STRLCPY(3)

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 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). 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, 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.

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. While this may seem somewhat confusing it was done to make truncation detection simple.

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