Get rid of warning in sscanf - c

Question

I have a little problem with my code. At the moment I just have a line (char* string with \0 at the end) and I want the line to be checked on special characters. Therefore I used the following code:
char lineJunk;
if(sscanf(lineContent, "%*[ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+=\0/{}:]%c", &lineJunk)){
return 0;
}
Now my compiler will spit out the following warning:
Multiple markers at this line
- no closing ‘]’ for ‘%[’ format [-Wformat=]
- embedded ‘\0’ in format [-Wformat-contains-nul]
- too many arguments for format [-Wformat-extra-args]
These warnings only appear when I have \0 in my sscanf. Yet otherwise the code won't work, because the Line I am checking on has \0 at its end. When I use \\0 instead of \0 the warnings disappear, but the code doesn't work anymore. Just \ doesn't work either.
Somebody know a solution?

Answer 1

You are not using sscanf() correctly, to explain the warnings
No closing ] means your format string has no closing ] which is required since you are passing a format with [.
The closeing ] in your format string is "not" there really, because you have an embeded '\0' in the format string, so the actual format string is
"%*[ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+="
This is because you have an explicit '\0' in the format string, which is causing the previous warning too.
There is one in the format string which is added by the compiler at the end of it, to mark the end and so it becomes a legitimate c string, in the sense that you can pass it to strlen() and other functions that expect the nul terminator to be present.
By embeding it in the format string, you are marking the end of the string at the position where you inserted, that's why the format string is the one I say in point 1.
You are discarding the matched value by using the * modifier, you need to remove it to make the passed parameter useful, because as is you are discarding the matched value and hence no parameter is required.
You can't match the '\0' with sscanf() if you want that you need to traverse the string one byte at a time until you find a '\0', and in that case the length should be known beforehand.

Answer 2

There is no need for '\0' in the format of sscanf(char *src, char *format, ...). sscanf() will stop scanning when it reaches the '\0' in src. So sscanf() will never provide '\0' for scanning.
As mention by #iharob, the '\0' in the format is trouble as sscanf() see that as the end of the format. That is what the compiler is warning about.
// Eliminate `\0` from the format.
#define SKIP "%*[ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+=/{}:]"
if(sscanf(lineContent, SKIP "%c" , &lineJunk) == 1) {
return 0;
}
Should A-Z be consecutive as with typical encoding of ASCII, a short-cut would be: #define SKIP "%*[A-Za-z0-9+=/{}:]"
--
Note better to check sscanf() results withe what code wants: 1 rather than non-zero. Under select situations sscanf() will return EOF