Hey there! Let's break down why you're hitting this error and how to fix it.

The Root Cause #

_gcvt_s is a Microsoft-specific safe C runtime function—it's not part of standard C++ or the POSIX libraries that g++ (GCC) uses on your Raspberry Pi. That's why adding -std=c++11 or similar flags doesn't help; those flags enable C++ standards support, but this function never existed in the GCC toolchain.

Solutions to Try #

1. Use Standard C++ String Formatting (Recommended)

For a cross-platform, safe approach, ditch platform-specific functions and use std::stringstream from the C++ standard library. This avoids buffer overflow risks and works everywhere:

#include <iostream>
#include <sstream>
#include <iomanip>
#include <cmath>
using namespace std;

int main(){
    float lo = 144814814.1736111;
    lo = floorf((lo / 1000000) * 100) / 100;
    
    stringstream ss;
    ss << fixed << setprecision(8) << lo; // Set 8 decimal places, matching your original _gcvt_s call
    string vOut = ss.str();
    
    cout << vOut << endl;
    return 0;
}

2. Use GCC's gcvt (Unsafe but Direct Replacement)

If you specifically need a C-style char buffer, GCC provides gcvt—a non-safe equivalent to _gcvt_s (note the missing underscore and _s). Just make sure your buffer is large enough to hold the converted string:

#include <iostream>
#include <cmath>
#include <cstdio> // Required header for gcvt
using namespace std;

int main(){
    float lo = 144814814.1736111;
    lo = floorf((lo / 1000000) * 100) / 100;
    char vOut [17];
    gcvt(lo, 8, vOut); // Converts lo to string with 8 significant digits
    cout << vOut;
    return 0;
}

⚠️ Heads up: gcvt doesn't check buffer bounds, so you need to ensure vOut is big enough to hold the result (including the decimal point, digits, and null terminator).

3. Use snprintf (Safe C-Style Alternative)

Another safe option is snprintf, a standard C function that lets you specify the buffer size to prevent overflow:

#include <iostream>
#include <cmath>
#include <cstdio>
using namespace std;

int main(){
    float lo = 144814814.1736111;
    lo = floorf((lo / 1000000) * 100) / 100;
    char vOut [17];
    // Format to 8 decimal places, matching your original intent
    snprintf(vOut, sizeof(vOut), "%.8f", lo);
    cout << vOut;
    return 0;
}

Final Notes #

• The std::stringstream method is the best long-term choice because it's fully standard-compliant and works on any C++ compiler, not just GCC.
• If you need to stick with C-style strings, snprintf is safer than gcvt thanks to the built-in buffer size check.

内容的提问来源于stack exchange，提问作者TepMaster