---
name: "strlcpy"
section: "3"
platform: "freebsd"
package: "base"
version: "16.0-CURRENT-20261027-61c70dc16a79"
release: "FreeBSD 16.0-CURRENT (2026-10-27)"
platforms: ["linux", "freebsd", "macos"]
url: "https://new.carta.tech/strlcat"
---

# strlcpy(3)

size-bounded string copying and concatenation

Part of [strlcat(3)](/strlcat/3.md).

## NAME

**strlcpy**, **strlcat** – size-bounded string copying and concatenation

## LIBRARY

Standard C Library (libc, -lc)

## SYNOPSIS

**#include \<string.h\>**

*size\_t*\

**strlcpy**(*char \* restrict dst*, *const char \* restrict src*, *size\_t dstsize*);

*size\_t*\

**strlcat**(*char \* restrict dst*, *const char \* restrict src*, *size\_t dstsize*);

## DESCRIPTION

The **strlcpy**() and **strlcat**() functions copy and concatenate strings with the same input parameters and output result as [strcpy(3)](/strcpy/3.md) and [strcat(3)](/strcat/3.md) with proper overflow protection. They are designed to be safer, more consistent, and less error prone replacements for the easily misused functions [strncpy(3)](/strncpy/3.md) and [strncat(3)](/strncat/3.md).

**strlcpy**() and **strlcat**() take the full size of the destination buffer and guarantee NUL-termination if there is room. Note that room for the NUL should be included in *dstsize*.

**strlcpy**() copies up to *dstsize* - 1 characters from the string *src* to *dst*, NUL-terminating the result if *dstsize* is not 0.

**strlcat**() appends string *src* to the end of *dst*. It will append at most *dstsize* - strlen(dst) - 1 characters. It will then NUL-terminate, unless *dstsize* is 0 or the original *dst* string was longer than *dstsize* (in practice this should not happen as it means that either *dstsize* is incorrect or that *dst* is not a proper string).

If the *src* and *dst* strings overlap, the behavior is undefined.

## RETURN VALUES

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*.

If the return value is **\>=** *dstsize*, the output string has been truncated. It is the caller's responsibility to handle this.

## EXAMPLES

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.

## SEE ALSO

[snprintf(3)](/snprintf/3.md), [strncat(3)](/strncat/3.md), [strncpy(3)](/strncpy/3.md), [wcslcpy(3)](/wcslcpy/3.md) 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, June 6-11, 1999, http://www.usenix.org/publications/library/proceedings/usenix99/full\_papers/millert/millert.pdf.

## HISTORY

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