STRLCPY(3) Library Functions Manual STRLCPY(3)

strlcpy, strlcat — size-bounded string copying and concatenation

Standard C Library (libc, -lc)

#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 NUL-terminated strings respectively.

The () function computes the length (like strlen(3)) of src, which MUST be NUL-terminated, and copies up to size - 1 bytes from src to dst, NUL-terminating the result.

If the bytes dst[0], dst[1], ..., dst[size - 1] are all non-NUL, then the () function returns size + (src) without writing anything to dst.

Otherwise, the () function computes the sum of the lengths of dst and src, which MUST be NUL-terminated, and copies the content of src to the position of the first NUL byte in dst, NUL-terminating the result. strlcat() will append at most size - strlen(dst) - 1 non-NUL bytes from src, followed by one NUL byte.

strncpy(3) and strncat(3)

Unlike strncpy(3), strlcpy() is guaranteed to NUL-terminate the result (as long as size is larger than 0). Note that you should include a byte for the NUL in size.

Unlike strncat(3), () is guaranteed to NUL-terminate the result if dst is NUL-terminated to begin with.

WARNING: () and strlcat() are not guaranteed to initialize all size bytes of dst — strlcpy() leaves bytes past dst[strlen(src) + 1] uninitialized, and strlcat() leaves bytes past dst[strlen(dst) + strlen(src) + 1] uninitialized. This can lead to security vulnerabilities such as leaking secrets from uninitialized stack or heap buffers. You MUST NOT simply replace strncpy(3) and strncat(3) by strlcpy() and strlcat() without proving it is safe to leave some of the output uninitialized.

WARNING: () does not guarantee to NUL-terminate dst even if there is space for it. In particular, if dst is not NUL-terminated on entry, then strlcat() will leave it without a NUL-terminator on return.

WARNING: The src argument MUST be NUL-terminated. Both () and strlcat() will read through src until they find a NUL terminator, reading src[size], src[size + 1], src[size + 2], and beyond if there was no earlier NUL terminator. Applications handling fixed-width fields with (possibly empty) NUL padding, instead of NUL-terminated C strings, MUST use strncpy(3) and strncat(3) instead. Attempting to use strlcpy() or strlcat() for these cases can lead to crashes or security vulnerabilities from buffer overruns.

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 bytes 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];

...

strlcpy(buf, s, sizeof(buf));
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 bytes 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 conform to IEEE Std 1003.1-2024 (“POSIX.1”).

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

March 30, 2025 NetBSD 11.0