neeru neeru - 4 months ago 18
Linux Question

pipe : write to front of pipe

I came across

TIMM_OSAL_WriteToFrontOfPipe
and
TIMM_OSAL_WriteToPipe
in the link

In
TIMM_OSAL_WriteToPipe
looks like, data is simply written to write end of pipe.
where as,
In
TIMM_OSAL_WriteToFrontOfPipe
, data is written at the write end, then again read and again written.

I tried to reproduce the same using a sample program below. But could not understand the point of reading and writing again. Could someone please throw some light on it ?

int main(void)
{
int fd[2];

int status = pipe (fd);
if (status) {
fprintf(stderr, "** pipe create failed");
return 0;
}

status = fork();
if (status < 0)
fprintf (stderr, "** fork failure!");

if (status == 0){
//close (fd[1]);
char buf[5] = {0,};
int ret_size;
fprintf(stderr, "child going to sleep...\n");
sleep (10);
fprintf(stderr, "\nchild woken up...\n");
ret_size = read (fd[0], buf, 5);
fprintf (stderr, "child printing ...\n");
fprintf (stderr, "%s\n", buf);
} else {
char buf[5] = {0};
int read_sz;
int write_sz;
//close (fd[0]);
strcpy(buf,"1");
write_sz = write (fd[1], buf, 1);
strcpy(buf,"2");
write_sz = write (fd[1], buf, 1);
strcpy(buf,"3");
write_sz = write (fd[1], buf, 1);
strcpy(buf,"4");
write_sz = write (fd[1], buf, 1);
if (write_sz < 0)
fprintf (stderr, "write failed");
else
fprintf(stderr, "written 1234\n");

memset(buf, 0, sizeof(buf));

strcpy(buf,"5");
write(fd[1], buf, 1);

memset(buf, 0, sizeof(buf));

read_sz = read (fd[0], buf, 5);
if (read_sz > 0)
fprintf(stderr, "read data :\n%s\n", buf);

//write back to pipe
write (fd[1], buf, 5);

}
return 0;
}


Output:

written 1234
read data :
12345
child going to sleep...
child woken up...
child printing ...
12345


I also tried by not
fork
ing and reading and writing in the same process.

Answer

WriteToFrontOfPipe means:

|----------------CURRENT_DATA| -> |------CURRENT_DATA YOUR_DATA|

Because pipe acts like queue, it takes 3 operations:

|------YOUR_DATA CURRENT_DATA| // write your data

|-------------------YOUR_DATA| // read tail to put your data in front

|------CURRENT_DATA YOUR_DATA| // save previous data back

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