I' ll attach here the code I wrote, it doesn't work the way it should, it doesn't read properly from fifo. It was sending the username correctly before adding more code, it makes me think I wrote the code bad from beginning. If it's helpful I'll post the client code too, but I think the problem is here. When I run the program it prints the length correctly, but it doesn't print the username at all.

//SERVER

void copyUsername(int fd, char *&username, int &length)
{
  printf("Waiting for client login...\n");

  if (read(fd, &length, sizeof(int)) == -1)
  {
    printf("Could not read in the fifo 1\n");
    return;
  }
  printf("Client wrote an username\n");

  printf("%d\n", length);
  if (read((char)fd, &username, sizeof(char)* length) == -1)
  {
    printf("Could not read in the fifo 2\n");
    return;
  }
  printf("%s\n", username);
  username[strlen(username)] = '\0';
  printf("Copied successfully from FIFO %s\n", username);
}

int main()
{
    if (mkfifo("fifo1", 0666) == -1)
    {
        if (errno != EEXIST)
        {
            printf("Could not create fifo file\n");
            return 1;
        }
    }

  int fd = open("fifo1", O_RDWR);
  if (fd == -1)
  {
    printf("Could not open fifo file\n");
    return 2;
  }

  char *username;
  int length;
  bool connected;

  int pfd[2];
  if(pipe(pfd) == -1)
  {
    printf("Could not open the pipe\n");
    return 3;
  }

  int id = fork();
  if(id == -1)
  {
    printf("Could not execute the fork\n");
    return 4;
  }


  if(id == 0)
  {
    // child process
    close(pfd[0]);
    copyUsername(fd, username, length);
    bool match = matchUsername(username, length);
    write(pfd[1], &match, sizeof(match));
    close(pfd[1]);
    exit(0);
  }
  else
  {
    // parent process
    close(pfd[1]);
    read(pfd[0], &connected, sizeof(connected));
    close(pfd[0]);
    wait(NULL);
  }

CodePudding user response：

A few issues ...

1. Doing printf for debug can interfere with the pipe data. Better to use stderr. A macro (e.g. prte and dbgprt) can help
2. In copyUsername, doing char *&username is overly complicated. It can/should be char *username.
3. Also, doing int &length is also complicated. Just pass back length as a return value.
4. In copyUsername, doing username[strlen(username)] = '\0'; is broken. It is a no-op. It is broken because it assumes the buffer is already 0 terminated. It is misplaced after the printf Replace with username[length] = 0;
5. matchUsername not provided. I had to synthesize this.
6. client code not provided. It would have helped to show it, so the server can be tested. I've synthesized one in the code below.

Here is the corrected code:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <sys/wait.h>

#define prte(_fmt...) \
    fprintf(stderr,_fmt)

#if DEBUG
#define dbgprt(_fmt...) \
    prte(_fmt)
#else
#define dbgprt(_fmt...) \
    do { } while (0)
#endif

bool
matchUsername(const char *username, int length)
{
    // NOTE: we can ignore the length because copyUsername has 0 terminated the
    // string
    bool match = strcmp(username,"neo") == 0;

    return match;
}

int
copyUsername(int fd, char *username)
{
    int length;

    dbgprt("Waiting for client login...\n");

    if (read(fd, &length, sizeof(int)) == -1) {
        prte("Could not read in the fifo 1\n");
        return -1;
    }
    dbgprt("Client wrote an username\n");

    dbgprt("%d\n", length);
    if (read(fd, username, length) == -1) {
        dbgprt("Could not read in the fifo 2\n");
        return -1;
    }
#if 0
    username[strlen(username)] = '\0';
#else
    username[length] = '\0';
#endif
    dbgprt("%s\n", username);
    dbgprt("Copied successfully from FIFO %s\n", username);

    return length;
}

int
main(void)
{
    if (mkfifo("fifo1", 0666) == -1) {
        if (errno != EEXIST) {
            printf("Could not create fifo file\n");
            return 1;
        }
    }

    int fd = open("fifo1", O_RDWR);

    if (fd == -1) {
        prte("Could not open fifo file\n");
        return 2;
    }

    char *username;
    int length;
    bool connected;

    int pfd[2];

    if (pipe(pfd) == -1) {
        prte("Could not open the pipe\n");
        return 3;
    }

    int id = fork();
    if (id == -1) {
        prte("Could not execute the fork\n");
        return 4;
    }

    if (id == 0) {
        // child process
        close(pfd[0]);
        length = copyUsername(fd, username);

        bool match = 0;
        if (length > 0)
            length = matchUsername(username, length);

        write(pfd[1], &match, sizeof(match));
        close(pfd[1]);
        exit(0);
    }
    else {
        // parent process
        close(pfd[1]);
        read(pfd[0], &connected, sizeof(connected));
        close(pfd[0]);
        wait(NULL);
        prte("main: connected=%d\n",connected);
    }

    return 0;
}

In the code above, I've used cpp conditionals to denote old vs. new code:

#if 0
// old code
#else
// new code
#endif

#if 1
// new code
#endif

Note: this can be cleaned up by running the file through unifdef -k

Here is the client code I synthesized to test the program:

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

int
main(int argc,char **argv)
{

    --argc;
      argv;

    if (argc != 1) {
        fprintf(stderr,"no name specified\n");
        exit(1);
    }

    int fd = open("fifo1",O_RDWR);
    if (fd < 0) {
        perror("open");
        exit(1);
    }

    int length = strlen(*argv);
    write(fd,&length,sizeof(length));

    write(fd,*argv,length);

    close(fd);

    return 0;
}

Here is the server output with the command: ./client bork:

main: connected=0

Here is the server output with the command: ./client neo:

main: connected=1