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NAME    [Toc]    [Back]

     strlcpy, strlcat - size-bounded string copying and  concatenation

SYNOPSIS    [Toc]    [Back]

     #include <string.h>

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

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

DESCRIPTION    [Toc]    [Back]

     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 you should include a byte
for the NUL
     in size.  Also note that strlcpy() and strlcat() only  operate on true
     ``C''  strings.   This  means that for strlcpy() src must be
     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    [Toc]    [Back]

     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    [Toc]    [Back]

     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 we know how many characters 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().  As a
matter of fact,
     the first version of this manual page got it wrong.

SEE ALSO    [Toc]    [Back]

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

OpenBSD     3.6                           June      22,      1998
[ Back ]
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