[SOLVED] Lost output of printf when using conditional breakpoint in gdb

sisrnb · 12-07-2013, 07:35 AM

Version of gdb is 7.4.1-debian.

Just a simple program below:

Code:

  1 #include <stdio.h>
  2 
  3 int main() {
  4     int i;
  5     for (i = 0; i < 3; i++) {
  6         printf("i=%d\n", i);
  7         sleep(1);
  8         if (i == 2) i = -1;
  9     }
 10     return 0;
 11 }

Compile and debug using gdb.

When set a breakpoint using command "b 7", run, and then continue if break, there are "i=0", "i=1", "i=2" in output:

Code:

(gdb) b 7
Breakpoint 1 at 0x400571: file hello2.c, line 7.
(gdb) r
Starting program: /home/si/s/__for_vm/p/hello_app/a.out 
i=0

Breakpoint 1, main () at hello2.c:7
7	        sleep(1);
(gdb) c
Continuing.
i=1

Breakpoint 1, main () at hello2.c:7
7	        sleep(1);
(gdb) 
Continuing.
i=2

Breakpoint 1, main () at hello2.c:7
7	        sleep(1);

However, when set a breakpoint using command "b 7 if i=2", run, and then continue if break, there is only "i=0" in output, no "i=1" and "i=2":

Code:

(gdb) b 7 if i=2
Breakpoint 1 at 0x400571: file hello2.c, line 7.
(gdb) r
Starting program: /home/si/s/__for_vm/p/hello_app/a.out 
i=0

Breakpoint 1, main () at hello2.c:7
7	        sleep(1);
(gdb) c
Continuing.
i=0

Breakpoint 1, main () at hello2.c:7
7	        sleep(1);
(gdb) 
Continuing.
i=0

Breakpoint 1, main () at hello2.c:7
7	        sleep(1);

Why there is no "i=1" and "i=2" in the output?

NevemTeve · 12-07-2013, 11:12 AM

for a start, add fflush(stdout); after printf

sisrnb · 12-07-2013, 08:03 PM

Quote:

Originally Posted by NevemTeve

for a start, add fflush(stdout); after printf

Thanks for the reply, NevemTeve!

I tried adding fflush(stdout); after printf, but still I can't see "i=1" and "i=2" in output:

Code:

(gdb) b 8 if i=2
Breakpoint 1 at 0x400620: file hello2.c, line 8.
(gdb) r
Starting program: /home/si/s/__for_vm/p/hello_app/a.out 
i=0

Breakpoint 1, main () at hello2.c:8
8	        sleep(1);
(gdb) c
Continuing.
i=0

Breakpoint 1, main () at hello2.c:8
8	        sleep(1);
(gdb) 
Continuing.
i=0

Breakpoint 1, main () at hello2.c:8
8	        sleep(1);

NevemTeve · 12-08-2013, 12:58 AM

Then try command 'next' instead of 'continue'. Also, use command 'display i'.

sisrnb · 12-08-2013, 04:22 AM

Quote:

Originally Posted by NevemTeve

Then try command 'next' instead of 'continue'. Also, use command 'display i'.

I tried, and it helped me found the reason.

Here is the output in terminal:

Code:

(gdb) b 8 if i=2
Breakpoint 1 at 0x400620: file h.c, line 8.
(gdb) r
Starting program: /home/si/s/__for_vm/p/hello_app/a.out 
i=0

Breakpoint 1, main () at h.c:8
8	        sleep(1);
(gdb) display i
1: i = 2
(gdb) next
9	        if (i == 2) i = -1;
1: i = 2
(gdb) next
5	    for (i = 0; i < 3; i++) {
1: i = -1
(gdb) next
6	        printf("i=%d\n", i);
1: i = 0
(gdb) next
i=0
7	        fflush(stdout);
1: i = 0
(gdb) next

Breakpoint 1, main () at h.c:8
8	        sleep(1);
1: i = 2
(gdb) next
9	        if (i == 2) i = -1;
1: i = 2
(gdb) next
5	    for (i = 0; i < 3; i++) {
1: i = -1
(gdb) next
6	        printf("i=%d\n", i);
1: i = 0
(gdb) next
i=0
7	        fflush(stdout);
1: i = 0

When first stepped to line 'fflush(stdout);', and stepped again.
Something strange happened, value of 'i' became 2, and it shouldn't encounter the breakpoint.
So I was thinking what changed value of 'i', therefore, I recalled that there is 'i=2' in command 'b 8 if i=2'.
Then I tried command 'b 8 if i==2' instead of 'b 8 if i=2', the issue disappeared.
And then I found the tutorial I referenced has such mistake.

Thanks for your help, NevemTeve!