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C users must avoid using dangerous functions that do not check bounds unless they've ensured that the bounds will never get exceeded. Functions to avoid in most cases (or ensure protection) include the functions strcpy(3), strcat(3), sprintf(3) (with cousin vsprintf(3)), and gets(3). These should be replaced with functions such as strncpy(3), strncat(3), snprintf(3), and fgets(3) respectively, but see the discussion below. The function strlen(3) should be avoided unless you can ensure that there will be a terminating NIL character to find. The scanf() family (scanf(3), fscanf(3), sscanf(3), vscanf(3), vsscanf(3), and vfscanf(3)) is often dangerous to use; do not use it to send data to a string without controlling the maximum length (the format %s is a particularly common problem). Other dangerous functions that may permit buffer overruns (depending on their use) include realpath(3), getopt(3), getpass(3), streadd(3), strecpy(3), and strtrns(3). You must careful with getwd(3); the buffer sent to getwd(3) must be at least PATH_MAX bytes long.
Unfortunately, snprintf()'s variants have additional problems. Officially, snprintf() is not a standard C function in the ISO 1990 (ANSI 1989) standard, though sprintf() is, so not all systems include snprintf(). Even worse, some systems' snprintf() do not actually protect against buffer overflows; they just call sprintf directly. Old versions of Linux's libc4 depended on a ``libbsd'' that did this horrible thing, and I'm told that some old HP systems did the same. Linux's current version of snprintf is known to work correctly, that is, it does actually respect the boundary requested. The return value of snprintf() varies as well; the Single Unix Specification (SUS) version 2 and the C99 standard differ on what is returned by snprintf(). Finally, it appears that at least some versions of snprintf don't guarantee that its string will end in NIL; if the string is too long, it won't include NIL at all. Note that the glib library (the basis of GTK, and not the same as the GNU C library glibc) has a g_snprintf(), which has a consistent return semantic, always NIL-terminates, and most importantly always respects the buffer length.