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STRLCPY(3)                 Library Functions Manual                 STRLCPY(3)

     strlcpy, strlcat - size-bounded string copying and concatenation

     Standard C Library (libc, -lc)

     #include <string.h>

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

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

     The strlcpy() and strlcat() functions copy and concatenate NUL-terminated
     strings respectively.

     The strlcpy() function copies up to size - 1 bytes 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.

   Relation to strncpy(3) and strncat(3)
     Unlike strncpy(3) and strncat(3), strlcpy() and strlcat() are guaranteed
     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 you should include a byte for the NUL in size.

     WARNING: Also unlike strncpy(3) and strncat(3), strlcpy() and strlcat()
     are not guaranteed to initialize all size bytes of dst -- bytes past
     dst[strlen(src) + 1] are left uninitialized.  This can lead to security
     vulnerabilities such as leaking secrets from uninitialized stack or heap

     WARNING: 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.  Applications handling fixed-
     width fields with (possibly empty) NUL padding, instead of NUL-terminated
     C strings, MUST use strncpy(3) and strncat(3) instead.  Attempting to use
     strlcpy() or strlcat() for these cases can lead to crashes or security
     vulnerabilities from buffer overruns.

     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 bytes 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.

     The following code fragment illustrates the simple case:

           char *s, *p, buf[BUFSIZ];


           strlcpy(buf, s, sizeof(buf));
           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 we know how many bytes we copied the first time, we can speed
     things 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().

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

     Todd C. Miller and Theo de Raadt, "strlcpy and strlcat -- Consistent,
     Safe, String Copy and Concatenation", Proceedings of the FREENIX Track:
     1999 USENIX Annual Technical Conference, USENIX Association,
     June 6-11, 1999.

     The strlcpy() and strlcat() functions first appeared in OpenBSD 2.4, then
     in NetBSD 1.4.3 and FreeBSD 3.3.

NetBSD 10.99                    August 11, 2023                   NetBSD 10.99