I know strncpy is a safer version of strcpy as said here.
However, when I want to copy from src to dst and dst is not a clean buffer, I get unwanted results, which can be avoided by strcpy:
char *p = "123";
char a[10] = "aaaaaa";
strncpy(a,p,strlen(p));
printf("%s\n", a); // 123aaa
strcpy(a,p);
printf("%s\n", a); // 123 <- desired output, as the trailing a's are garbage
In my actual case, I know strlen(src) < sizeof(dst) (at least, if that isn't the case, the program will crash a lot sooner), so I can safely strcpy.
However, if strncpy is what I should use, then must I add after dst[strlen(src)] = '\0' to avoid garbage (or maybe better yet, init the buffer beforehand?)?
The third argument of strncpy is meant to represent the size of the target buffer. And when it fills it up, it doesn't add a null terminating character by design.
If you have sufficient space for the terminator and you insist on strncpy, just pass strlen(p) + 1 so it will not assume it exhausted the target buffer.
Like many already noted by now. This use of strncpy defeats the purpose, and is really no better than than a simple call to strcpy. The only practical use for strncpy, is if you want to overwrite a part of the string in-place (which is the use case you stumbled upon). Though that too is questionable use...