Pointer from C is removed from memory - c

Question

In this situation, I pass a pointer from Go to a C function, the C function modifies that pointer value (fills in an array) and I use the same pointer again on the Go code, making sure to call C.free to release it after I am done.
I am sometimes getting a nil reference on that pointer, I am having a hard time understanding why.
Here, the object in Go is cpuTimesC, in C it is the cputicks.
I have also tried making the function return the pointer, with the same results. The weirdest thing is that if I put a printf statement at the end of the function, it takes longer before I eventually get the nil error.
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
}
The error is:
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
And I know it happens here, because for some reason cpuTimesC is nil:
cput := (*[maxCPUTimesLen]C.u_longlong_t)(unsafe.Pointer(cpuTimesC))[:cpuTimesLength:cpuTimesLength]
Why would this object be nil sometimes, and how do I make it remain in memory until I call that C.free?
This is on AIX PPC64 with cgo if that makes any difference.

Answer 1

This had nothing to do with pointers being removed from memory, the cputicks was NULL because ncpus was -1 so the malloc call returned NULL, ncpu was -1 because the first parameter of the perfstat_cpu call was not initialized properly, adding:
strcpy(firstcpu.name, FIRST_CPU);
Before the call to perfstat_cpu fixed the issue, as suggested by #kostik