How can the parent process perceive the modification of the shared memory

Question

I want to write log files using mmap in multi-process server, the processes can write to the initial log file, but if one process modify the shared address to a new file, other processes can not perceive this. The codes are like this:

firstly: open a file and map it to shared memory;

secondly: child process writes something to the file

thirdly: child process open a new log and modify the shared memory to the new file

fourthly: parent process writes something to the log file. I want the parent process write to the new log, but it writes to the old file. This means the parent process does not know the modification of child process.

my question is : How can the parent(or other children) process perceive the modification of the shared memory, and write to the new file?

typedef struct shared_mgs_s {
    log_atomic_t lock;
    int          offset;    
} shared_msg_t;
char         *write_addr;

shared_msg_t    *sm;
log_atomic_t    *log_mutex_ptr;

#define FILE_SIZE 1 * 1024 * 1024

int map_log_file(char *file_name)
{
    // create a log file and map it to shared memory
    int fd = -1;
    if ((fd = open(file_name, O_RDWR | O_CREAT | O_EXCL, S_IRUSR | S_IWUSR)) < 0)
    {
        printf("create %s failed %d\n",file_name, errno);
        return -1;
    }
    
    if (ftruncate(fd, FILE_SIZE) < 0)
    {
        printf("ftruncate %d failed", errno);
        return -1;
    }
    write_addr = (char*)mmap(NULL, FILE_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
    close(fd);
    sm->offset = 0;
    
    return 0;
}

void write_log(char *msg)
{    
    memcpy(write_addr + sm->offset, msg, strlen(msg));
    sm->offset += strlen(msg);
}

int create_shared_info()
{
    sm = mmap(NULL, 64, PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS, -1, 0);
    if (!sm) 
        return -1;

    sm->offset = 0;
    sm->lock = 0;
    log_mutex_ptr = &(sm->lock);
}

int main()
{
    if(create_shared_info() < 0)
    {
        printf("create shared info failed\n");
        return -1;
    }

    if(map_log_file("./old.log") != 0)
    {
        printf("map old log failed\n");
        return -1;
    }
    if(0 == fork())
    {
        log_spinlock(log_mutex_ptr, 1, 2048);
        munmap(write_addr, FILE_SIZE);
        if(map_log_file("./new.log") != 0)
        {
            printf("create new log failed\n");
            return -1;
        }
        write_log("from child\n");
        log_unlock(log_mutex_ptr);
        exit(0);
    } else {
        usleep(10); // make sure child process get lock firstly, just test use the new file
        log_spinlock(log_mutex_ptr, 1, 2048);
        write_log("from parent\n");
        log_unlock(log_mutex_ptr);
    }

    return 0;
}

in the way, the parent writes to old.log, this is not what I want, I want it write to the new one.

Answer 1

but if one process modify the shared address to a new file, other processes can not perceive this.

Another way to address this:

1. Let another process change the log file name with something like the mv command-line utility or the rename() POSIX function. For example, mv /log/file.log /log/file.log.1. As you noticed, this will mean all the processes that were logging to /log/file.log will now be logging to /log/file.log.1, which you don't want.

2. Each process using your logging functionality will have a signal handler (historically, Unix processes have used SIGHUP) that will cause it to reopen its log file, thus picking up the new log file with a simple open() call. If your log functionality is fully formatting each log message internally and using a single write() call with the log file opened with the O_APPEND flag set, even in a multithreaded process you can replace the active log file descriptor atomically with open( "/log/file.log", O_WRITE | O_CREAT | O_APPEND, ...) to a temporary file descriptor and then you replace the log file descriptor with a call to dup2(): dup2( tempLogFD, logFileFD ); And since both open() and dup2() are async-signal-safe, you can do both calls in your SIGHUP signal handler:

   // needs headers and error-checking
   int logFileFD;
   
   void sighupHandler( int sig )
   {
       int tempLogFD = open( "/log/file.log", O_WRONLY | O_CREAT | O_APPEND,
           0644 );
       dup2( tempLogFD, logFileFD );
       close( tempLogFD );
   }
   

3. You can use the Linux command fuser -SIGHUP /log/file.log.1 (the renamed original log file) to send SIGHUP to all the processes that are still writing to /log/file.log.1 and they will swap writing their logs from /log/file.log.1 to the new /log/file.log.

Note that there's no complex synchronization nor any shared resources needed between your processes.