MirOS Manual: strlcat(3), strlcpy(3)

STRLCPY(3)                 BSD Programmer's Manual                  STRLCPY(3)


     strlcpy, strlcat - size-bounded string copying and concatenation


     #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 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.


     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 find-
     ing a NUL, the length of the string is considered to be size and the des-
     tination 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 in-
     correct 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];


           (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 de-
     feat 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)


     The strlcpy() and strlcat() functions first appeared in OpenBSD 2.4.

MirOS BSD #10-current           June 22, 1998                                1

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