Buffers & Boundaries

Two numbers, one pointer

A C string is char * plus a convention: bytes until '\0'. Its length is discovered by walking (strlen). A C buffer is char * plus a capacity someone must track: how many bytes the allocation actually holds. The classic disasters come from conflating them:

char buf[8];
strcpy(buf, "0123456789");   /* writes 11 bytes into 8 — gone */

strcpy knows the source length but not the destination capacity — it cannot be safe. The functions that can be safe take both numbers:

/* strncpy: copies at most n bytes; NO '\0' if source fills n! */
strncpy(buf, src, sizeof buf);
/* snprintf: always terminates, returns what it WOULD have written */
int need = snprintf(buf, sizeof buf, "%s/%s", a, b);
if (need < 0 || (size_t)need >= sizeof buf) /* truncated — handle it */;

snprintf is the workhorse: bounded, terminating, and it tells you when the output did not fit. That return value is the truncation detector most code forgets to check.

The boundary question, every time

Before touching a buffer, answer three questions in a comment: capacity? current length? who terminates? Code that cannot answer those three questions in its own comments is code that overflows under maintenance.