How do I pass an array of structures to the *nix read() function.

This is a discussion on How do I pass an array of structures to the *nix read() function. within the C Programming forums, part of the General Programming Boards category; I'm trying to pass an array of structures to the *nix read() function. Here is the code... Code: #include <stdio.h> ...

  1. #1
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    How do I pass an array of structures to the *nix read() function.

    I'm trying to pass an array of structures to the *nix read() function. Here is the code...

    Code:
    #include <stdio.h>
    #include <stdlib.h>
    
    #include <sys/types.h>
    #include <sys/stat.h>
    #include <fcntl.h>
    #include <unistd.h>
    #include <errno.h>
    
    
    struct bbs {
      char *s;
      int i;
    };
    
    struct bbs user[1];
    
    int main(void)
    {
    
      user[0].s = "chad\n";
      user[0].i = 1;
    
      /*user[1].s = "was\n";
      user[1].i = 1;
    
      user[2].s = "here\n";
      user[2].i = 1;
      */
    
    
      ssize_t n;
      int fd;
    
      if ((fd = open("/home/cdalten/oakland/ho", O_RDWR)) < 0) {
        perror("Can't open file\n");
        exit(EXIT_FAILURE);
      }
    
      write(fd, &user, sizeof(struct bbs));
    
      while((n = read(fd, &user, sizeof(struct bbs))) > 0 ) {
        printf("The string is: %s\n", user[1].s);
      }
    
      close(fd);
     
      exit(EXIT_SUCCESS);
    }
    And here is what I get when I compile and run the code.
    [cdalten@localhost oakland]$ gcc -g -Wall wr.c -o wr
    [cdalten@localhost oakland]$ ./wr
    The string is: (null)
    The string is: (null)
    [cdalten@localhost oakland]$

  2. #2
    Guest Sebastiani's Avatar
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    1) You haven't allocated any memory to the bbs objects.
    2) Even if you had, you would need to pass the address of the memory (not the structure) to the read routine (otherwise, you'd just be overwriting the pointer).
    3) When you do finally allocate some memory, just remember to pass the size of the buffer (not the structure) as the third parameter of the read function.

  3. #3
    Captain Crash brewbuck's Avatar
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    Portland, OR
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    Writing out a structure that contains pointer values just writes meaningless data to the disk. The pointers will not be useful (or even necessarily valid) in another invocation of the program. You need to write out the data which is pointed to, not the pointer value itself.
    Code:
    //try
    //{
    	if (a) do { f( b); } while(1);
    	else   do { f(!b); } while(1);
    //}

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