Truncation of a unix file from outside ...

truncate-from-outside.c

/* Can we truncate a file from outside? */
#include <stdio.h>
#include <fcntl.h>
#include <string.h>
#include <errno.h>

int main(int argc, char** argv) {
  /* opens the given argv[1] and writes a statement to it every 5 seconds */
  int fd;

  if ((fd = open(argv[1], O_RDWR|O_CREAT)) == -1) {
    printf("open error: %s\n", strerror(errno));
    return 1;
  }
  char buf[15];
  int i = 0;
  while (i < 10) {
    sprintf(buf, "statement: %3d\n", i++);
    if (write(fd, buf, 15) < 0) {
      printf("write error: %s\n", buf);
      return 1;
    }
    printf("sleeping, truncate the file with something like 'cat /dev/null > argv[1]'\n");
    sleep(5);
  }
  if (close(fd) < 0) {
    printf("close error: %s\n", strerror(errno));
    return 1;
  }
  return 0;
}
/* On Linux, you'll observe that a "hole will be created */