c - C 中的指针从内存中删除

Question

在这种情况下，我将指针从 Go 传递到 C 函数，C 函数修改该指针值(填充数组)，然后我在 Go 代码中再次使用相同的指针，确保调用 C.free 在我完成后释放它。

我有时在该指针上获得零引用，我很难理解为什么。

这里，Go中的对象是cpuTimesC，在C中是cputicks。

我也尝试过让函数返回指针，结果相同。最奇怪的是，如果我在函数末尾放置 printf 语句，则需要更长的时间才能最终得到 nil 错误。

package collector

/*
#cgo LDFLAGS: -lperfstat
#include <stdlib.h>
#include <stdio.h>
#include <libperfstat.h>
#include <string.h>
#include <time.h>

u_longlong_t **ref;

int getCPUTicks(uint64_t **cputicks, size_t *cpu_ticks_len) {
    int i, ncpus, cputotal;
    perfstat_id_t firstcpu;
    perfstat_cpu_t *statp;

    cputotal = perfstat_cpu(NULL, NULL, sizeof(perfstat_cpu_t), 0);
    if (cputotal <= 0){
        return -1;
   }

    statp = calloc(cputotal, sizeof(perfstat_cpu_t));
    if(statp==NULL){
            return -1;
    }
    ncpus = perfstat_cpu(&firstcpu, statp, sizeof(perfstat_cpu_t), cputotal);
    *cpu_ticks_len = ncpus*4;

    *cputicks = (uint64_t *) malloc(sizeof(uint64_t)*(*cpu_ticks_len));
    for (i = 0; i < ncpus; i++) {
        int offset = 4 * i;
        (*cputicks)[offset] = statp[i].user;
        (*cputicks)[offset+1] = statp[i].sys;
        (*cputicks)[offset+2] = statp[i].wait;
        (*cputicks)[offset+3] = statp[i].idle;
    }
    return 0;
}
*/
import "C"

import (
    "errors"
    "unsafe"
    "fmt"
    "github.com/prometheus/client_golang/prometheus"
)

const ClocksPerSec = float64(C.CLK_TCK)
const maxCPUTimesLen = 1024 * 4

type statCollector struct {
    cpu *prometheus.Desc
}

func init() {
    registerCollector("cpu", true, NewCPUCollector)
}

func NewCPUCollector() (Collector, error) {
    return &statCollector{
        cpu: nodeCPUSecondsDesc,
    }, nil
}

func (c *statCollector) Update(ch chan<- prometheus.Metric) error {
    var fieldsCount = 4
    cpuFields := []string{"user", "sys", "wait", "idle"}

    var (
        cpuTimesC       *C.uint64_t
        cpuTimesLength  C.size_t
    )

    if C.getCPUTicks(&cpuTimesC, &cpuTimesLength) == -1 {
        return errors.New("could not retrieve CPU times")
    }
    defer C.free(unsafe.Pointer(cpuTimesC))
    cput := (*[maxCPUTimesLen]C.u_longlong_t)(unsafe.Pointer(cpuTimesC))[:cpuTimesLength:cpuTimesLength]

    cpuTicks := make([]float64, cpuTimesLength)
    for i, value := range cput {
        cpuTicks[i] = float64(value) / ClocksPerSec
    }

    for i, value := range cpuTicks {
        cpux := fmt.Sprintf("CPU %d", i/fieldsCount)
        ch <- prometheus.MustNewConstMetric(c.cpu, prometheus.CounterValue, value, cpux, cpuFields[i%fieldsCount])
    }

    return nil
}

错误是:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x33 addr=0x0 pc=0x1003fcec0]

goroutine 940 [running]:
github.com/dlopes7/aix-prometheus-exporter/collector.(*statCollector).Update(0xa000100000d21b8, 0xa0001000028c480, 0x0, 0x0)
        /home/david/go/src/github.com/dlopes7/aix-prometheus-exporter/collector/cpu_aix.go:81 +0xf0
github.com/dlopes7/aix-prometheus-exporter/collector.execute(0x10043e7e7, 0x3, 0x1101120e0, 0xa000100000d21b8, 0xa0001000028c480)
        /home/david/go/src/github.com/dlopes7/aix-prometheus-exporter/collector/collector.go:95 +0x6c
github.com/dlopes7/aix-prometheus-exporter/collector.AIXCollector.Collect.func1(0xa0001000028c480, 0xa000100000cd440, 0x10043e7e7, 0x3, 0x1101120e0, 0xa000100000d21b8)
        /home/david/go/src/github.com/dlopes7/aix-prometheus-exporter/collector/collector.go:115 +0x4c
created by github.com/dlopes7/aix-prometheus-exporter/collector.AIXCollector.Collect
        /home/david/go/src/github.com/dlopes7/aix-prometheus-exporter/collector/collector.go:114 +0xf8

我知道它发生在这里，因为由于某种原因 cpuTimesC 为零:

cput := (*[maxCPUTimesLen]C.u_longlong_t)(unsafe.Pointer(cpuTimesC))[:cpuTimesLength:cpuTimesLength]

为什么这个对象有时会为零，如何让它保留在内存中，直到我调用 C.free？

这是在 AIX PPC64 上使用 cgo(如果有什么区别的话)。

Answer 1

这与从内存中删除指针无关，cputicks 为 NULL，因为 ncpus 为 -1，因此 malloc 调用返回 NULL，ncpu 为 -1，因为 perfstat_cpu 调用的第一个参数未正确初始化，添加:

strcpy(firstcpu.name, FIRST_CPU);

按照 @kostik 的建议，在调用 perfstat_cpu 修复问题之前