比較兩個文件的第二個字節時出現分段錯誤

Question

我正在測試命令fread（）和fseek，我制作了一個程序來比較程序的第二個字節。 該程序在指示的行（最后一個fread（））給出了分段錯誤。

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;
}

雖然我通過定義來糾正程序

char temp1, temp2;

和寫作

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

我仍然不知道是什么給了第二個fread的seg錯誤，而第一個運行正確。 可能是分段錯誤的原因是什么？

Answer 1

1）以二進制模式打開文件`fp = fopen（name，“rb”）;

2）檢查文件打開的結果if(fp != NULL)

3）為數據分配內存： char *temp1; 是不夠的，它只是指針，你需要這樣的東西：

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

編輯：

4）不要忘記關閉文件（即使它不是分段錯誤的原因）。

我只是進行更改，以下代碼可以正常工作：

#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;
}

EDIT2：

5）使用后取消分配內存（增加兩行）

Answer 2

第一個版本的問題是temp指針不指向任何東西：

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

有了這個，文件中的字符被放置在temp指針所指向的隨機位置。

對於第二個版本，字符放在temp變量中，這沒關系。