ggstrlcpy(3) size-bounded string copying and concatenation

Other Alias



#include <ggi/gg.h>
size_t ggstrlcpy(char *dst, const char *src, size_t siz);
size_t ggstrlcat(char *dst, const char *src, size_t siz);


The ggstrlcpy and ggstrlcat 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, ggstrlcpy and ggstrlcat 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 ggstrlcat, 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 ggstrlcpy and ggstrlcat only operate on true C strings. This means that for ggstrlcpy src must be NUL-terminated and for ggstrlcat both src and dst must be NUL-terminated.

The ggstrlcpy function copies up to siz - 1 characters from the NUL-terminated string src to dst, NUL-terminating the result.

The ggstrlcat function appends the NUL-terminated string src to the end of dst. It will append at most siz - strlen(dst) - 1 bytes, NUL-terminating the result.


The ggstrlcpy and ggstrlcat functions return the total length of the string they tried to create. For ggstrlcpy that means the length of src. For ggstrlcat 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 ggstrlcat 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 ggstrlcat 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];
(void)ggstrlcpy(buf, s, sizeof(buf));
(void)ggstrlcat(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 (ggstrlcpy(pname, dir, sizeof(pname)) >= sizeof(pname))
        goto toolong;
if (ggstrlcat(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 = ggstrlcpy(pname, dir, sizeof(pname));
if (n >= sizeof(pname))
        goto toolong;
if (ggstrlcpy(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 ggstrlcpy and ggstrlcat.


strlcpy and strlcat first appeared in OpenBSD 2.4, then in NetBSD 1.4.3 and FreeBSD 3.3.0. ggstrlcpy and ggstrlcat has been added to libgg for portability.