What is the meaning of the 'Unknown Field' in a Go Panic Stack Trace?

Understanding the "Unknown Field" in Panic Stack Traces

When debugging stack traces from panics, it's important to understand the various elements displayed. In the provided example:

panic: nil

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

The second number in the main.F() line, 0x10436000, has puzzled some users. This is because it doesn't follow the expected pattern of arguments passed to the function.

Decoding the Second Number

The data printed in the stack trace is the arguments passed to the function, but the values are not represented directly. Instead, they are raw data printed in pointer-sized values. The playground being used operates on a 64-bit word architecture with 32-bit pointers (GOARCH=amd64p32).

Due to this configuration, you'll always see an even number of values printed in the frame arguments.

Pointer Sizes and Data Representation

The function main.F() only has one argument of type int, which is 4 bytes long. However, the pointer size is 8 bytes, meaning that the entire 64-bit word is used to hold the argument. The first 4 bytes contain the actual value of the argument (1 in this case), and the remaining 4 bytes are unused.

Therefore, the 0x10436000 in the stack trace is simply the unused portion of the first 64-bit word.

Other Considerations

The second number in the stack trace will vary depending on the types and number of arguments passed to the function. For example, if main.F() had two arguments of type uint8, the stack trace would show:

main.F(0x97301, 0x10436000)

In this case, the 0x97301 is the actual value of the first argument, and the 0x10436000 is the unused portion of the first 64-bit word, as before.

Return values are also allocated on the stack. For example, if main.F() had a signature of func F(a int64) (int, int), the stack trace would show:

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

In this case, the 0xa is the argument, and the 0x1054d60 and 0xc420078058 are the return values.

Understanding the representation of arguments and return values in panic stack traces is crucial for effective debugging.

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