(name); /* possibly you've saved the length previously */
name[namelen - 3] = 0;

If you need to copy it (because it's a string literal or you want to keep the original around)

/* make sure strlen(name) >= 3 */
namelen = strlen(name); /* possibly you've saved the length previously */
strncpy(copy, name, namelen - 3);
/* add a final null terminator */
copy[namelen - 3] = 0;

If you know the length of the string you can use pointer arithmetic to get a string with the last three characters:

const char* mystring = "abc123";
const int len = 6;

const char* substring = mystring + len - 3;

Please note that substring points to the same memory as mystring and is only valid as long as mystring is valid and left unchanged. The reason that this works is that a c string doesn't have any special markers at the beginning, only the NULL termination at the end.

I interpreted your question as wanting the last three characters, getting rid of the start, as opposed to how David Heffernan read it, one of us is obviously wrong.

I think some of your post was lost in translation.

To truncate a string in C, you can simply insert a terminating null character in the desired position. All of the standard functions will then treat the string as having the new length.

#include <stdio.h>
#include <string.h>

int main(void)
{
    char string[] = "one one two three five eight thirteen twenty-one";

    printf("%s\n", string);

    string[strlen(string) - 3]  = '\0';

    printf("%s\n", string);

    return 0;
}