github.com/cnboonhan/delve@v0.0.0-20230908061759-363f2388c2fb/Documentation/cli/expr.md (about) 1 # Expressions 2 3 Delve can evaluate a subset of go expression language, specifically the following features are supported: 4 5 - All (binary and unary) on basic types except <-, ++ and -- 6 - Comparison operators on any type 7 - Type casts between numeric types 8 - Type casts of integer constants into any pointer type and vice versa 9 - Type casts between string, []byte and []rune 10 - Struct member access (i.e. `somevar.memberfield`) 11 - Slicing and indexing operators on arrays, slices and strings 12 - Map access 13 - Pointer dereference 14 - Calls to builtin functions: `cap`, `len`, `complex`, `imag` and `real` 15 - Type assertion on interface variables (i.e. `somevar.(concretetype)`) 16 17 # Special Variables 18 19 Delve defines two special variables: 20 21 * `runtime.curg` evaluates to the 'g' struct for the current goroutine, in particular `runtime.curg.goid` is the goroutine id of the current goroutine. 22 * `runtime.frameoff` is the offset of the frame's base address from the bottom of the stack. 23 24 # Nesting limit 25 26 When delve evaluates a memory address it will automatically return the value of nested struct members, array and slice items and dereference pointers. 27 However to limit the size of the output evaluation will be limited to two levels deep. Beyond two levels only the address of the item will be returned, for example: 28 29 ``` 30 (dlv) print c1 31 main.cstruct { 32 pb: *struct main.bstruct { 33 a: (*main.astruct)(0xc82000a430), 34 }, 35 sa: []*main.astruct len: 3, cap: 3, [ 36 *(*main.astruct)(0xc82000a440), 37 *(*main.astruct)(0xc82000a450), 38 *(*main.astruct)(0xc82000a460), 39 ], 40 } 41 ``` 42 43 To see the contents of the first item of the slice `c1.sa` there are two possibilities: 44 45 1. Execute `print c1.sa[0]` 46 2. Use the address directly, executing: `print *(*main.astruct)(0xc82000a440)` 47 48 # Elements limit 49 50 For arrays, slices, strings and maps delve will only return a maximum of 64 elements at a time: 51 52 ``` 53 (dlv) print ba 54 []int len: 200, cap: 200, [0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,...+136 more] 55 ``` 56 57 To see more values use the slice operator: 58 59 ``` 60 (dlv) print ba[64:] 61 []int len: 136, cap: 136, [0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,...+72 more] 62 ``` 63 64 For this purpose delve allows use of the slice operator on maps, `m[64:]` will return the key/value pairs of map `m` that follow the first 64 key/value pairs (note that delve iterates over maps using a fixed ordering). 65 66 These limits can be configured with `max-string-len` and `max-array-values`. See [config](https://github.com/go-delve/delve/tree/master/Documentation/cli#config) for usage. 67 68 # Interfaces 69 70 Interfaces will be printed using the following syntax: 71 ``` 72 <interface name>(<concrete type>) <value> 73 ``` 74 75 For example: 76 77 ``` 78 (dlv) p iface1 79 (dlv) p iface1 80 interface {}(*struct main.astruct) *{A: 1, B: 2} 81 (dlv) p iface2 82 interface {}(*struct string) *"test" 83 (dlv) p err1 84 error(*struct main.astruct) *{A: 1, B: 2} 85 ``` 86 87 To use the contents of an interface variable use a type assertion: 88 89 ``` 90 (dlv) p iface1.(*main.astruct).B 91 2 92 ``` 93 94 Or just use the special `.(data)` type assertion: 95 96 ``` 97 (dlv) p iface1.(data).B 98 2 99 ``` 100 101 If the contents of the interface variable are a struct or a pointer to struct the fields can also be accessed directly: 102 103 ``` 104 (dlv) p iface1.B 105 2 106 ``` 107 108 # Specifying package paths 109 110 Packages with the same name can be disambiguated by using the full package path. For example, if the application imports two packages, `some/package` and `some/other/package`, both defining a variable `A`, the two variables can be accessed using this syntax: 111 112 ``` 113 (dlv) p "some/package".A 114 (dlv) p "some/other/package".A 115 ``` 116 117 # Pointers in Cgo 118 119 Char pointers are always treated as NUL terminated strings, both indexing and the slice operator can be applied to them. Other C pointers can also be used similarly to Go slices, with indexing and the slice operator. In both of these cases it is up to the user to respect array bounds. 120 121 122 # CPU Registers 123 124 The name of a CPU register, in all uppercase letters, will resolve to the value of that CPU register in the current frame. For example on AMD64 the expression `RAX` will evaluate to the value of the RAX register. 125 126 Register names are shadowed by both local and global variables, so if a local variable called "RAX" exists, the `RAX` expression will evaluate to it instead of the CPU register. 127 128 Register names can optionally be prefixed by any number of underscore characters, so `RAX`, `_RAX`, `__RAX`, etc... can all be used to refer to the same RAX register and, in absence of shadowing from other variables, will all evaluate to the same value. 129 130 Registers of 64bits or less are returned as uint64 variables. Larger registers are returned as strings of hexadecimal digits. 131 132 Because many architectures have SIMD registers that can be used by the application in different ways the following syntax is also available: 133 134 * `REGNAME.intN` returns the register REGNAME as an array of intN elements. 135 * `REGNAME.uintN` returns the register REGNAME as an array of uintN elements. 136 * `REGNAME.floatN` returns the register REGNAME as an array of floatN elements. 137 138 In all cases N must be a power of 2. 139