Why Does Printing a Nested Struct Cause a Stack Overflow?

Excessive Stack Usage During Nested Struct Printing

Nested Struct and String() Method

Consider the following nested struct:

<code class="go">type ConfigOne struct {
    Daemon daemon
}
type daemon struct {
    Loglevel int
    Logfile string
}</code>

A String() string method converts the struct elements into a string:

<code class="go">func (c ConfigOne) String() string {
    return fmt.Sprintf("%+v\n", c)
}</code>

Recursion and Stack Overflow

When printing the nested struct with c.String(), the following error occurs:

runtime: goroutine stack exceeds 1000000000-byte limit
fatal error: stack overflow

This error indicates excessive stack usage due to recursion. The recursion is caused by the % v format, which invokes String() for the nested daemon struct. This process repeats indefinitely, leading to a stack overflow.

Solution

To resolve the recursion, avoid using % v in the String() method. Instead, construct the string manually, as demonstrated below:

<code class="go">func (c ConfigOne) String() string {
    //return fmt.Sprintf("%+v\n", c.Daemon.Loglevel)
    return fmt.Sprintf("%+v\n", c.Daemon)
}</code>

By explicitly formatting the daemon struct, we avoid the infinite recursion and ensure that the intended data is output.

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