c - 出现段错误(核心已转储)

Question

我在执行下面的代码时遇到段错误。程序编译成功，但在执行时出现错误。什么是段错误以及如何纠正它？

#include<stdio.h>
#include<malloc.h>
#include "sel.h"
#include<sys/time.h>
main(int argc,char *argv[])
{

    struct timeval t1,t2;
    struct timezone tz;
    int i,n;
    int *a;

    char *num;
    FILE *fp;
    unsigned long long time1,time2;

    fp=fopen("file1.txt","w");
    sscanf(argv[1],"%d",&n);
    a=(int*)malloc (sizeof(int)*n);
    for(i=n;i>0;i--)
    {
        sprintf(num,"%d\n",i);
        fputs(num,fp);
    }
    fclose(fp);
    fp=fopen("file1.txt","r");
    i=0;
    while(fgets(num,255,fp)!=NULL)
    {
        sscanf(num,"%d",&a[i++]);
    }

    gettimeofday(&t1,&tz);
    time1=t1.tv_sec*1000000+t1.tv_usec;
    sel(a,n);
    gettimeofday(&t2,&tz);
    time2=t2.tv_sec*1000000+t2.tv_usec;
    fclose(fp);
    fp=fopen("file2","w");
    for(i=0;i<n;i++)
    {
        sprintf(num,"%d\n",a[i]);
        fputs(num,fp);
    }
    free(a);
    printf("\n %llu",time2-time1);
}

Answer 1

段错误意味着您的内存访问错误。在这种情况下，您从未为 num 分配任何内存，因此您正在写入未初始化的指针。