Debugging with GDB: Variables

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10.3 Program Variables

The most common kind of expression to use is the name of a variable in your program.

Variables in expressions are understood in the selected stack frame (see Selecting a Frame); they must be either:

or

This means that in the function 

foo (a)
     int a;
{
  bar (a);
  {
    int b = test ();
    bar (b);
  }
}

you can examine and use the variable a whenever your program is executing within the function foo, but you can only use or examine the variable b while your program is executing inside the block where b is declared.

There is an exception: you can refer to a variable or function whose scope is a single source file even if the current execution point is not in this file. But it is possible to have more than one such variable or function with the same name (in different source files). If that happens, referring to that name has unpredictable effects. If you wish, you can specify a static variable in a particular function or file by using the colon-colon (::) notation: 

file::variable
function::variable

Here file or function is the name of the context for the static variable. In the case of file names, you can use quotes to make sure GDB parses the file name as a single word—for example, to print a global value of x defined in f2.c: 

(gdb) p 'f2.c'::x

The :: notation is normally used for referring to static variables, since you typically disambiguate uses of local variables in functions by selecting the appropriate frame and using the simple name of the variable. However, you may also use this notation to refer to local variables in frames enclosing the selected frame: 

void
foo (int a)
{
  if (a < 10)
    bar (a);
  else
    process (a);    /* Stop here */
}
int
bar (int a)
{
  foo (a + 5);
}

For example, if there is a breakpoint at the commented line, here is what you might see when the program stops after executing the call bar(0): 

(gdb) p a
$1 = 10
(gdb) p bar::a
$2 = 5
(gdb) up 2
#2  0x080483d0 in foo (a=5) at foobar.c:12
(gdb) p a
$3 = 5
(gdb) p bar::a
$4 = 0

These uses of ‘::’ are very rarely in conflict with the very similar use of the same notation in C++. When they are in conflict, the C++ meaning takes precedence; however, this can be overridden by quoting the file or function name with single quotes.

For example, suppose the program is stopped in a method of a class that has a field named includefile, and there is also an include file named includefile that defines a variable, some_global. 

(gdb) p includefile
$1 = 23
(gdb) p includefile::some_global
A syntax error in expression, near `'.
(gdb) p 'includefile'::some_global
$2 = 27

Warning: Occasionally, a local variable may appear to have the wrong value at certain points in a function—just after entry to a new scope, and just before exit.

You may see this problem when you are stepping by machine instructions. This is because, on most machines, it takes more than one instruction to set up a stack frame (including local variable definitions); if you are stepping by machine instructions, variables may appear to have the wrong values until the stack frame is completely built. On exit, it usually also takes more than one machine instruction to destroy a stack frame; after you begin stepping through that group of instructions, local variable definitions may be gone.

This may also happen when the compiler does significant optimizations. To be sure of always seeing accurate values, turn off all optimization when compiling.

Another possible effect of compiler optimizations is to optimize unused variables out of existence, or assign variables to registers (as opposed to memory addresses). Depending on the support for such cases offered by the debug info format used by the compiler, GDB might not be able to display values for such local variables. If that happens, GDB will print a message like this: 

No symbol "foo" in current context.

To solve such problems, either recompile without optimizations, or use a different debug info format, if the compiler supports several such formats. See Compilation, for more information on choosing compiler options. See C and C++ , for more information about debug info formats that are best suited to C++ programs.

If you ask to print an object whose contents are unknown to GDB, e.g., because its data type is not completely specified by the debug information, GDB will say ‘<incomplete type>’. See incomplete type, for more about this.

If you try to examine or use the value of a (global) variable for which GDB has no type information, e.g., because the program includes no debug information, GDB displays an error message. See unknown type, for more about unknown types. If you cast the variable to its declared type, GDB gets the variable’s value using the cast-to type as the variable’s type. For example, in a C program: 

  (gdb) p var
  'var' has unknown type; cast it to its declared type
  (gdb) p (float) var
  $1 = 3.14

If you append @entry string to a function parameter name you get its value at the time the function got called. If the value is not available an error message is printed. Entry values are available only with some compilers. Entry values are normally also printed at the function parameter list according to set print entry-values. 

Breakpoint 1, d (i=30) at gdb.base/entry-value.c:29
29	  i++;
(gdb) next
30	  e (i);
(gdb) print i
$1 = 31
(gdb) print i@entry
$2 = 30

Strings are identified as arrays of char values without specified signedness. Arrays of either signed char or unsigned char get printed as arrays of 1 byte sized integers. -fsigned-char or -funsigned-char GCC options have no effect as GDB defines literal string type "char" as char without a sign. For program code 

char var0[] = "A";
signed char var1[] = "A";

You get during debugging 

(gdb) print var0
$1 = "A"
(gdb) print var1
$2 = {65 'A', 0 '\0'}

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