Weird behaviour when debugging C programs in Eclipse using GDB

Question

I was trying to debug a C program that requires user inputs, using Eclipse IDE and GDB debugger. Every time I entered a user input in the console, my input was displayed twice. In other words, my input was echoed with the same value. For example, if I debug the following C program:

#include <stdio.h>

int main(void) {
    int length, width, height;

    printf("Enter length: ");
    scanf("%d", &length);
    printf("Enter width: ");
    scanf("%d", &width);
    printf("Enter height: ");
    scanf("%d", &height);
    printf("Volume = %d\n", length*width*height);

    return 0;
}

, then the console will look something like this:

Enter length: 2
2
Enter width: 3
3
Enter height: 4
4
Volume = 24

.

As you can see, the values 2, 3 and 4 are displayed twice respectively. Running this program (instead of debugging) gives the expected result:

Enter length: 2
Enter width: 3
Enter height: 4
Volume = 24

In addition, I notice that the execution of printf() statements are delayed: although I have already clicked "step over", there is nothing shown in the console. To make this issue more obvious, let us change the code to:

#include <stdio.h>

int main(void) {
    int length, width, height;

    printf("Enter length: ");
    printf("Enter length: "); // newly added line
    printf("Enter length: "); // newly added line
    scanf("%d", &length);
    printf("Enter width: ");
    scanf("%d", &width);
    printf("Enter height: ");
    scanf("%d", &height);
    printf("Volume = %d\n", length*width*height);

    return 0;
}

In this case, the string "Enter length: Enter length: Enter length: " will be displayed all at once, after I stepped over line 9: scanf("%d", &length);. However, if I add a new line character '\n' at the end every time I call printf(), then there will be no problem and the strings will be immediately printed to the console.

I know that these two issues are trivial, but being OCD I really want to know what is going on here. I guess it has something to do with how Eclipse handles input and output? But then again I don't see these issues when debugging java programs in Eclipse.

PS. I am running on Mac OS X El Capitan, Eclipse (Mars) IDE for C/C++ Developers, and I installed GDB with Homebrew according to the instruction given here: http://ntraft.com/installing-gdb-on-os-x-mavericks/.

Thanks in advance for answering.

Try a better IDE: CLion from JetBrains. I don't understand why anybody uses Eclipse. — duffymo
@duffymo: Because it's free? :) I use Eclipse for java so I tried their C/C++ version. I actually use text editors like Sublime Text for C/C++.. — David
It's the only reason I ever used it. Glad I learned about something better. — duffymo
If you're on mac, can't you use x-code? Otherwise, have a look at Qt creator. — juanchopanza
In the Run Configurations window, is there a checkbox labeled Connect process input & output to a terminal? Is it checked? — Mark Plotnick

ebby94 ebby94 · Accepted Answer · 2016-03-20T15:11:06

The output is being buffered. A new line character flushes the buffer and displays the result on the screen. That's why you're not having the issule when you add \n.

Alternatively, you can try adding fflush(stdout); after your printf statements.

Also, I would also suggest you to use Xcode for Mac instead of Eclipse.

Weird behaviour when debugging C programs in Eclipse using GDB

1 Answers