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

Question

#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.

Answer 1

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.

Answer 2

Your format string expects characters 64 to follow the integer. %lu is the format specifier; 64 are literal characters.

Answer 3

Your format string is invalid. It should go like this on 64 bit builds: "%lu%1[BKMG]" and "%llu%1[BKMG]" when building for 32 bits.

The universal solution uses SCNu64 macro: "%" SCNu64 "%1[BKMG]". This macro is defined in cinttypes header file.

Also your last parameter is passed incorrectly. It have to be an array of characters since zero will be appended at the end. Your code leads to undefined behavior since when c value will be written, something beyond this variable also will be written.

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

int main() {
    std::uint64_t ui;
    char s[32];
    auto ret = std::sscanf("111K", "%lu%1[BKMG]", &ui, s);

    std::cout << "ret=" << ret << "  ui=" << ui << " s=" << s << "\n";

    return 0;
}

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