c++ - 一个关于fread的奇怪问题，offset与文档不符

Question

//encrypt data
void EncryptBlock(unsigned char*& blockdata, size_t n)
{
}

//encrypt file
bool EncryptFile(const char* path, size_t blocksize)
{
    FILE* fp = NULL;
    auto erno = fopen_s(&fp, path, "rb+");
    if (erno != 0)
    {
        printf("openfile:[%s] fail!!, errno=%d\n", path, erno);
        return false;
    }

    //get file size
    _fseeki64(fp, 0, SEEK_END);
    int64_t filesize = _ftelli64(fp);
    _fseeki64(fp, 0, SEEK_SET);
    printf("filesize:%lld, %lldmb\n", filesize, filesize / (1024 * 1024));

    int64_t processed = 0;
    //buffer
    unsigned char* blockdata = new  unsigned char[blocksize];
    while (1)
    {
        int64_t currpos = _ftelli64(fp);
        //printf("curr:%lld, size=%lldmb\n", currpos, currpos / (1024 * 1024));

        //read and set offset 
        size_t readsize = fread(blockdata, 1, blocksize, fp);
        int64_t currpos2 = _ftelli64(fp);
        if (currpos + readsize != currpos2)
        {
            return false;
        }
        _fseeki64(fp, -1 * readsize, SEEK_CUR);

        //encrypt
        EncryptBlock(blockdata, readsize);

        //write
        size_t writesize = fwrite(blockdata, 1, readsize, fp);
        if (writesize != readsize)
        {
            printf("write:writesize=%llu, readsize=%llu, warning!!!!!!!!!!!\n", writesize, readsize);
        }
        processed += writesize;

        if (readsize != blocksize)
        {
            printf("readsize=%llu, block=%llu, break[processd:%llu]\n", readsize, blocksize, processed);
            break;
        }
    }
    delete[] blockdata;
    fclose(fp);

    //error
    if (processed != filesize)
    {
        printf("processed(%lld) != filesize(%lld), warning!!!!!!!!!!!\n", processed, filesize);
        return false;
    }
    else
    {
        return true;
    }
}
int main(int argc, char *argv[])
{
    auto tv1 = GetTickCount64();
    auto suc = EncryptFile("d:\\soft.msi", 1024 * 102);
    auto tv2 = GetTickCount64();

    auto used = (tv2 - tv1);
    printf("encrypt：%s, cost：%llums\n", suc ? " succ" : " fail", used);
    getchar();
    return 0;
}

请使用大于20MB的文件

EncryptFile("path", 1024 * 1024) 或 EncryptFile("path", 1024 * 4) 正确

EncryptFile("path", 1024 * 102) 不正确

第二个周期将进入if (currpos + readsize != currpos2)

我使用windows10(vs2015)和Ubuntu20(gcc 9.3.0)也是同样的现象

https://i.loli.net/2021/07/08/Tdt4ocLsw9Mgn3R.gif

我想错误的原因应该是我对fread、fwrite和ftell的误解。 microsoft.docs.fread说The file pointer associated with stream (if there is one) is increased by the number of bytes actually read.

Answer 1

您需要在 fwrite 和 fread 之间使用 fseek。

The standard says

output shall not be directly followed by input without an intervening call to the fflush function or to a file positioning function (fseek, fsetpos, or rewind), and input shall not be directly followed by output without an intervening call to a file positioning function, unless the input operation encounters end- of-file.