Question

I am trying to debug the following C code with eclipse-Juno-CDT, and cygwin-gcc (cygwin version=1.7.16, gcc version=3.4.4, gdb version=7.5.50), on 64bit windows. The code works fine in normal mode. Initially debugger was not running, because the source file was not found. Then I searched around and added the path mapping information (from /cygdrive/c to C:\). Now it is running but with the following problems:

1. I have put a breakpoint before the “hello c 1” line, and then single stepping. But nothing gets printed on the console.

2. after single stepping on the last line (“exit”), I get the error: “No source available for _cygwin_exit_return() at …”

   // stdio.h and stdlib.h are included, but when I put a #include the code
   // they dont show up, so I deleted those lines in this code fragment.
   int main(void) {
        int a=10;
        int b=5; // breakpoint on this line, single step after this
        printf("hello c 1\n"); // these outputs are not printed in console
        // fflush(stdout);
        printf("A=%d, B=%d\n", a, b); // but debugger shows the correct values in data window
        // fflush(stdout);
        return EXIT_SUCCESS; // error on this line
   }
   

Added later: After some more debugging, I figured that even after the exit-error, if I do a “continue”, then I am getting the lines on the console after the program terminates. So I added extra “fflush(stdout)” lines, and now I can see the outputs when they are being printed.

But how to fix the exit-error problem? Also, editing the file to add fflush to see debug outputs is a pain – is there a way to avoid this? Can somebody help me with this very basic problem, or point me to a place where the solution is given? Thanks in advance.

Answer 1

While logically a C program begins at int main() and ends when that function returns, environments (like Windows or Cygwin) frequently add pre- and post-code, for initializing / breaking down memory management, opening / closing standard streams, and other such bookkeeping. An executable compiled with Cygwin, after returning from int main(), switches to a cleanup function _cygwin_exit_return(), provided by the Cygwin runtime – for which you don’t have sources, so your debugger complains.

As for getting the output immediately, you could use an unbuffered output stream.

Option one, use fprintf( stderr, ... ) (since stderr is by definition unbuffered). This, however, also affects the non-debugging behaviour of your program.

Option two:

int main()
{
// Using NDEBUG as also used by <assert.h>; feel free to use a different define
#ifndef NDEBUG
    // For debugging, set stdout to unbuffered
    setbuf( stdout, NULL );
#endif
    ....