c - 使用 windows.h、C 截断文件

Question

我的任务是创建一个随机访问/二进制列表，并允许程序使用 windows.h 库及其 HANDLE 函数从中删除目标记录(结构)。 我编写了以下代码来尝试截断文件，虽然删除文件末尾的某些内容似乎有效，但中间处理文件的部分似乎遇到了麻烦，我被难住了。

void erase_record(FILE *file, stct buffer, char *target)
{
HANDLE h;
int jump=0;
int mark;
stct s;
file = fopen("list.txt","rb+");
s.artist = (char*)malloc(20*(sizeof(char)));
s.name = (char*)malloc(20*(sizeof(char)));
while(1)
{
    jump++;
    fread(&buffer,sizeof(buffer),1,file);
    if(!strcmp(target,buffer.name)) // did we find a match? 
    {
        jump--;
        fread(&buffer,sizeof(buffer),1,file); // then check one more time.
        if(feof(file)) // is the match right before the eof?
        {
            fclose(file);
            h=CreateFile(L"list.txt",GENERIC_WRITE | GENERIC_READ,0,0,OPEN_ALWAYS,FILE_ATTRIBUTE_NORMAL,0);
            SetFilePointer(h,(jump)*sizeof(buffer),NULL,FILE_BEGIN); // truncate from last
            SetEndOfFile(h);
            CloseHandle(h);
            break;
        }
        else // is it in middle of the file?
        {
            mark = jump; // we mark the struct we wanted to delete
            while(1)
            {
                s.shelf = buffer.shelf;
                s.artist = buffer.artist;
                s.name = buffer.name;
                jump++;
                fread(&buffer,sizeof(buffer),1,file); // find the end of the file first
                if(feof(file))// when we DO find the end..
                {
                    break;
                }
            }
            fclose(file);
            h=CreateFile(L"list.txt",GENERIC_WRITE | GENERIC_READ,0,0,OPEN_ALWAYS,FILE_ATTRIBUTE_NORMAL,0);
            SetFilePointer(h,(jump)*sizeof(buffer),NULL,FILE_BEGIN); //deleting it!
            SetEndOfFile(h);
            CloseHandle(h);
            file = fopen("list.txt","rb+");
            fseek(file,mark*sizeof(buffer),SEEK_SET); // we erase the marked struct
            fwrite(&s,sizeof(s),1,file);
            break;
        }
    }
}

}

任何人都可以分享有关问题所在的任何见解吗？

Answer 1

我认为这个问题与以下内容有关。当您说出以下内容时，一分钱都会掉下来:

Well, i checked with ferror() but it returned 0. The program just crashes after i try running the section i mentioned and after running through a debug tool i get "Unhandled exception at 0x577514cf (msvcr100d.dll) in Project 5.exe: 0xC0000005: Access violation reading location 0xcdcdcdcd."

值0xcdcdcdcd为

"...a debugging value that comes from the bowels of the C runtime library. When you allocate a block of memory in a debug build, it's initialized to this spurious value in the hope of catching bugs. 0xCDCDCDCD is non-NULL and is never a valid memory pointer..." (http://www.microsoft.com/msj/1198/c/c1198.aspx).

这意味着某些东西无法分配内存。

我要做的第一件事是检查 malloc() 语句是否返回 NULL。

接下来，检查fopen()的返回值是否为NULL。

接下来是以下几行...

s.artist = buffer.artist;
s.name = buffer.name;

他们引入了内存泄漏。您在这里不是复制字符串，而是复制指针。因此，您之前使用 malloc() 为 artist 和 name 分配的内存现已丢失，您将永远无法释放() 它。例如，我认为您可能打算使用 strcpy() 将字符串从 buffer 复制到 s.name 中？

此外，您是否正在将指针写入文件？如果您读回 buffer.name 并且它是一个指针(我认为这一定是因为您这样对待它)，那么您正在读回创建文件时有效的任何旧地址， 但在这个实例中，您的程序可能完全无效!尝试取消引用该指针会让您头疼!

我建议，当您将结构写入文件时，不要写入指针s.name和s.artist，而是写入实际的字符串到文件。然后从文件中读回字符串，然后为它们创建指针。

fwrite(&s,sizeof(s),1,file); // OOPS! You wrote pointer values to a file. 
// These pointers will be invalid addresses in the next program instance!

而是做一些类似的事情..

fwrite(all non pointer members of struct)
fwrite(&s.label, LABEL_LEN, 1, file); // Note, include the string null-terminator
fwrite(&s.name, NAME_LEN, 1, file);   // Note, include the string null-terminator

然后，当您阅读时，将标签和名称读回为实际字符串......

fread(all non pointer members of struct)
fread(&stringBuffer, LABEL_LEN, 1, file);
s.label = strdup(stringBuffer);
fread(&stringBuffer, NAME_LEN, 1, file);
s.name= strdup(stringBuffer);

然后记住将 free() s.name 和 s.label 作为 strdup()已创建一个副本，您有责任销毁该副本。