Zxdb can evaluate and print values from simple C, C++, and Rust
expressions. The most common use case to evaluate an expression in zxdb is with
the print verb. Expressions can also be used for commands that take a memory
location as an argument such as stack or mem-read.
When you evaluate an expression you need a stack frame, which in turn requires a process with a paused thread. If the process is currently running, you can use the pause verb to pause the thread.
You can use the print command to show the current value of a variable in the current stack frame:
For example, to see the value of a variable i:
print i
34
You can also evaluate expressions in the context of another stack frame without switching to that frame.
For example, to see the value of argv[0] in stack frame 2:
frame 2 print argv[0]
"/bin/cowsay"
Print with expressions
The print command accepts the following arguments:
--max-array=<number>: Specifies the maximum array size to print. By default this is 256. Specifying large values slows down expression evaluation.--rawor-r: Bypass pretty-printers and show the raw type information.--typesor-t: Force type printing on. The type of every value printed is explicitly shown. Implies-v.--verboseor-v: Don't omit type names. Show reference addresses and pointer types.
To write an expression beginning with a hyphen, use -- to mark the end of
arguments. Hyphens after -- are treated as part of the expression:
print -- -i
Number formatting options
The print command accepts the following options to force numeric values
to be of specific types:
-b: Binary-c: Character-d: Signed decimal-u: Unsigned decimal-x: Unsigned hexadecimal
Special variables
When you work with variables in zxdb you may have an identifier name that is
not parseable in the current language. This is often the case for
compiler-generated symbols. Make sure to enclose such strings in $(<symbols>).
Parentheses inside the escaped contents can be literal as long as they are
balanced, otherwise, escape them by preceding with a backslash. Include any
literal backslash with two backslashes.
These are all valid examples:
$(something with spaces)$({{impl}})$(some_closure(data))$(line\)noise\\)
Additionally, zxdb also supports:
CPU registers
You can refer to CPU registers with the $reg(register name) syntax. For
example, to display the ARM register v3:
print $reg(v3)
0x573a420f128
CPU registers can also be used unescaped as long as no variable in the current scope has the same name. Registers can also be used like any other variable in more complex expressions:
print -x rax + rbx
0x2108aa0032a
Vector registers
Vector registers can be treated as arrays based on the setting of
vector-format.
print ymm1
{3.141593, 1.0, 0, 0}
[zxdb] print ymm[0] * 2
6.28319
List vector registers
You can list the vector registers with regs.
For example, to list all of the vector registers:
regs --vector
(Use "print $registername" to show a single one, or
"print $registername = newvalue" to set.)
Vector Registers
mxcsr 0x1fa0 = 8096
Name [3] [2] [1] [0]
ymm0 0 0 -3.72066e-103 -3.72066e-103
ymm1 0 0 3.79837e-312 2.63127e-312
ymm2 0 0 0 -3.72066e-103
ymm3 0 0 1.26218e-311 1.26218e-311
ymm4 0 0 1.26218e-311 1.26218e-311
ymm5 0 0 1.26218e-311 1.26218e-311
ymm6 0 0 5.96337e-321 5.87938e-321
ymm7 0 0 2.56125e-311 2.4891e-311
ymm8 0 0 0 0
ymm9 0 0 0 0
ymm10 0 0 0 0
ymm11 0 0 0 0
ymm12 0 0 0 0
ymm13 0 0 0 0
ymm14 0 0 0 0
ymm15 0 0 0 0
(Use "get/set vector-format" to control vector register interpretation.
Currently showing vectors of "double".)
Use display
When stepping through a function, it can be useful to automatically print one
or more expressions each time the program stops. The display command adds a
given expression to this list:
display status
Added to display for every stop: status
[zxdb] next
🛑 main(…) • main.cc:48
[code dump]
status = 5;
Use locals
The locals command shows all local variables in the current stack frame. It
accepts the same arguments as print (see Print with expressions):
locals
argc = 1
argv = (const char* const*) 0x59999ec02dc0