Why am I getting a 'panic: runtime error: invalid memory address or nil pointer dereference' error in my Go code?

Go: panic: runtime error: invalid memory address or nil pointer dereference

Issue

When invoking Go, it explicitly panics with the following message:

panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x38 pc=0x26df]

goroutine 1 [running]:
main.getBody(0x1cdcd4, 0xf800000004, 0x1f2b44, 0x23, 0xf84005c800, ...)
        /Users/matt/Dropbox/code/go/scripts/cron/fido.go:65 +0x2bb
main.getToken(0xf84005c7e0, 0x10)
        /Users/matt/Dropbox/code/go/scripts/cron/fido.go:140 +0x156
main.main()
        /Users/matt/Dropbox/code/go/scripts/cron/fido.go:178 +0x61

goroutine 2 [syscall]:
created by runtime.main
        /usr/local/Cellar/go/1.0.3/src/pkg/runtime/proc.c:221

goroutine 3 [syscall]:
syscall.Syscall6()
        /usr/local/Cellar/go/1.0.3/src/pkg/syscall/asm_darwin_amd64.s:38 +0x5
syscall.kevent(0x6, 0x0, 0x0, 0xf840085188, 0xa, ...)
        /usr/local/Cellar/go/1.0.3/src/pkg/syscall/zsyscall_darwin_amd64.go:199 +0x88
syscall.Kevent(0xf800000006, 0x0, 0x0, 0xf840085188, 0xa0000000a, ...)
        /usr/local/Cellar/go/1.0.3/src/pkg/syscall/syscall_bsd.go:546 +0xa4
net.(*pollster).WaitFD(0xf840085180, 0xf840059040, 0x0, 0x0, 0x0, ...)
        /usr/local/Cellar/go/1.0.3/src/pkg/net/fd_darwin.go:96 +0x185
net.(*pollServer).Run(0xf840059040, 0x0)
        /usr/local/Cellar/go/1.0.3/src/pkg/net/fd.go:236 +0xe4
created by net.newPollServer
        /usr/local/Cellar/go/1.0.3/src/pkg/net/newpollserver.go:35 +0x382

Answer

The panic: runtime error: invalid memory address or nil pointer dereference arises when a resource lacks a memory address, known as a nil pointer dereference. This typically happens when you try to use a pointer that points to nothing or when accessing a field of a nil pointer.

To troubleshoot this issue:

1. Check for nil pointers: Ensure you haven't assigned any variables or structure fields to nil values.
2. Validate memory addresses: Make sure you're not trying to access a memory address that doesn't exist. Utilize the & operator to get the address of a variable or structure field.
3. Debug the code: Use a debugger like delve or the built-in debug package to step through your code and identify which line is causing the panic. The stack trace provided in the error message can assist in pinpointing the problematic code.
4. Inspect the stack trace: The stack trace provides valuable information about the function calls leading up to the panic, which can help in identifying the source of the issue.

In the given code, the panic occurred in the getBody function when attempting to close the res.Body. According to the docs for func (*Client) Do, an error is only returned if caused by client policy or an HTTP protocol error, and successful responses will always contain a non-nil body.

The suggested modification is:

res, err := client.Do(req)

if err != nil {
    return nil, err
}
defer res.Body.Close()

By checking for the err before attempting to use res.Body, you can gracefully handle any errors that might have occurred during the request and avoid the panic.

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