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Writing to and reading from a common file simultaneously via two different processes

Dear All,

There are two processes. Process 1 writes to a file. Process 2 reads from the same file. This is simultaneous. But, the first process locks the file. So I am unable to do it. Here is my code.

// Writer

Code:

#include <stdio.h>

#include <unistd.h>

#include <stdlib.h>



typedef struct parameter

{

        int count;

}param;



int main()

{

FILE *fp;

int i;

param p;



fp = fopen("data.txt","w+");

        

if(fp == NULL)

{

        printf("\nCannot open file.");

        exit(1);

}

                

i = 0;

while(1)

{

p.count = i++;                

printf("\nWriting count = %d",p.count);

                fwrite(&p,sizeof(p),1,fp);                

sleep(5);

}

        

fclose(fp);

return(0);

}

//Reader

Code:

#include <stdio.h>

#include <unistd.h>

#include <stdlib.h>



typedef struct parameter

{

int count;

}param;



int main()

{

FILE *fp;

int x;

param p;        

                

fp = fopen("data.txt","r");

        

if(fp == NULL)

{

        printf("\nCannot open file.");

        exit(1);

}        

        

while(1)

{                

x = fread(&p,sizeof(p),1,fp );                

printf("\nReading:count =%d",p.count);                

sleep(1);

}        

        

printf("\nOut of while...");

fclose(fp);

return(0);

}

I have searched about it and stumbled across file-locking, but don't know how to code it. Then I came across one process writing and renaming the file etc. Any help will be appreciated. Thanks.

PS: I want two processes to communicate via a common file. I know it can be done via sockets, but this time I want to do it via common file because I want to learn how to do it this way properly.

In the Writer application, after you perform the fwrite(), perform an fflush(). See if that makes a difference.

Thanks dwhitney67 for the reply.
Yes, it works after fflush(fp).
// I searched and learned that fflush flushes out the buffer associated with a stream.
Thanks.

The writer process opens a file and then enters an infinite loop. So the other process should not be allowed to open the file and get a file handle (?). But I guess that it was allowed in this case. Could you please explain it? Also, please tell me if there is a better way to do this via common file?

Thanks in advance.

Is there a reason you aren't using a named pipe? A named pipe will allow the reader to block for more data (instead of requiring a spinlock), with the drawback being that only one process can read from it. If you stick with the regular file, you should use advisory locks (see fcntl F_GETLK, etc.), anyway; otherwise, you might get partial-line reads from the reader while the writer is writing. Lastly, consider using "a+" instead of "w+".
Kevin Barry