LinuxQuestions.org - [SOLVED] Lost output of printf when using conditional breakpoint in gdb

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Lost output of printf when using conditional breakpoint in gdb

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?

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

Quote:

Originally Posted by NevemTeve (Post 5076986)

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);

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

Quote:

Originally Posted by NevemTeve (Post 5077239)

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!