gdb: show typeinfo of some data

Question

Basically, I want to get typeid(*this).name(), i.e. the real type of this.

I want to get this in GDB (without modifying the source code). I tried print typeid(*this) but it says that typeid is unknown (because I didn't included it there in the source file).

Answer 1

Use ptype command, like this:

(gdb) ptype 42
type = int

Answer 2

Use ptype, whatis, and explore (my favorite!):

Where you have a variable named value which is defined as:

uint32_t value = 1234;

...the following all work:

1. ptype value shows unsigned int
2. whatis value shows uint32_t
3. explore value (my favorite!) shows:

   The value of 'value' is of type 'uint32_t' which is a typedef of type 'unsigned int'
   'value' is a scalar value of type 'unsigned int'.
   value = 1234
   

Example:

(gdb) ptype value
type = unsigned int
(gdb) ptype &value
type = unsigned int *
(gdb) whatis value
type = uint32_t
(gdb) explore value
The value of 'value' is of type 'uint32_t' which is a typedef of type 'unsigned int'
'value' is a scalar value of type 'unsigned int'.
value = 1234

Thanks to @o11c's comment below for pointing out the existence of the whatis command.

I discovered the explore command by running help all inside gdb. See my comment in the "References" section below.

Try it yourself:

# download the file "type_punning.c"
wget https://raw.githubusercontent.com/Generalsimus/eRCaGuy_hello_world/master/c/type_punning.c

# build it with optimization OFF (`-O0`) and debugging symbols ON (`-ggdb`), 
# and output all intermediary files (`-save-temps=obj`), and run it in the 
# gdb debugger (`gdb bin/type_punning`)
mkdir -p bin && gcc -Wall -Wextra -Werror -O0 -ggdb -std=c11 -save-temps=obj \
type_punning.c -o bin/type_punning && gdb bin/type_punning

Now, with gdb running, do the following:

# set breakpoint to a line just after `u.value = 1234;`
b type_punning.c:52
# run to that point
r
# Now run these various commands to see what type `u.value` is:
ptype u.value
whatis u.value 
explore u.value  

Full example commands and output:

eRCaGuy_hello_world/c$ mkdir -p bin && gcc -Wall -Wextra -Werror -O0 -ggdb -std=c11 -save-temps=obj type_punning.c -o bin/type_punning && gdb bin/type_punning
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Reading symbols from bin/type_punning...done.
(gdb) b type_punning.c:52
Breakpoint 1 at 0x70b: file type_punning.c, line 52.
(gdb) r
Starting program: /home/gabriel/GS/dev/eRCaGuy_hello_world/c/bin/type_punning 
Type punning and ptr-based serialization demo
TECHNIQUE 1: union-based type punning:

Breakpoint 1, main () at type_punning.c:53
53          printf("1st byte = 0x%02X\n", (u.bytes)[0]);
(gdb) ptype u.value
type = unsigned int
(gdb) whatis u.value 
type = uint32_t
(gdb) explore u.value  
The value of 'u.value' is of type 'uint32_t' which is a typedef of type 'unsigned int'
'u.value' is a scalar value of type 'unsigned int'.
u.value = 1234
(gdb) 

My old/original answer

As @Star Brilliant says here, this:

ptype my_var

returns things like type = unsigned short, but I want it to return type = uint16_t instead, so I can truly know how many bytes it is when inspecting memory. The best I can figure out to get this effect is to do:

print &my_var

which prints (uint16_t *) 0x7ffffffefc2c, thereby revealing that its pointer type is uint16_t*, meaning its type is uint16_t.

I find this to be more-useful than ptype my_var, but a more direct way to get this effect is desired in case you have any suggestions.

Sample gdb commands and output:

(gdb) ptype my_var
type = unsigned short
(gdb) print &my_var
$27 = (uint16_t *) 0x7ffffffefc2c

Again, notice ptype my_var reveals it is an unsigned short, whereas print &my_var reveals the more-detailed and desired answer which is that it is a uint16_t.

References:

1. @o11c's comment below
2. @Star Brilliant's answer
3. help all - I used this command while running gdb, copy-pasted the output all to a text editor, and searched for "type" to discover the explore command.

See also:

1. My answer on How to view a pointer like an array in GDB?
2. My answer on How to use printf in GDB in order to write a custom description around your variable output
3. [my ans.] "gdb" debugger skips a break point weirdly
4. [my Q&A] What's the difference between a compiler's `-O0` option and `-Og` option?
5. [my Q&A] https://askubuntu.com/questions/1349047/where-do-i-find-core-dump-files-and-how-do-i-view-and-analyze-the-backtrace-st

_{Keywords: how to run gdb; how to view variable types and values in gdb; how to build and compile for gdb debugging symbols}

Answer 3

The ptype [ARG] command will print the type.

Answer 4

This question may be related: vtable in polymorphic class of C++ using gdb:

(gdb) help set print object
Set printing of object's derived type based on vtable info. 

It's not exactly typeid() but it should show the real object type when inspecting a polymorphic pointer (e.g. this in a base class). Naturally works only for classes with a vtable (i.e. at least one virtual method) but so does typeid.