Manpages - strlcpy.3bsd

(See

for include usage.)

The

and

functions copy and concatenate strings respectively. They are designed to be safer, more consistent, and less error prone replacements for

and

Unlike those functions,

and

take the full size of the buffer (not just the length) and guarantee to NUL-terminate the result (as long as

is larger than 0 or, in the case of

as long as there is at least one byte free in

Note that a byte for the NUL should be included in

Also note that

and

only operate on true

strings. This means that for

must be NUL-terminated and for

both

and

must be NUL-terminated.

The

function copies up to

1 characters from the NUL-terminated string

NUL-terminating the result.

The

function appends the NUL-terminated string

to the end of

It will append at most

strlen(dst) - 1 bytes, NUL-terminating the result.

The

and

functions return the total length of the string they tried to create. For

that means the length of

For

that means the initial length of

plus the length of

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

Note, however, that if

traverses

characters without finding a NUL, the length of the string is considered to be

and the destination string will not be NUL-terminated (since there was no space for the NUL). This keeps

from running off the end of a string. In practice this should not happen (as it means that either

is incorrect or that

is not a proper

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];

…

(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

and

As a matter of fact, the first version of this manual page got it wrong.

The

and

functions first appeared in

and made their appearance in