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strlcpy(3)               BSD Library Functions Manual               strlcpy(3)


     strlcpy, strlcat -- size-bounded string copying and concatenation


     Standard C Library (libc, -lc)


     #include <string.h>

     strlcpy(char * restrict dst, const char * restrict src, size_t size);

     strlcat(char * restrict dst, const char * restrict src, size_t size);


     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-termi-
     nated and for strlcat() both src and dst must be NUL-terminated.

     The strlcpy() function copies up to size - 1 characters from the NUL-ter-
     minated 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-termi-
     nating the result.

     The source and destination strings should not overlap, as the behavior is


     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 trunca-
     tion 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 destina-
     tion 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 incor-
     rect or that dst is not a proper ``C'' string).  The check exists to pre-
     vent 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 strlcpy() and strlcat().  As a matter of
     fact, the first version of this manual page got it wrong.


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


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

BSD                              June 22, 1998                             BSD

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