Examine a variable with GDB

I need to examine the value of a pointer at a certain point in my program, and I'm not quite sure how to do it. I tried 'print ptr', but GDB just throws back this message:

Code:

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No symbol "ptr" in current context.

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It doesn't work for any other variable either. I've compiled the program with the -g switch.

Did you get other messages when you started gdb <filename> ?

What that error means is that the variable name you entered is not in scope.
Try running the code and breaking on the line where you think the variable has a problem.

Code:

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gdb <filename>
br  22
r
print *ptr

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So if I put the break inside a function call, I can't examine variables in main()?

hmm, you were actually using print ptr?
or were you using *ptr?

ptr would be a variable in itself.
*ptr is the pointer. and you would need to know which variable is being accessed.

I wanted to find out what address and possible also what memory label ptr was pointing at.

you can "br main" or br on a line in main just after your function call.

If you step through the function, you will eventually come back to main.

When you are in another function, the calling function's variables are not in scope.

You just need to go "up" in the stack to bring ptr back in scope.

Code:

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jdeckard@bender ~/test $ cat gdbfun.c
void myfunc( void )
{
  int x;

  x = 1 + 1;
}

int main( void )
{
  char *ptr = "Greetings from the text segment!";

  myfunc();

  return 0;
}

jdeckard@bender ~/test $ gcc -g -o gdbfun gdbfun.c -std=c99
jdeckard@bender ~/test $ gdb gdbfun
GNU gdb 6.3
Copyright 2004 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "x86_64-pc-linux-gnu"...Using host libthread_db library "/lib/libthread_db.so.1".

(gdb) b myfunc
Breakpoint 1 at 0x40046c: file gdbfun.c, line 5.
(gdb) r
Starting program: /home/jdeckard/test/gdbfun

Breakpoint 1, myfunc () at gdbfun.c:5
5          x = 1 + 1;
(gdb) up
#1  0x000000000040048a in main () at gdbfun.c:12
12        myfunc();
(gdb) p ptr
$1 = 0x400590 "Greetings from the text segment!"
(gdb)

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Ah right, so 'up' moves up one stack frame? That's good to know. :)

Correct, and it doesn't change the flow of execution. I'm sure you can guess what command takes you back down the stack.