| STRLCPY(3) | Library Functions Manual | STRLCPY(3) | 
strlcpy, strlcat —
#include <string.h>
size_t
  
  strlcpy(char
    *dst, const char
    *src, size_t
  size);
size_t
  
  strlcat(char
    *dst, const char
    *src, size_t
  size);
strlcpy() and strlcat()
  functions copy and concatenate strings respectively. They are designed to be
  safer, more consistent, and less error prone replacements for
  strncpy(3) and
  strncat(3). Unlike those
  functions, strlcpy() and
  strlcat() take the full size of the buffer (not just
  the length) and guarantee to NUL-terminate the result (as long as
  size is larger than 0 or, in the case of
  strlcat(), as long as there is at least one byte free
  in dst). Note that you should include a byte for the NUL
  in size. Also note that
  strlcpy() and strlcat() only
  operate on true “C” strings. This means that for
  strlcpy() src must be
  NUL-terminated and for strlcat() both
  src and dst must be
  NUL-terminated.
The strlcpy() function copies up to
    size - 1 characters from the NUL-terminated string
    src to dst, NUL-terminating the
    result.
The strlcat() function appends the
    NUL-terminated string src to the end of
    dst. It will append at most size
    - strlen(dst) - 1 bytes, NUL-terminating the result.
strlcpy() and strlcat()
  functions return the total length of the string they tried to create. For
  strlcpy() that means the length of
  src. For strlcat() that means
  the initial length of dst plus the length of
  src. While this may seem somewhat confusing it was done
  to make truncation detection simple.
Note however, that if strlcat() traverses
    size characters without finding a NUL, the length of
    the string is considered to be size and the
    destination string will not be NUL-terminated (since there was no space for
    the NUL). This keeps strlcat() from running off the
    end of a string. In practice this should not happen (as it means that either
    size is incorrect or that dst is
    not a proper “C” string). The check exists to prevent
    potential security problems in incorrect code.
char *s, *p, buf[BUFSIZ]; ... (void)strlcpy(buf, s, sizeof(buf)); (void)strlcat(buf, p, sizeof(buf));
To detect truncation, perhaps while building a pathname, something like the following might be used:
char *dir, *file, pname[MAXPATHLEN]; ... if (strlcpy(pname, dir, sizeof(pname)) ≥ sizeof(pname)) goto toolong; if (strlcat(pname, file, sizeof(pname)) ≥ sizeof(pname)) goto toolong;
Since we know how many characters we copied the first time, we can speed things up a bit by using a copy instead of an append:
char *dir, *file, pname[MAXPATHLEN]; size_t n; ... n = strlcpy(pname, dir, sizeof(pname)); if (n ≥ sizeof(pname)) goto toolong; if (strlcpy(pname + n, file, sizeof(pname) - n) ≥ sizeof(pname) - n) goto toolong;
However, one may question the validity of such optimizations, as
    they defeat the whole purpose of strlcpy() and
    strlcat().
Todd C. Miller and Theo de Raadt, strlcpy and strlcat -- Consistent, Safe, String Copy and Concatenation, Proceedings of the FREENIX Track: 1999 USENIX Annual Technical Conference, USENIX Association, http://www.usenix.org/publications/library/proceedings/usenix99/full_papers/millert/millert.pdf, June 6-11, 1999.
strlcpy() and strlcat()
  functions first appeared in OpenBSD 2.4, then in
  NetBSD 1.4.3 and FreeBSD 3.3.
| March 1, 2001 | NetBSD 9.3 |