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* report a bug of gcc implementation of std::vsnprintf()
@ 2023-03-30 8:29 Qiang Ren
2023-03-30 9:42 ` Jakub Jelinek
0 siblings, 1 reply; 2+ messages in thread
From: Qiang Ren @ 2023-03-30 8:29 UTC (permalink / raw)
To: gcc-bugs
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Hi,
I found a bug in the result of std::vsnprintf(), here is the test app that can reproduce it.
I tested with g++ 11.2 and 12.2 and both have the bug, and this issue does not happen with visual c++.
#include <stdio.h>
#include <string>
#include <iostream>
#include <cstdarg>
int Sprintf(std::string& s, const char* pszFormat, ...) {
va_list argptr;
va_start(argptr, pszFormat);
auto n = std::vsnprintf(nullptr, 0, pszFormat, argptr);
char* buf = (char*)malloc(n + 1);
n = std::vsnprintf(buf, n + 1, pszFormat, argptr);
s = buf;
free(buf);
va_end(argptr);
return n;
}
int main() {
std::string s, s0, s1, s2;
double d = 3.14;
//the result is wrong
Sprintf(s, "abc%.0f", d);
std::cout << "s=" << s << "\n\n";
//the result is wrong
Sprintf(s0, "%.0f", d);
std::cout << "s0=" << s0 << "\n\n";
//the printf result is correct
printf( "%.0f\n\n", d);
//now it becomes correct
Sprintf(s1, "%.0f", d);
std::cout << "s1=" << s1 << "\n\n";
//and this becomes correct too.
Sprintf(s2, "abc%.0f", d);
std::cout << "s2=" << s2 << "\n\n";
}
In a short word, printf() get the correct result, after calling it, vsnprintf() became correct too.
my build command: g++ -o t1 t1.cpp
Thanks,
Qiang Ren
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: report a bug of gcc implementation of std::vsnprintf()
2023-03-30 8:29 report a bug of gcc implementation of std::vsnprintf() Qiang Ren
@ 2023-03-30 9:42 ` Jakub Jelinek
0 siblings, 0 replies; 2+ messages in thread
From: Jakub Jelinek @ 2023-03-30 9:42 UTC (permalink / raw)
To: Qiang Ren; +Cc: gcc-bugs
On Thu, Mar 30, 2023 at 08:29:45AM +0000, Qiang Ren via Gcc-bugs wrote:
> I found a bug in the result of std::vsnprintf(), here is the test app that can reproduce it.
> I tested with g++ 11.2 and 12.2 and both have the bug, and this issue does not happen with visual c++.
gcc-bugs is not the right way to report GCC bugs, though in this case
the only bug is in your program.
You can't use the same va_list multiple times as you pass it to
std::vsnprintf, either you need to use va_copy before the first call
and use the copy in one of the two calls, or you need to va-end after
the first vsnprintf call and va_start again before the second call.
>
> #include <stdio.h>
> #include <string>
> #include <iostream>
> #include <cstdarg>
>
> int Sprintf(std::string& s, const char* pszFormat, ...) {
> va_list argptr;
> va_start(argptr, pszFormat);
>
> auto n = std::vsnprintf(nullptr, 0, pszFormat, argptr);
> char* buf = (char*)malloc(n + 1);
> n = std::vsnprintf(buf, n + 1, pszFormat, argptr);
> s = buf;
> free(buf);
>
> va_end(argptr);
> return n;
> }
Jakub
^ permalink raw reply [flat|nested] 2+ messages in thread
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2023-03-30 9:42 ` Jakub Jelinek
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