runtime error: invalid memory address or nil pointer dereference

本篇文章主要是结合我之前面试的各种经历和实战开发中遇到的问题解决经验整理的，希望这篇《runtime error: invalid memory address or nil pointer dereference》对你有很大帮助！欢迎收藏，分享给更多的需要的朋友学习~

问题内容

agent.go文件

package db

import (
    "bytes"
    "fmt"
    "github.com/open-falcon/common/model"
    "github.com/open-falcon/hbs/g"
    "log"
    "net/http"
)

func UpdateAgent(agentInfo *model.AgentUpdateInfo) {
    sql := ""
    if g.Config().Hosts == "" {
        sql = fmt.Sprintf(
            "insert into host(hostname, ip, agent_version, plugin_version) values ('%s', '%s', '%s', '%s') on duplicate key update ip='%s', agent_version='%s', plugin_version='%s'",
            agentInfo.ReportRequest.Hostname,
            agentInfo.ReportRequest.IP,
            agentInfo.ReportRequest.AgentVersion,
            agentInfo.ReportRequest.PluginVersion,
            agentInfo.ReportRequest.IP,
            agentInfo.ReportRequest.AgentVersion,
            agentInfo.ReportRequest.PluginVersion,
        )
    } else {
        // sync, just update
        sql = fmt.Sprintf(
            "update host set ip='%s', agent_version='%s', plugin_version='%s' where hostname='%s'",
            agentInfo.ReportRequest.IP,
            agentInfo.ReportRequest.AgentVersion,
            agentInfo.ReportRequest.PluginVersion,
            agentInfo.ReportRequest.Hostname,
        )
    }

    defer func() {
        if err := recover(); err != nil {
            log.Panicln("出错了:", err)
        }
    }()
    _, err := DB.Exec(sql)
    if err != nil {
        log.Println("exec", sql, "fail", err)
    } else {
        url := g.Config().GroupHostAddUrl
        fmt.Println(url)
        var hosts, group_name string
        hosts = agentInfo.ReportRequest.Hostname
        group_name = g.Config().DefaultHostGroup
        var jsonStr = []byte(`{"hosts":"` + hosts + `","group_name":"` + group_name + `"}`)
        req, err := http.NewRequest("POST", url, bytes.NewBuffer(jsonStr))
        req.Header.Set("Content-Type", "application/json")
        client := &http.Client{}
        resp, err := client.Do(req)
        defer resp.Body.Close()
        if err != nil {
            log.Println("Post To Portal API Fail:", sql, err)
        }
    }
}

程序运行一段时间后报错：

panic: 出错了: runtime error: invalid memory address or nil pointer dereference

[signal 0xb code=0x1 addr=0x0 pc=0x462123]

goroutine 13174364 [running]:
log.Panicln(0xc208a11690, 0x2, 0x2)
        /usr/local/go/src/log/log.go:321 +0xb9
github.com/open-falcon/hbs/db.func·001()
        /home/work/workspace/src/github.com/open-falcon/hbs/db/agent.go:38 +0x12f
github.com/open-falcon/hbs/db.UpdateAgent(0xc2084aff50)
        /home/work/workspace/src/github.com/open-falcon/hbs/db/agent.go:55 +0xc73
github.com/open-falcon/hbs/cache.(*SafeAgents).Put(0xc20806a0a0, 0xc2085af5c0)
        /home/work/workspace/src/github.com/open-falcon/hbs/cache/agents.go:31 +0xa4
github.com/open-falcon/hbs/rpc.(*Agent).ReportStatus(0xc20813fa60, 0xc2085af5c0, 0xc209e3ede8, 0x0, 0x0)
        /home/work/workspace/src/github.com/open-falcon/hbs/rpc/agent.go:31 +0x73
reflect.Value.call(0x755320, 0x7b4500, 0x13, 0x7f8b50, 0x4, 0xc208a11f28, 0x3, 0x3, 0x0, 0x0, ...)
        /usr/local/go/src/reflect/value.go:419 +0x10e5
reflect.Value.Call(0x755320, 0x7b4500, 0x13, 0xc208a11f28, 0x3, 0x3, 0x0, 0x0, 0x0)
        /usr/local/go/src/reflect/value.go:296 +0xbc
net/rpc.(*service).call(0xc2080a0280, 0xc2080a0240, 0xc208109620, 0xc2080ae200, 0xc208917100, 0x769160, 0xc2085af5c0, 0x16, 0x769520, 0xc209e3ede8, ...)
        /usr/local/go/src/net/rpc/server.go:382 +0x1f7
created by net/rpc.(*Server).ServeCodec
        /usr/local/go/src/net/rpc/server.go:476 +0x44a

正确答案

问题出在

        resp, err := client.Do(req)
        defer resp.Body.Close()

很简单，client.Do失败了，err不为nil，此时resp.Body为nil，但是又要强制调用它的close函数，于是报错。

只讨论代码的话，应该这样改：

        resp, err := client.Do(req)
        if err != nil {
            resp.Body.Close()
        }

但是，根本原因是，你的服务端出问题了。

去看一下是没启动还是怎么了……

另外，有可能不是“运行一段时间后报错”，
有可能是刚开始就会出错，只不过还没等到client.Do(req)的超时，于是刚开始“看上去是正常的”。

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