NAME

       strlcpy, strlcat — size-bounded string copying and concatenation

LIBRARY

       Utility functions from BSD systems (libbsd, -lbsd)

SYNOPSIS

       #include <string.h>
       (See libbsd(7) for include usage.)

       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 or, in the case of strlcat(), as long as
       there is at least one byte free in dst).  Note that a byte for the NUL should be included in size.   Also
       note  that  strlcpy()  and strlcat() only operate 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.

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.

       Note, however, that if strlcat() traverses size characters without finding  a  NUL,  the  length  of  the
       string is considered to be size and the destination string will not be NUL-terminated (since there was no
       space  for the NUL).  This keeps strlcat() from running off the end of a string.  In practice this should
       not happen (as it means that either size is incorrect or that dst is not 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, 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), strncat(3), strncpy(3)

HISTORY

       The strlcpy() and strlcat() functions first appeared  in  OpenBSD  2.4,  and  made  their  appearance  in
       NetBSD 1.4.3, FreeBSD 3.3 and glibc 2.38.

Debian                                           January 7, 2024                                   strlcpy(3bsd)