GDB: break if variable equal value - c

Question

I like to make GDB set a break point when a variable equal some value I set, I tried this example:
#include <stdio.h>
main()
{
int i = 0;
for(i=0;i<7;++i)
printf("%d\n", i);
return 0;
}
Output from GDB:
(gdb) break if ((int)i == 5)
No default breakpoint address now.
(gdb) run
Starting program: /home/SIFE/run
0
1
2
3
4
5
6
Program exited normally.
(gdb)
Like you see, GDB didn't make any break point, is this possible with GDB?

Answer 1

in addition to a watchpoint nested inside a breakpoint
you can also set a single breakpoint on the 'filename:line_number' and use a condition.
I find it sometimes easier.
(gdb) break iter.c:6 if i == 5
Breakpoint 2 at 0x4004dc: file iter.c, line 6.
(gdb) c
Continuing.
0
1
2
3
4
Breakpoint 2, main () at iter.c:6
6 printf("%d\n", i);
If like me you get tired of line numbers changing, you can add a label
then set the breakpoint on the label like so:
#include <stdio.h>
main()
{
int i = 0;
for(i=0;i<7;++i) {
looping:
printf("%d\n", i);
}
return 0;
}
(gdb) break main:looping if i == 5

Answer 2

You can use a watchpoint for this (A breakpoint on data instead of code).
You can start by using watch i.
Then set a condition for it using condition <breakpoint num> i == 5
You can get the breakpoint number by using info watch

Answer 3

First, you need to compile your code with appropriate flags, enabling debug into code.
$ gcc -Wall -g -ggdb -o ex1 ex1.c
then just run you code with your favourite debugger
$ gdb ./ex1
show me the code.
(gdb) list
1 #include <stdio.h>
2 int main(void)
3 {
4 int i = 0;
5 for(i=0;i<7;++i)
6 printf("%d\n", i);
7
8 return 0;
9 }
break on lines 5 and looks if i == 5.
(gdb) b 5
Breakpoint 1 at 0x4004fb: file ex1.c, line 5.
(gdb) rwatch i if i==5
Hardware read watchpoint 5: i
checking breakpoints
(gdb) info b
Num Type Disp Enb Address What
1 breakpoint keep y 0x00000000004004fb in main at ex1.c:5
breakpoint already hit 1 time
5 read watchpoint keep y i
stop only if i==5
running the program
(gdb) c
Continuing.
0
1
2
3
4
Hardware read watchpoint 5: i
Value = 5
0x0000000000400523 in main () at ex1.c:5
5 for(i=0;i<7;++i)

Answer 4

There are hardware and software watchpoints. They are for reading and for writing a variable. You need to consult a tutorial:
http://www.unknownroad.com/rtfm/gdbtut/gdbwatch.html
To set a watchpoint, first you need to break the code into a place where the varianle i is present in the environment, and set the watchpoint.
watch command is used to set a watchpoit for writing, while rwatch for reading, and awatch for reading/writing.