Two problems to watch for:

1) strcpy writes the terminating NUL, so it requires the destination to be large enough. The literal "This is a very long string" is 26 bytes of characters + 1 NUL = 27 bytes. If buf has fewer than 27 bytes, this causes a buffer overflow and undefined behavior $andasecurityrisk$.

2) strcpy does no bounds checking, so avoid it when you don't know the destination size.

Safer alternatives $pickonedependingonyourplatformandneeds$:

If buf is an array and you know its size at compile time:
char buf[sizeof "This is a very long string"];
strcpy(buf, "This is a very long string"); // safe because sizes match

Prefer snprintf $standardandsimple$:
char buf$$100$$;
snprintf(buf, sizeof buf, "%s", "This is a very long string");

Use strlcpy if available $BSDandmanysystems$:
char buf$$100$$;
strlcpy(buf, "This is a very long string", sizeof buf);

If you must use strncpy, ensure NUL termination:
char buf$$100$$;
strncpy(buf, "This is a very long string", sizeof buf - 1);
buf$$sizeofbuf-1$$ = '\0';

Dynamically allocate exactly what you need:
const char s = "This is a very long string";
char buf = malloc$strlen(s)+1$;
if $buf$ { strcpy$buf,s$; / ... / free$buf$; }

If buf is just a pointer and you don't know its capacity, don't use strcpy — either pass the buffer size into the function or allocate the destination yourself.

Want me to show a fixed version for your specific code? If so, tell me how buf is declared $sizeorpointer$.