What is the significance of the "Unknown field" in a Go panic stack trace, and how can we decode the arguments in the traceback?

Delving into Unknown Fields in Panic Stack Traces

Encountering the enigmatic "Unknown field in panic stack trace" is not uncommon when debugging panic errors in Go. To unravel this enigma, let's delve into the complexities of stack traces.

In the provided code snippet:

package main

func F(a int) {
    panic(nil)
}

func main() {
    F(1)
}

Running the program reveals a panic stack trace:

panic: nil

goroutine 1 [running]:
main.F(0x1, 0x10436000)
    /tmp/sandbox090887108/main.go:4 +0x20
main.main()
    /tmp/sandbox090887108/main.go:8 +0x20

What is the significance of the cryptic value 0x10436000 mentioned alongside F(0x1)?

The Mystery Unraveled

The value 0x10436000 is not a second argument to the function. Instead, it represents the raw data values passed in as the first argument, expressed as pointer-sized values. This is a result of the playground architecture, which features 64-bit word size but 32-bit pointers (GOARCH=amd64p32).

Decoding the Arguments

In the traceback.go file, the values are printed by iteratively accessing the arguments based on their pointer size:

for i := uintptr(0); i < frame.arglen/sys.PtrSize; i++ {

This means that since the word size is double the pointer size in the playground, pairs of values are printed in each frame's arguments.

Examples for Clarity

To illustrate further:

• Consider the following function:

func F(a uint8) {
    panic(nil)
}

Calling F(1) results in:

main.F(0x97301, 0x10436000)

Only the first 8 bits of the 64-bit word are used (1 in this case), while the remainder is unused.

• On amd64 systems, a function with three uint32 arguments would produce a stack trace:

main.F(0x100000001, 0xc400000001)

The three arguments occupy two words, and the extra unused values are printed.

• Functions with return values also allocate space on the stack, as demonstrated by:

func F(a int64) (int, int)

On amd64, the stack frame arguments would appear as:

main.F(0xa, 0x1054d60, 0xc420078058)

The first word is for the argument, while the other two are for the uninitialized return values.

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