strlcpy()

NAME

strlcpy, strlcat - 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);

DESCRIPTION

The strlcpy() and strlcat() functions copy and concatenate strings respectively. They are designed to be safer, more consistent, and less error prone replacements for strcpy(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). Note that you should include a byte for the NUL in size.

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

RETURN VALUES

The strlcpy() and strlcat() functions return the total length in bytes of the string they tried to create, not including the NUL terminator.

For strlcpy() that means the length of src.

For strlcat() that means the initial length of dst plus the length of src.

Because this return value is the total potential length, it is easy to detect when truncation happens in dst by comparing this value to size.

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[MAXPATHNAMELEN];
...
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 on an append:

char *dir, *file, pname[MAXPATHNAMELEN];
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

snprntf(3)

strcat(3)

strncpy(3)