Write() to file descriptor disappearing when read()

Question

So I'm working on the server side of my program right now, and I want to do the following:

1) open a file in read/write mode

2) append a word (WORD) to the end of the file

3) [I believe I have all of this part down already] open a pipe, create a child process, have it read directly from the file (file descriptor), execute a command, and send the result into the write/output of the pipe. The parent process reads from the read/input of the pipe and puts the info into a buffer to send back to the client.

What I'm having trouble with is the appending part. I'm pretty sure it appends to the file (with a newline in between the existing text and my WORD) because when I directly open the text file it's there. But when I try to print it from my buffer, it's not there. I have tried closing the file descriptor after writing and reopening and it's not there. I've tried strcat instead of writing to the file descriptor and it's not there.

  #define WORD "WORD"
  #define BUFFERLENGTH 512
  char buffer[BUFFERLENGTH];

  int fileDesc = open (filePath, O_RDWR|O_APPEND, 0660);
  if (fileDesc <= 0){
    write(clientDesc, ERRORMSG, BUFFERLENGTH);
    exit(EXIT_FAILURE);
  }

  read(fileDesc,buffer,BUFFERLENGTH);

  long length  = lseek(fileDesc,0,SEEK_END);
  int status  = write(fileDesc,WORD,sizeof(WORD)-1);

  read(fileDesc, buffer, BUFFER_LEN+1);
  printf("new text: %s\n", buffer); //WORD does not show up at the end of file as intended

Is there something I'm really misunderstanding?

Perhaps I don't fully understand how open(), read(), write(), and lseek() work, but if anyone could help explain to me why this isn't working as intended that'd be greatly appreciated. I've been struggling with this for the past week and the number of tabs I currently have open to searching for a solution is tragic.

Answer 1

After your write() call you're going to be at the end of the file, so read() isn't going to be able to read anything. You'll need to lseek() to a point earlier in the file if you want to be able to read anything from it.

You should be checking the return from read() (and almost all other system calls, for that matter) and use perror() or similar in the case of error, and this will do wonders for helping you to understand what's going on when you see behavior you don't expect.

Modifying your program:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <string.h>

#define WORD "WORD"
#define BUFFERLENGTH 512

int main(void)
{
    char * filePath = "testfile.txt";
    char buffer[BUFFERLENGTH] = {0};

    // Open file.

    int fd = open(filePath, O_RDWR | O_APPEND, 0660);
    if (fd < 0) {
        perror("couldn't open file");
        return EXIT_FAILURE;
    }

    // Write word to end.

    int status = write(fd, WORD, strlen(WORD));
    if ( status < 0 ) {
        perror("couldn't write");
        return EXIT_FAILURE;
    }

    // Seek to start of file.

    long length = lseek(fd, 0, SEEK_SET);
    if ( length < 0 ) {
        perror("couldn't lseek");
        return EXIT_FAILURE;
    }

    // Read contents of file.

    status = read(fd, buffer, BUFFERLENGTH - 1);
    if ( status < 0 ) {
        perror("couldn't read");
        return EXIT_FAILURE;
    }

    // Print buffer.

    printf("file contents: %s\n", buffer);

    return 0;
}

yields:

paul@mac:scratch$ touch testfile.txt
paul@mac:scratch$ ./file
file contents: WORD
paul@mac:scratch$ ./file
file contents: WORDWORD
paul@mac:scratch$ ./file
file contents: WORDWORDWORD
paul@mac:scratch$ ./file
file contents: WORDWORDWORDWORD
paul@mac:scratch$

If you want to actually see only the new contents, then you'll need to lseek() to some point other than the start of the file. Since a successful write() will return the number of bytes written, you can use this value to offset back from the end of the file:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <string.h>

#define BUFFERLENGTH 512

int main(int argc, char * argv[])
{
    if ( argc < 2 ) {
        fprintf(stderr, "you need to enter a word argument\n");
        return EXIT_FAILURE;
    }

    char * filePath = "testfile.txt";
    char buffer[BUFFERLENGTH] = {0};

    // Open file.

    int fd = open(filePath, O_RDWR | O_APPEND, 0660);
    if ( fd < 0 ) {
        perror("couldn't open file");
        return EXIT_FAILURE;
    }

    // Write word to end.

    int status = write(fd, argv[1], strlen(argv[1]));
    if ( status < 0 ) {
        perror("couldn't write");
        return EXIT_FAILURE;
    }

    // Seek to point before last write.

    long length = lseek(fd, -status, SEEK_END);
    if ( length < 0 ) {
        perror("couldn't lseek");
        return EXIT_FAILURE;
    }

    // Read from there to end of file.

    status = read(fd, buffer, BUFFERLENGTH - 1);
    if ( status < 0 ) {
        perror("couldn't read");
        return EXIT_FAILURE;
    }

    // Print buffer.

    printf("new text: %s\n", buffer);

    return 0;
}

yielding:

paul@mac:scratch$ rm testfile.txt
paul@mac:scratch$ touch testfile.txt
paul@mac:scratch$ ./file2 these
new text: these
paul@mac:scratch$ ./file2 are
new text: are
paul@mac:scratch$ ./file2 some
new text: some
paul@mac:scratch$ ./file2 words
new text: words
paul@mac:scratch$ cat testfile.txt
thesearesomewordspaul@mac:scratch$