strlcpy(3bsd) 3bsd strlcpy(3bsd)

strlcpy, strlcat — size-bounded string copying and concatenation

library “libbsd”

#include <string.h> (See libbsd(7) for include usage.)
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 a byte for the NUL should be included 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 it is known how many characters were copied the first time, things can be sped 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(). As a matter of fact, the first version of this manual page got it wrong.

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

The strlcpy() and strlcat() functions first appeared in OpenBSD 2.4, and made their appearance in NetBSD 1.4.3, FreeBSD 3.3 and glibc 2.38.

January 7, 2024 Linux 6.10.10-arch1-1