unistd read（）不起作用

Question

I am trying to write / read text files using the unistd library. I will later deploy it to a file in the / dev path. I'm trying to use std::vector type instead of char buffer[] . So I wrote two implementations, one using vector and another char buffer. Both write well in the file, but neither can read the file.

Here is my test code:

using buffer of char

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;
}

using buffer of vector

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;

}

Thanks everyone!

Answer 1

You have to reset your file descriptor to be at the beginning of the file before trying to reuse it for reading after you used it for writing. You could close the file and reopen it, or you can seek to the beginning.

lseek(file_descriptor_vector, SEEK_SET, 0);

Don't clear your vector before reading into it. the clear method will essentially deallocate the memory associated with the vector.

Answer 2

1] Just use fclose and reopen the file.

2] fseek(fp, 0, SEEK_SET);

3] rewind(fp);

You are pointing to the end of the file with file pointer .