I tried to use the lseek function in the next program to read from any position.
Although the return value of close () is specifically ignored in the event of an error (where we should probably just omit closing the descriptor and let the operating system handle it), we implicitly ignore it in the “success” path as well. I think I should have a more robust program and always check the return value of file operations (with a possible exception for printf () for user messages, where failure does not affect the correctness of the primary function of the program).
How can I do that?

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>

#define FILE    "test"

int
main(void)
{
   int fd;
   char buf[10] = { 0 };
   
   printf("lseek(2) test for RW file with append flag\n\n");

   /* open the file */
   printf("open(2)...");
   if( (fd = open(FILE, O_RDWR | O_APPEND | O_CREAT | O_TRUNC, S_IRWXU)) == -1 ) {
      printf(" KO\n");
      return EXIT_FAILURE;
   }
   printf("OK\n");

   /* write something */
   printf("write(2)...");
   if( write(fd, "{first write call}", 18) != 18 ) {
      printf(" KO\n");
      (void)close(fd);
      return EXIT_FAILURE;
   }
   printf(" OK\n");

   printf("\nTrying to seek 5 bytes from the beginning of the file..\n\n");

   /* move somewhere */
   printf("lseek(2)...");
   if( lseek(fd, 5, SEEK_SET) < 0 ) {
      printf(" KO\n");
      (void)close(fd);
      return EXIT_FAILURE;
   }
   printf(" OK\n");

   /* read something */
   printf("read(2)...");
   if( read(fd, buf, 5) == -1 ) {
      printf(" KO\n");
      (void)close(fd);
      return EXIT_FAILURE;
   }
   printf(" OK\n");

   /* write something, again */
   printf("write(2)...");
   if( write(fd, "[2nd write]", 11) != 11 ) {
      printf(" KO\n");
      (void)close(fd);
      return EXIT_FAILURE;
   }
   printf(" OK\n");

   close(fd);

   printf("\nTest finished. Check the '%s' file.\n", FILE);

   return EXIT_SUCCESS;
  }