How do I SOLVE the issue of cout / cerr / perror synchronisation?

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I have posted this on different forums and here, I KNOW what is the issue ( no need to repeat that here) , but I really do not have a solution.

I am using cout to track code process, works fine.
I am using perror to identify code erros - where applicable.
Even when perror returns "Invalid argument" it helps with debugging.

perror / cerr DO NOT follow the code flow.

According to responses I have received both cout and perror/cerr are BUFFERED , but perror/cerr actual outputs to console (terminal) is NEVER put in same "place" , but mostly at the end of the code run.

perror/cerr are output in red - that helps , but I really would like to have them in correct code sequence.

I DO NOT KNOW HOW TO buffer/ unbuffer perror/ cerr so it does output in correct code sequence.

I have been putting off the solution for long time, now I am open to suggestions HOW to fix this,

Attached code and output demonstrate the issue.

<pre>	if (FileDescriptor_socket) {
		cout << "Synchronize  @line " << __LINE__ << endl;
		perror("socket allocated SUCCESS");
		cerr << "socket allocated SUCCESS" << endl;
		cout << "Synchronize  @line " << __LINE__ << endl;
		return 0;


STOP  @line 1437
Synchronize  @line 1445

I like to see perror/cerr here 


Synchronize  @line 1448
STOP  @line 744
SUCCESS SERVER.AllocateSocket() 
function Socket_RX_Composite
STOP  @line 749
TASK @line 753
function Socket_RX_Composite
STOP  @line 755
Not here 
socket allocated SUCCESS: Success
socket allocated SUCCESS

What I have tried:

Posted in efferent forums, this is just 30 characters fill as required.

Posted 21-Jan-20 5:51am

Vaclav_

Updated 21-Jan-20 6:37am

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Richard MacCutchan 21-Jan-20 12:01pm

When I run the above code I get the following output:

Synchronize  @line 31
socket allocated SUCCESS: No error
socket allocated SUCCESS
Synchronize  @line 34

So where is all the other text generated?

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k5054 · Accepted Answer · 2020-01-21T06:38:00

Solution 2

In addition to using flush, have you considered using strerror(), instead of perror()

e.g.

C++

#include <cstring>
#include <cerrno>
#include <iostream>

void perror_cpp(const char *message)
{
    std::cerr << message << ": " << strerror(errno) << std::endl << std::flush;
}

You could add you colorizing in there, too, to clean up your debug code, and is simple enough it could be marked inline.

Posted 21-Jan-20 6:38am

k5054

Updated 21-Jan-20 6:42am

v2

Comments

Vaclav_ 21-Jan-20 12:40pm

I am sorry, since this has been a long time issue, I totally forgot about "flush". Yes, I did use it and it did not help. But I'll try it again.
I'll give strerror a try too. Thanks

k5054 21-Jan-20 12:47pm

I haven't tried this, but maybe turn off buffering for both cerr and cout: more info here https://stackoverflow.com/a/156413

Vaclav_ 21-Jan-20 18:21pm

I feel this may shed some light on the issue.
Here are quotes from elsewhere:
"by default cout is buffered " and "new line at the end of cout will clear the buffer"
With that - the issue is NOT with cout anyway , but with cerr / perror.

I can add endl or /n to cerr or add flush to whole cerr line.

Not sure how to "add" such to perror - since it prints the optional message first then the actual text of the "errno" .

k5054 21-Jan-20 21:21pm

well, you could fflush(stderr).
Or you could use the code snippet above. sterror() returns the error string associated with the current errno, so my perror_cpp duplicates the functionality of perror(), but sends the output through std::cerr. Note that in the code given, the final << std::flush probably isn't strictly necessary, since std::endl is supposed to do a flush, too.

Vaclav_ 21-Jan-20 23:04pm

SOLVED
Purfect...perror_cpp prints EXACTLY where it does most good.
Tracking socket behaviour in REAL time!
Thanks