Question about different behaviour in C and Java when using stdout and stderr

[Radosław Smogura]

Hi,

This is quite interesting.

I wonder if this can be somehow related to terminal used to run both programs.

Is the first one run in different terminal and with different shell? If yes, would it be possible to try running both programs in the same terminal and in the same shell with the same basic configuration of prompts and output? 

Best regards,
Rado Smogura

> On 21 Nov 2023, at 06:00, 刘希晨 <benrush0705 at gmail.com> wrote:
> 
> 
> Hi I noticed a different behaviour in C and Java about using stdout and stderr, and I want to know what causes the difference.
> 
> I wrote a function to print some strings to both stdout and stderr in C as below:
> 
> #include <stdio.h>
> #include <string.h>
> 
> void test_out(char* outBuffer, size_t outSize, char* errBuffer, size_t errSize) {
>     if(outSize > 0) {
>         fwrite(outBuffer, 1, outSize, stdout);
>         fflush(stdout);
>     }
>     if(errSize > 0) {
>         fwrite(errBuffer, 1, errSize, stderr);
>         fflush(stderr);
>     }
> }
> 
> int main() {
>     char* hello1 = "out world1";
>     char* hello2 = "err world2";
>     size_t len1 = strlen(hello1);
>     size_t len2 = strlen(hello2);
>     for(int i = 0; i < 5; i++) {
>         test_out(hello1, len1, hello2, len2);
>     }
> }
> 
> 
> 
> the output, as expected should be : 
> <image.png>
> 
> 
> the stdout and stderr prints the message by turn, which is what we want
> 
> however, when I am calling the same function in java to implement the same logic like below:
> public class Test {
>     private static final MethodHandle testHandle;
> 
>     static {
>         SymbolLookup symbolLookup = NativeUtil.loadLibrary(Constants.TENET);
>         testHandle = NativeUtil.methodHandle(symbolLookup, "test_out", FunctionDescriptor.ofVoid(ValueLayout.ADDRESS, ValueLayout.JAVA_LONG, ValueLayout.ADDRESS, ValueLayout.JAVA_LONG));
>     }
>     public static void main(String[] args) {
>         try(Arena arena = Arena.ofConfined()) {
>             MemorySegment s1 = arena.allocateUtf8String("out world1");
>             MemorySegment s2 = arena.allocateUtf8String("err world2");
>             for(int i = 0; i < 5; i++) {
>                 testHandle.invokeExact(s1, s1.byteSize(), s2, s2.byteSize());
>             }
>         } catch (Throwable e) {
>             throw new RuntimeException(e);
>         }
>     }
> }
> 
> the result is not as expected:
> <image.png>
> 
> 
> either the stdout finish all its output first, or the stderr finish its ouput first, and I tried using multiple thread to call test_out() function at the same time, the behaviour is still the same, the stdout and stderr are seperated.
> 
> I want to know what causes this, and how could I make stdout and stderr behave in Java just like in C. Thanks for your help!
> 
> 
>  
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