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That is independent of whether one >> does a dummy NULL,0 call or just calls it with a real, but possibly too >> small, buffer. >> >> This is true for userspace (as required by posix) as well as the kernel >> implementation of vsnprintf(). What makes you think otherwise? >> > > this snippet from PRINTF(3) man page > > RETURN VALUE > Upon successful return, these functions return the number of characters > printed (excluding the null byte used to end output to strings). > Assuming we have the same man pages installed, try reading the very next paragraph: The functions snprintf() and vsnprintf() do not write more than size bytes (including the terminating null byte ('\0')). If the output was truncated due to this limit, then the return value is the number of characters (excluding the terminating null byte) which would have been written to the final string if enough space had been available. Thus, a return value of size or more means that the output was truncated. How else would you even expect the vsnprintf(NULL, 0, ...) thing to work?