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STRLCPY(3) Library Functions Manual STRLCPY(3)

strlcpy, strlcat — size-bounded string copying and concatenation

library “libc”

#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);

The () 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 () function copies up to size - 1 characters from the NUL-terminated string src to dst, NUL-terminating the result.

The () 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.

The 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.

The following code fragment illustrates the simple case:

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().

snprintf(3), strncat(3), strncpy(3)

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.

The strlcpy() and strlcat() functions first appeared in OpenBSD 2.4, then in NetBSD 1.4.3 and FreeBSD 3.3.

March 1, 2001 NetBSD-9.2