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](https://fuchsia.dev/fuchsia-src/development/debugger/execution.md.txt) 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=`*: Specifies the maximum array size to print. By default this is 256. Specifying large values slows down expression evaluation. - *`--raw` or `-r`*: Bypass pretty-printers and show the raw type information. - *`--types` or `-t`* : Force type printing on. The type of every value printed is explicitly shown. Implies `-v`. - *`--verbose` or `-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 `$()`. Parentheses inside the escaped contents can be literal as long as they are balanced, otherwise, escape them by preceding with a backslash. Include any literal backslash with two backslashes. These are all valid examples: - `$(something with spaces)` - `$({{impl}})` - `$(some_closure(data))` - `$(line\)noise\\)` Additionally, zxdb also supports: - [CPU registers](https://fuchsia.dev/fuchsia-src/development/debugger/expressions.md.txt#cpu-registers) - [Vector registers](https://fuchsia.dev/fuchsia-src/development/debugger/expressions.md.txt#vector-registers) ### 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: > [!NOTE] > **Note:** You can use the `-x` option to display the unsigned hexadecimal value. For additional options, see [Number formatting options](https://fuchsia.dev/fuchsia-src/development/debugger/expressions.md.txt#number-formatting-options). print -x rax + rbx 0x2108aa0032a On ARM64 targets that support the Scalable Vector Extension (SVE), you can also refer to SVE registers (`z0`-`z31`, `p0`-`p15`, `ffr`, and `vg`) in expressions. ### 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: > [!NOTE] > **Note:** You can also use `-v` instead of `--vector`. 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: > [!NOTE] > **Note:** This example uses a variable named `status` and its value is returned when the program stops. 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](https://fuchsia.dev/fuchsia-src/development/debugger/expressions.md.txt#print-expressions)): locals argc = 1 argv = (const char* const*) 0x59999ec02dc0 [](https://issues.fuchsia.dev/issues/new?component=1439230&template=1881640&description=Issue+on:+https://fuchsia.dev/docs/development/debugger/expressions.md "Report a bug or suggest improvements on this page") [](https://cs.opensource.google/fuchsia/fuchsia/+/main:docs/development/debugger/expressions.md "View the source code of this page") [](https://ci.android.com/edit?repo=fuchsia/fuchsia/main&file=docs/development/debugger/expressions.md&cloneRepo=fuchsia/fuchsia:refs/heads/main&fullPath=fuchsia/docs/development/debugger/expressions.md "Edit the source code of this page")