83 lines
2.6 KiB
Markdown
83 lines
2.6 KiB
Markdown
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# Expressions
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Delve can evaluate a subset of go expression language, specifically the following features are supported:
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- All (binary and unary) on basic types except <-, ++ and --
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- Comparison operators on any type
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- Type casts between numeric types
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- Type casts of integer constants into any pointer type
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- Struct member access (i.e. `somevar.memberfield`)
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- Slicing and indexing operators on arrays, slices and strings
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- Map access
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- Pointer dereference
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- Calls to builtin functions: `cap`, `len`, `complex`, `imag` and `real`
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- Type assertion on interface variables (i.e. `somevar.(concretetype)`)
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# Nesting limit
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When delve evaluates a memory address it will automatically return the value of nested struct members, array and slice items and dereference pointers.
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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:
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```
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(dlv) print c1
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main.cstruct {
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pb: *struct main.bstruct {
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a: (*main.astruct)(0xc82000a430),
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},
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sa: []*main.astruct len: 3, cap: 3, [
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*(*main.astruct)(0xc82000a440),
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*(*main.astruct)(0xc82000a450),
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*(*main.astruct)(0xc82000a460),
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],
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}
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```
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To see the contents of the first item of the slice `c1.sa` there are two possibilities:
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1. Execute `print c1.sa[0]`
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2. Use the address directly, executing: `print *(*main.astruct)(0xc82000a440)`
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# Elements limit
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For arrays, slices, strings and maps delve will only return a maximum of 64 elements at a time:
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```
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(dlv) print ba
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[]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]
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```
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To see more values use the slice operator:
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```
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(dlv) print ba[64:]
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[]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]
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```
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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).
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# Interfaces
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Interfaces will be printed using the following syntax:
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```
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<interface name>(<concrete type>) <value>
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```
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For example:
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```
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(dlv) p iface1
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(dlv) p iface1
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interface {}(*struct main.astruct) *{A: 1, B: 2}
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(dlv) p iface2
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interface {}(*struct string) *"test"
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(dlv) p err1
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error(*struct main.astruct) *{A: 1, B: 2}
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```
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To use a field of a struct contained inside an interface variable use a type assertion:
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```
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(dlv) p iface1.(*main.astruct).B
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2
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```
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