[SOLVED] Why sscanf can't read an uint64_t and a char from one string?

Why sscanf can't read an uint64_t and a char from one string?

#include <cstdio>
#include <cstdint>
#include <cassert>

int main() {
    std::uint64_t ui;
    char c;
    auto ret = std::sscanf("111K", "%lu64%[B, K, M, G]", &ui, &c);

    assert(ret == 2);
    assert(ui == 111);
}

I tried to use sscanf to read a uint64_t and a char from one string, but it only read it ui (assertion ret == 2 fails) every time I tried this.

Solution

You have two issues here. First

%lu64

should be

"%" SCNu64

to read in a 64 bit integer.

The second issue is

%[B, K, M, G]

requires a char* or wchar_t* as its output parameter as it populates a c-string. You need to change

char c;

to at least

char c[2] // 2 because it adds a null terminator

in order capture K. We put that all together and we get

auto ret = std::sscanf("111K", "%" SCNu64 "%[B, K, M, G]", &ui, c);

Do note that

%[B, K, M, G]

is actually trying to match all of the spaces and comma inside the brackets. You could also write it as

%[BKMG]

and get the same results.