lseek() in C/C++ to read the alternate nth byte and write it in another file
From a given file (e.g. input.txt) read the alternate nth byte and write it on another file with the help of “lseek”.
lseek (C System Call): lseek is a system call that is used to change the location of the read/write pointer of a file descriptor. The location can be set either in absolute or relative terms.
Function Definition
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.
For example, say our Input file is as follows:
// C program to read nth byte of a file and // copy it to another file using lseek #include <stdio.h> #include <unistd.h> #include <sys/types.h> #include <fcntl.h> 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; } |
Output file (end.txt)
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