Code in "If" block doesn't run, even though condition to make it false is inside the block

Question

I am running a socket application which listens to realtime data and processes it continuously. I have separate piece of code which is supposed to execute every second. But that piece of code does not execute. Let me clarify with the code as follows:

In main.cpp, I have

Client client;
//Code to connect to socket and subscribe to realtime data.
while(client.isConnected){
    client.executeRealTime();
}

In Client.cpp, I have the following piece of code:

void Client::executeRealTime(){
    time_t now = time(NULL);
    int currSeconds=now%86400;
    if(currSeconds>13800){
        printf("Before Condition %d %d\n",currSeconds,strategyTimer);
        if(currSeconds>strategyTimer){
            printf("After Condition %d %d\n",currSeconds,strategyTimer);
            for(int i=0;i<numStrategies;i++){
                if((strategies[i]->isTradeable)==1){
                    strategies[i]->execute(currSeconds);
                }
            }
            strategyTimer=currSeconds;
        }
    }
}

Variable strategyTimer is of type int and it is not accessed anywhere else in the code.

The code prints out the printf of "before Condition" continously when it is run. I can verify from the output that at each second, there occurs a print statement where currSeconds is more than strategyTimer by 1 followed by print statement of "after condition". Then I comment out the "before condition" print statement and run the code again. This time I expect the print statement "After Condition" to run every second. But that statement is never printed. Unless the code enters the inner "if" block, the strategyTimer variable cannot be changed. I do not understand why having the "before condition" print statement is changing the behaviour of "after condition" print statement.

Please let me understand how to fix this.

I am using eclipse workspace in windows 7 and compiling using cygwin toolchain.

Before Condition 20198 20198
Before Condition 20198 20198
Before Condition 20198 20198
Before Condition 20198 20198
Before Condition 20198 20198
Before Condition 20198 20198
Before Condition 20199 20198
After Condition 20199 20198
Before Condition 20199 20199
Before Condition 20199 20199
Before Condition 20199 20199

But when I comment out the Before Condition statement, I do not see any output.

First, obvious question: what is the output of 'before condition' line? specifically, what are currSeconds and strategyTimer? Do they ever Satisfy the condition currSeconds > strategyTimer? - Oops, sorry, I just saw that you indicate that it _does_ exceed it by 1. Are they _both_ signed integers? — Arunas, Feb 27 '15 at 05:32
What is strategyTimer initialized to? If you can run Valgrind, you might also find that something is over-writing it, perhaps through a buffer overflow or vagrant pointer. — Arunas, Feb 27 '15 at 05:38
@Arunas, please see the edit. strategyTimer is initialised to 0. Behaviour of "after condition" print statement changes when I comment out "before condition" print statement. Both variables have been initialised as int. — lonstud, Feb 27 '15 at 05:44
It's possible that the output is being buffered. This is a classic way of not finding where your program is really crashing, because not all the printfs actually get their data to your console. Consider this question: http://stackoverflow.com/questions/12450066/flushing-buffers-in-c — Arunas, Feb 27 '15 at 05:48
Thank you Arunas. fflush(stdout); solved the issue. Now I can see the printf appropriately. You may put this as the answer. — lonstud, Feb 27 '15 at 05:54

score 2 · Accepted Answer · edited May 23 '17 at 10:24

As I discovered while debugging c programs over 20 years ago, printf puts things into stdout's buffer. But stdout is under no obligation to actually put that to your screen. It will do so when it's ready.

This has the nasty effect of making you look in entirely the wrong place for your bugs.

The solution, as is pointed out in Flushing buffers in C is to use fflush after your debug printf statements, or to use a different logging mechanism that doesn't suffer from this.

Code in "If" block doesn't run, even though condition to make it false is inside the block

1 Answers1