C / C++中的lseek()读取备用的第n个字节并将其写入另一个文件

从给定的文件(例如input.txt)中读取第n个备用字节，并借助“ lseek”将其写入另一个文件中。
lseek(C系统调用) ：lseek是一个系统调用，用于更改文件描述符的读/写指针的位置。可以以绝对或相对方式设置位置。
函数定义

off_t lseek(int fildes, off_t offset, int whence);

Field Description
int fildes : The file descriptor of the pointer that is going to be moved
off_t offset : The offset of the pointer (measured in bytes).
int whence : The method in which the offset is to be interpreted
(rela, absolute, etc.). Legal value r this variable are provided at the end.
return value : Returns the offset of the pointer (in bytes) from the
beginning of the file. If the return value is -1,
then there was an error moving the pointer.

例如，说我们的输入文件如下：

// C program to read nth byte of a file and
// copy it to another file using lseek
#include 
#include 
#include 
#include 
  
void func(char arr[], int n)
{
    // Open the file for READ only.
    int f_write = open("start.txt", O_RDONLY);
  
    // Open the file for WRITE and READ only.
    int f_read = open("end.txt", O_WRONLY);
  
    int count = 0;
    while (read(f_write, arr, 1))
    {
        // to write the 1st byte of the input file in
        // the output file
        if (count < n)
        {
            // SEEK_CUR specifies that
            // the offset provided is relative to the
            // current file position
            lseek (f_write, n, SEEK_CUR);
            write (f_read, arr, 1);
            count = n;
        }
  
        // After the nth byte (now taking the alternate
        // nth byte)
        else
        {
            count = (2*n);
            lseek(f_write, count, SEEK_CUR);
            write(f_read, arr, 1);
        }
    }
    close(f_write);
    close(f_read);
}
  
// Driver code
int main()
{
    char arr[100];
    int n;
    n = 5;
  
    // Calling for the function
    func(arr, n);
    return 0;
}

输出文件(end.txt)

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