Format string with multiple percent signs

Question

I know %% is used to escape actual % signs in a string, so %%%ds will end up with %10s in the following format string, but I don't know why I need %%5s in this string?

After all, there are only two additional arguments (BUFFSIZE / 10).

#define BUFFSIZE 100
char buf[100]={0}
sprintf(buf, "%%5s %%%ds %%%ds", BUFFSIZE / 10, BUFFSIZE / 10);

After running the code above, the buf will contain the string,

%10s %10s 

Answer 1

The purpose is to get a format string to use it in another function that needs a format string like sscanf().

With your code you get: %5s %10s %10s written to your buf, see online, which means it will accept three strings with a length identifier.

%%5s          --> %5s
%%%ds with 10 --> %10s (read it that way: {%%}{%d}{s})

That buffer %5s %10s %10s could now be used in a sscanf() call like shown here.

But there is a best practice to prevent a buffer overflow caused by sscanf() which is also described by Kernighan and Pike in their book The Practice of Programming, see here on SO.

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The reason why you maybe can't use %*s may be, see here on SO:

For printf, the * allows you to specify minimum field width through an extra parameter, i.e. printf("%*d", 4, 100); specifies a field width of 4.

For scanf, the * indicates that the field is to be read but ignored, so that i.e. scanf("%*d %d", &i) for the input "12 34" will ignore 12 and read 34 into the integer i.

Answer 2

% in itself is a valid conversion specifier. The prescribed syntax is, as mentioned in C11, chapter §7.21.6.1/P2, (emphasis mine)

Each conversion specification is introduced by the character %. After the %, the following appear in sequence:

• Zero or more flags [...]

• An optional minimum field width.

• An optional precision [...]

• An optional length modifier [...]

• A conversion specifier character that specifies the type of conversion to be applied.

Then, from P8, for conversion specifiers

The conversion specifiers and their meanings are:

......

%

A % character is written. No argument is converted. The complete conversion specification shall be %%.

So, based on greedy approach, compiler will group a syntax like

 ....   %%%ds, BUFFSIZE / 10 ....

as

 {%%}{%d}{s}
  ^^--------------------------Replaced as %
      ^^----------------------Actual conversion specification happens, argument is used
         ^^------------------just part of final output

which finally produces

  %Xs    //where X is the value of (BUFFSIZE / 10)

which is a valid format string (%, minimum field width, conversion specifier, all in order), again, to be used later.