c++ - fopen - 不能写超过 16K？

Question

我目前正在使用 fopen 来写入/读取二进制文件。对于小文件，一切都很好。但在某些情况下，当“恰好”内容大于 16K 时，文件的其余部分无效!!!

代码很简单，fopen ... fread/fwrite ... fflush ... fclose !

我尝试过使用 C++。但是现在我在“阅读”期间遇到了问题

在 BinaryDefaultRead 中返回 -1 !!!但是真的不知道为什么!我一次只写 4 个字节!!!

在 Win7 64 位下使用 MSVC 2008 编译器。

#include <fstream>

using namespace std;

size_t BinaryDefaultRead(ifstream& stream, void* buffer, unsigned int bufferSize)
{
    //return fread(buffer, 1, (size_t) bufferSize, file);
    stream.read((char*)buffer, bufferSize);
    if (!stream)
        return -1;

    return bufferSize;
}

size_t BinaryDefaultWrite(ofstream& stream, const void* buffer, unsigned int bufferSize)
{
    //return fwrite(buffer, 1, (size_t) bufferSize, file);
    stream.write((char*)buffer, bufferSize);
    if (!stream)
        return -1;

    return bufferSize;
}

// Read an unsigned integer from a stream in a machine endian independent manner (for portability).
size_t BinaryReadUINT(ifstream& stream, unsigned int* value)
{
    unsigned char buf[4];
    size_t result = BinaryDefaultRead(stream, (void *)buf, 4);

    if (result < 0)
        return result;

    *value = ((unsigned int) buf[0]) |
    (((unsigned int) buf[1]) << 8) |
    (((unsigned int) buf[2]) << 16) |
    (((unsigned int) buf[3]) << 24);

    return result;
}

// Write an unsigned integer to a stream in a machine endian independent manner (for portability).
size_t BinaryWriteUINT(ofstream& stream, unsigned int aValue)
{
    unsigned char buf[4];
    buf[0] = aValue & 0x000000ff;
    buf[1] = (aValue >> 8) & 0x000000ff;
    buf[2] = (aValue >> 16) & 0x000000ff;
    buf[3] = (aValue >> 24) & 0x000000ff;

    return BinaryDefaultWrite(stream, (void*)buf, 4);
}

// Read a floating point value from a stream in a machine endian independent manner (for portability).
size_t BinaryReadFLOAT(ifstream& stream, float* value)
{
    union {
        float f;
        unsigned int  i;
    } u;
    size_t result = BinaryReadUINT(stream, &u.i);

    if (result < 0)
        return result;

    *value = u.f;

    return result;
}

// Write a floating point value to a stream in a machine endian independent manner (for portability).
size_t BinaryWriteFLOAT(ofstream& stream, float aValue)
{
    union {
        float f;
        unsigned int  i;
    } u;
    u.f = aValue;
    return BinaryWriteUINT(stream, u.i);
}

size_t BinaryReadUINTArray(ifstream& stream, unsigned int* buffer, unsigned int count)
{
    size_t result;
    for(unsigned int i = 0; i < count; i++)
    {
        result = BinaryReadUINT(stream, buffer + i);
        if (result < 0)
            return result;
    }

    return result;
}

size_t BinaryWriteUINTArray(ofstream& stream, unsigned int* buffer, unsigned int count)
{
    size_t result;
    for(unsigned int i = 0; i < count; i++)
    {
        result = BinaryWriteUINT(stream, buffer[i]);
        if (result < 0)
            return result;
    }

    return result;
}

size_t BinaryReadFLOATArray(ifstream& stream, float* buffer, unsigned int count)
{
    size_t result;
    for(unsigned int i = 0; i < count; i++)
    {
        result = BinaryReadFLOAT(stream, buffer + i);
        if (result < 0)
            return result;
    }

    return result;
}

size_t BinaryWriteFLOATArray(ofstream& stream, float* buffer, unsigned int count)
{
    size_t result;
    for(unsigned int i = 0; i < count; i++)
    {
        result = BinaryWriteFLOAT(stream, buffer[i]);
        if (result < 0)
            return result;
    }

    return result;
}

Answer 1

fopen 仅用于打开文件流，不用于读取或写入。 fread 和 fwrite 用于执行此操作。

fwrite 和 fread 不能确保您写入传递给它们的所有元素:它们返回写入的元素数量，这可能小于你传递给它的元素。

只需检查返回值，并保持 fwrite-ing 直到您写出所有元素或直到流出现错误:使用 ferror 来检查一个错误。

---

来自 fwrite 手册:

fread() and fwrite() return the number of items successfully read or written (i.e., not the number of characters). If an error occurs, or the end-of-file is reached, the return value is a short item count (or zero).

fread() does not distinguish between end-of-file and error, and callers must use feof(3) and ferror(3) to determine which occurred.