strlcat(), strlcpy()

Size-bounded string copying and concatenation

Synopsis:

#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 );

Arguments:

dst
A pointer to the destination string.
src
A pointer to the source string.
size
The size of the destination buffer.

Library:

libc

Use the -l c option to qcc to link against this library. This library is usually included automatically.

Description:

The strlcpy() and strlcat() functions copy and concatenate strings respectively. They're designed to be safer, more consistent, and less error-prone replacements for strncpy() and strncat() .

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's at least one byte free in dst).

You should include a byte for the NUL in size. Also note that strlcpy() and strlcat() operate only 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.

Returns:

The total length of the string:

  • 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.
If strlcat() traverses size characters without finding a NUL, the length of the string is considered to be size, and the destination string isn't NUL-terminated (since there was no space for the NUL). This keeps strlcat() from running off the end of a string. In practice this shouldn't happen (as it means that either size is incorrect or that dst isn't a proper C string). The check exists to prevent potential security problems in incorrect code.

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, you could use something like this: 

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().

Classification:

Unix

Safety:
Cancellation point No
Interrupt handler Yes
Signal handler Yes
Thread Yes

Contributing author:

OpenBSD

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