Question

我正在测试命令fread（）和fseek，我制作了一个程序来比较程序的第二个字节。该程序在指定的行（最后一个fread（））处给出了分段错误。

int main(int argc, char** argv)
{
    FILE *file1, *file2;
    long size1, size2;
    long size1a, size2a;
    char *name1, *name2;
    char *temp1, *temp2;

    if (argc != 3)
    {
        cout<<"Enter two file names"<<endl;
        exit(1);
    }

    name1 = argv[1];
    name2 = argv[2];

    //open files
    file1 = fopen(name1, "r");
    file2 = fopen(name2, "r");

    //get file size
    fseek(file1, 0, SEEK_END);
    size1 = ftell(file1);
    rewind(file1);

    fseek (file2,0,SEEK_END);
    size2 = ftell(file2);
    rewind(file2);

    fseek(file1,1,SEEK_SET);
    fseek(file2,1,SEEK_SET);
    cout<<"1"<<endl; //----cout worked
    fread(temp1,1,1,file1);
    cout<<"2"<<temp1<<endl; //---cout worked. 2nd byte of file1 was printed.
    fread(temp2,1,1,file2); //SEGMENTATION FAULT AT THIS LINE

    if(*temp1==*temp2)
        cout<<"same"<<endl;
    else
        cout<<"different"<<endl;    

    return 0;
}

虽然我通过定义

来纠正了程序

char temp1, temp2;

并撰写

fread(&temp1,1,1,file1);
fread(&temp2,1,1,file2);

我仍然不知道是什么在第二个fread中给出了seg错误，而第一个运行正确。可能是分段错误的原因是什么？

Answer 1

1）以二进制模式打开文件`fp = fopen（name，“rb”）;

2）检查文件打开if(fp != NULL)

的结果

3）为数据分配内存：char *temp1;是不够的，它只是指针，你需要这样的东西：

  temp = (char*) malloc(sizeof(char) * N); // N number of bytes

修改

4）不要忘记关闭文件（即使它不是分段错误的原因）。

我只是进行了更改，以下代码可以正常运行：

#include <stdio.h> #include <stdlib.h> #include <iostream> using namespace std; int main(int argc, char** argv) { FILE *file1, *file2; long size1, size2; long size1a, size2a; long greater; char *name1, *name2; char *temp1, *temp2; // memory allocation (two lines added) temp1 = (char*) malloc(sizeof(char)); temp2 = (char*) malloc(sizeof(char)); if (argc != 3) { cout<<"Enter two file names"<<endl; exit(1); } name1 = argv[1]; name2 = argv[2]; //open files file1 = fopen(name1, "rb"); if(file1 == NULL) { cout<<"File "<< name1 << " cannot be read" <<endl; exit(1); } file2 = fopen(name2, "rb"); if(file2 == NULL) { cout<<"File "<< name2 << " cannot be read" <<endl; exit(1); } //get file size fseek(file1, 0, SEEK_END); size1 = ftell(file1); rewind(file1); fseek (file2,0,SEEK_END); size2 = ftell(file2); rewind(file2); if(size1>size2) greater = size1; else greater = size2; fseek(file1,1,SEEK_SET); fseek(file2,1,SEEK_SET); cout<<"1"<<endl; //----cout worked fread(temp1,1,1,file1); cout<<"2"<<temp1<<endl; //---cout worked. 2nd byte of file1 was printed. fread(temp2,1,1,file2); //SEGMENTATION FAULT AT THIS LINE if(*temp1==*temp2) cout<<"same"<<endl; else cout<<"different"<<endl; fclose(file1); fclose(file2); // memory de-allocation (two lines added) free(temp1); free(temp2); return 0; }

<强> EDIT2：

5）使用后取消分配内存（再添加两行）

Answer 2

第一个版本的问题是temp指针并不指向任何内容：

char *temp1, *temp2;
...
fread(temp1,1,1,file1);

有了这个，文件中的字符被放置在temp指针所指向的随机位置。

对于第二个版本，字符放在temp变量内，这没关系。

比较两个文件的第二个字节时出现分段错误

2 个答案: