dongmaobeng7145
2019-01-17 10:44
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从Windows交叉编译到Linux时出错?

When cross-compiling my code from Win10 to Linux, the following code stopped using rows.next

    executed rows.Next()

    rows, err := DB.Query("SELECT * FROM `machines`")
    erh.Check(err)

    newUsers := make(map[string]User, count)
    defer erh.Check(rows.Close())    
    for rows.Next() {
        //tu is temp user
        tu := User{}
        err := rows.Scan(&tu.Id, &tu.Name, &tu.Apikey, &tu.Ip, &tu.Machine, &tu.State, &tu.DbAvailable)
        erh.Check(err)
        log.Println(tu)
        newUsers[tu.Apikey] = tu
    }

When I placed the defer under rows.Next the rows.next code was executed

rows, err := DB.Query("SELECT * FROM `machines`")
erh.Check(err)

newUsers := make(map[string]User, count)

for rows.Next() {
    //tu is temp user
    tu := User{}
    err := rows.Scan(&tu.Id, &tu.Name, &tu.Apikey, &tu.Ip, &tu.Machine, &tu.State, &tu.DbAvailable)
    erh.Check(err)
    log.Println(tu)
    newUsers[tu.Apikey] = tu
}
defer erh.Check(rows.Close())

Can someone confirm this is a bug, or am I missing something here?

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1条回答 默认 最新

  • douni9620 2019-01-17 14:05
    已采纳

    I'm not sure how this could work on Linux, it shouldn't. This line:

        defer erh.Check(rows.Close())    
    

    Will execute rows.Close() immediately, hold the return value, then defer only the call to erh.Check. That's how defer works - arguments are evaluated immediately. From the docs:

    Each time a "defer" statement executes, the function value and parameters to the call are evaluated as usual and saved anew but the actual function is not invoked.

    If you want to use a construct like this, it would have to be within a closure, something like:

    defer func() {
        erh.Check(rows.Close())
    }()
    

    So that there are no arguments to evaluate at the time of defer.

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