How to pass variable length width specifier in sscanf?

Question

sscanf(input_str, "%5s", buf); //reads at max 5 characters from input_str to buf

But I need to use something like %MACRO_SIZEs instead of %5s

A trivial solution is to create a format string for the same

char fmt_str[100] = "";    
snprintf(fmt_str, 100, "%%%ds", MACRO_SIZE);
sscanf(input_str, fmt_str, buf);

Is there a better way to achieve the same?

Answer 1

Like Stefan said, but for sscanf() to more properly answer the question, and with a bit more macro trickery:

#define MACRO_SIZE 5

#define FORMAT(S) "%" #S "s"
#define RESOLVE(S) FORMAT(S)

char buf[MACRO_SIZE + 1];
sscanf(input_str, RESOLVE(MACRO_SIZE), buf);

This uses C's automatic joining together of adjacent string literals, to form the required formatting string at compile-time. This only works if MACRO_SIZE is a preprocessor macro, not if it's a normal runtime variable.

The extra macro call through RESOLVE() is needed since otherwise the argument would not be resolved to its #defined value, and we'd end up with a formatting string of "%MACRO_SIZEs", which is not what we want.

Answer 2

if your MACRO_SIZE is const at compile time, you can try this:

#define MACRO_SIZE "5"
snprintf(fmt_str, 100, "%" MACRO_SIZE "s", buf);