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#define _LARGEFILE64_SOURCE /* See feature_test_macros(7) */

       //void *mmap(void *addr, size_t length, int prot, int flags,/int fd, off_t offset);

       //int munmap(void *addr, size_t length);

       //void *memmove(void *dest, const void *src, size_t n);

//size_t pagesize = getpagesize();

//printf("System page size: %lu bytes\n", pagesize);

//char * region = mmap(

#include <stdio.h>

#include <stdlib.h>

#include <sys/types.h>

#include <sys/stat.h>

#include <unistd.h>

#include <fcntl.h>

#include <sys/mman.h>

#include <string.h>

#include <errno.h>

#define EXTENDSIZE 8

#define FILESIZE (NUMINTS * sizeof(int))

int

main(

  int argc,

  char **argv)

{

  int i;

  int fd;

  char *map;  /* mmapped array of char * */

  long extendSize;

  if (argc!=3) {

    fprintf(stderr,"?Require 2 param: filename spaceToInsert\n");

    exit(EXIT_FAILURE);

  }

  /* Open a file for writing.

   *  - Creating the file if it doesn't exist.

   *  - Truncating it to 0 size if it already exists. (not really needed)

   *

   * Note: "O_WRONLY" mode is not sufficient when mmaping.

   */

  fd = open(argv[1], O_RDWR, 0);    // | O_CREAT | O_TRUNC, (mode_t)0600);

  if (fd == -1) {

    perror("Error opening file for writing");

    exit(EXIT_FAILURE);

  }

  extendSize = atoi( argv[2] );

  if ( extendSize <= 0 ) {

     perror( "Extend size must be positive integer" );

     exit( EXIT_FAILURE );

  }

  struct stat fsize { 0 };

  if (fstat(fd, &fsize) < 0) {

    int saveErrno = errno;

    fprintf(stderr, "fstat(%d) returned errno=%d.", fd, saveErrno);

    exit(EXIT_FAILURE);

  }

  fprintf(stderr, "%%sizeof fstat.off_t is %d\n", sizeof(fsize.st_size));

  off_t eopos = lseek(fd, 0, SEEK_END); 

  if (eopos == -1) {

    close(fd);

    perror("Error calling lseek() to get file length");

    exit(EXIT_FAILURE);

  }

  fprintf(stderr, "%%Sought size is %ld\n", eopos);

  fprintf(stderr, "%%Adding head-end %ld bytes\n", extendSize );

  //result = lseek(fd, 0, SEEK_END);

  off_t neweopos = eopos + extendSize - 1;

  off_t seekres = lseek(fd, neweopos, SEEK_SET);

  if (seekres == -1) {

    close(fd); 

    perror("Error calling lseek() to 'expand' the file");

    exit(EXIT_FAILURE);

  } 

  int result = write(fd, "", 1); //Write a null bytes to expand file

  if (result != 1) {

    close(fd);

    perror("Error writing last byte of the file");

    exit(EXIT_FAILURE);

  }

  neweopos++;

  /* Now the file is ready to be mmapped.

   */

  map = (char*)mmap(0, neweopos, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);

  if (map == MAP_FAILED) {

    close(fd);

    perror("Error mmapping the file");

    exit(EXIT_FAILURE);

  }

  /* Now move the file contents forward (just leave the original junk hanging around at beginning of file)

   */

  memmove(map + extendSize, map, eopos);

  /* free the mmapped memory

   */

  if (munmap(map, neweopos) == -1) {

    perror("Error un-mmapping the file");

    exit(EXIT_FAILURE);

    /* Decide here whether to close(fd) and exit() or not. Depends... */

  }

  // zero out the first extendSize bytes of file

  if ( fseek( fd, 0, SEEK_SET ) !== 0 ) {

     perror( "Error getting to beginning of file" );

     exit();

  }

  // write zeros

  while ( extendSize-- ) {

     fputc( 0, fd );

   }

  /* Un-mmaping doesn't close the file, so we still need to do that.

   */

  close(fd);

  return 0;

}