unistd read() 不起作用

Question

我正在尝试使用 unistd 库写入/读取文本文件。稍后我会把它部署到/dev路径下的一个文件中。我正在尝试使用 std::vector 类型而不是 char buffer[]。所以我写了两个实现，一个使用向量，另一个使用字符缓冲区。两者在文件中都写得很好，但都不能读取文件。

这是我的测试代码：

使用字符缓冲区

void tests_using_buffer_of_char(const std::string &path)
{
    int file_descriptor_char;   
    char buffer[8] ={0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08};
    std::cout << sizeof(buffer) << std::endl;       

    for(auto i = 0; i < sizeof(buffer); i++)
        std::cout << buffer[i] << std::endl;

    file_descriptor_char = open(path.c_str(), O_RDWR);
    if(file_descriptor_char != -1)
        std::cout << "open with success " << path << std::endl;

    std::size_t size = strlen(buffer);
    ssize_t write_bytes = write(file_descriptor_char, buffer, size);
    std::cout << size << " <-- size of buffer " << write_bytes << " <-- size of writed bytes" <<  std::endl;

    memset(&buffer[0], 0, sizeof(buffer));

    ssize_t read_bytes = read(file_descriptor_char, &buffer, size);
    std::cout << size << " <-- size of buffer " << read_bytes << " <-- size of readed bytes" <<  std::endl;

    for(auto i = 0; i < sizeof(buffer); i++)
        std::cout << buffer[i] << std::endl;

    if(close(file_descriptor_char) != -1)
        std::cout << "close with success "<< path << std::endl;
}

使用向量缓冲区

void tests_using_buffer_of_vector(const std::string &path)
{
    int file_descriptor_vector;
    std::vector<char> buffer = {0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08};
    std::cout << buffer.size() << std::endl;

    for(auto i : buffer)
        std::cout << i << std::endl;

    file_descriptor_vector = open(path.c_str(), O_RDWR);
    if(file_descriptor_vector != -1)
        std::cout << "open whit success " << path << std::endl;

    ssize_t write_bytes = write(file_descriptor_vector, buffer.data(), buffer.size());
    std::cout << buffer.size() << " <-- size of buffer " << write_bytes << " <-- size of writed bytes" <<  std::endl;   

    buffer.clear();     

    ssize_t read_bytes = read(file_descriptor_vector, buffer.data(), write_bytes);
    std::cout << buffer.size() << " <-- size of buffer " << read_bytes << " <-- size of readed bytes" <<  std::endl;    

    for(auto i : buffer)
        std::cout << i << std::endl;

    if(close(file_descriptor_vector) != -1)
        std::cout << "close with success " << path << std::endl;

}

谢谢大家！

Answer 1

您必须将文件描述符重置为文件的开头，然后才能在将其用于写入后尝试重新使用它进行读取。您可以关闭文件再重新打开，也可以从头搜索。

lseek(file_descriptor_vector, SEEK_SET, 0);

读入之前不要清除向量。 clear 方法实质上将释放与向量关联的内存。

Answer 2

1] 只需使用 fclose 并重新打开文件。

2] fseek(fp, 0, SEEK_SET);

3]rewind(fp);

您正指向带有 file pointer 的文件末尾。

unistd read() 不起作用

unistd read() doesn't work

c++

c++11

vector

unistd.h