strlcat(3) - NetBSD Manual Pages

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STRLCPY(3)              NetBSD Library Functions Manual             STRLCPY(3)


NAME

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


LIBRARY

     Standard C Library (libc, -lc)


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 NUL-terminated
     strings respectively.

     The strlcpy() function computes the length (like strlen(3)) of src, which
     MUST be NUL-terminated, and copies up to size - 1 bytes from src to dst,
     NUL-terminating the result.

     If the bytes dst[0], dst[1], ..., dst[size - 1] are all non-NUL, then the
     strlcat() function returns size + strlen(src) without writing anything to
     dst.

     Otherwise, the strlcat() function computes the sum of the lengths of dst
     and src, which MUST be NUL-terminated, and copies the content of src to
     the position of the first NUL byte in dst, NUL-terminating the result.
     strlcat() will append at most size - strlen(dst) - 1 non-NUL bytes from
     src, followed by one NUL byte.

   Relation to strncpy(3) and strncat(3)
     Unlike strncpy(3), strlcpy() is guaranteed 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.

     Unlike strncat(3), strlcat() is guaranteed to NUL-terminate the result if
     dst is NUL-terminated to begin with.

     WARNING: strlcpy() and strlcat() are not guaranteed to initialize all
     size bytes of dst -- strlcpy() leaves bytes past dst[strlen(src) + 1]
     uninitialized, and strlcat() leaves bytes past dst[strlen(dst) +
     strlen(src) + 1] uninitialized.  This can lead to security vulnerabili-
     ties such as leaking secrets from uninitialized stack or heap buffers.
     You MUST NOT simply replace strncpy(3) and strncat(3) by strlcpy() and
     strlcat() without proving it is safe to leave some of the output unini-
     tialized.

     WARNING: strlcat() does not guarantee to NUL-terminate dst even if there
     is space for it.  In particular, if dst is not NUL-terminated on entry,
     then strlcat() will leave it without a NUL-terminator on return.

     WARNING: The src argument MUST be NUL-terminated.  Both strlcpy() and
     strlcat() will read through src until they find a NUL terminator, reading
     src[size], src[size + 1], src[size + 2], and beyond if there was no ear-
     lier NUL terminator.  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.


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 trunca-
     tion 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 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.


EXAMPLES

     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().


SEE ALSO

     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,
     http://www.usenix.org/publications/library/proceedings/usenix99/full_papers/millert/millert.pdf,
     June 6-11, 1999.


STANDARDS

     The strlcpy() and strlcat() functions conform to IEEE Std 1003.1-2024
     (``POSIX.1'').


HISTORY

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

NetBSD 10.99                    March 30, 2025                    NetBSD 10.99