Named pipes Example in C

Question

Reader Side,

#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

int main()
{
        int fd;
        char buff[100];
        fd = open ("MyPipes",O_RDONLY);
        read (fd, buff, 100);
        printf ("%s\n",buff);
        close(fd);
}

Writer Side,

#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

char *ptr = "Akshit Soni";
int main()
{
        int fd;
        fd = open ("MyPipes",O_WRONLY);
        write (fd, ptr, strlen(ptr));
        close (fd);
}

Problem is that reader program outputs gets garbage value.

\0 not an issue ... i removed .. i put that for just testing purpose :) — toxic_boi_8041, Jan 27 '14 at 05:36
@EdHeal, "MyPipes" is probably a named pipe. See http://www.linuxjournal.com/article/2156. — paxdiablo, Jan 27 '14 at 05:39
See http://stackoverflow.com/a/2789967/1967396 for a working example of how to do this. — Floris, Jan 27 '14 at 05:41

paxdiablo · Accepted Answer · 2014-01-27T05:54:15.947

3

Your (first) problem lies here:

write (fd, ptr, strlen(ptr));

The strlen of "Akshit Soni" does not include the trailing NUL character. You need to use strlen (ptr) + 1 as the length.

You should also allow for the fact that read() may not return all the bytes you asked for (100) nor all the bytes that were sent (12 including the NUL). It's possible (for reasons such as timing or interrupts) that only part of the data may be read by a single call to read().

To allow for that, you could try something like:

int main()
{
    int fd;
    char buff[100];
    fd = open ("MyPipes",O_RDONLY);
    int sz = read (fd, buff, 100);
    while ((sz > 0) && (buff[sz-1] != '\0')) {
        printf ("%*.*s", sz-1, sz-1, buff);
        sz = read (fd, buff, 100);
    }
    if (sz > 0)
        printf ("%s\n",buff);
    close(fd);
}

As an aside, make sure you have actually created the named pipe before you run the code, with something like (from bash):

mkfifo MyPipes

edited Jan 27 '14 at 05:54

answered Jan 27 '14 at 05:37

paxdiablo

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Yes. Need `strlen(ptr) + 1`. Well spotted. – Floris Jan 27 '14 at 05:37
It is necessary but not sufficient. – Floris Jan 27 '14 at 05:38
1

@Axit `strlen(ptr) + 1 not working` is _not_ a very useful statement, especially when it works fine here. Are you still getting garbage at the end of the string? Is there a compile problem? Has your PC melted? A little more specificity would help :-) – paxdiablo Jan 27 '14 at 05:55

score 0 · Answer 2 · answered Jan 27 '14 at 05:36

0

Make sure that you open things in the right order; first open the pipe for write, then for read. You don't test that you don't have a NULL response to the opening of the pipe, and you may well be looking at garbage. Always test the return value of your functions...

answered Jan 27 '14 at 05:36

Floris

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Where is the pipe? It is just a file – Ed Heal Jan 27 '14 at 05:38
Named pipe is new concept for me... i see this example on internet and try to implement @Ed Heal – toxic_boi_8041 Jan 27 '14 at 05:39

score 0 · Answer 3 · answered Jan 27 '14 at 05:48

it is not correct to create named pipe. use mkfifo to create it
for create file, might need to use more flags, assuming there is no such file named MyPipes exist already. fd = open ("MyPipes",O_WRONLY|O_CREAT); no O_CREAT, there is no file to be created.
write (fd, ptr, strlen(ptr)); change it to write (fd, ptr, strlen(ptr)+1);

strlen will return the length withou "\0".

sujin · Answer 4 · 2014-01-27T06:43:35.643

0

You failed to add the proper flag when open.

Try this:

fd = open ("mypipe11",O_WRONLY | O_CREAT);

EDIT:

Create the named pipe in writer before open using mkfifo.

#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

char *ptr = "Akshit Soni";
int main()
{
        int fd;

        /* Create named pipe */
        mkfifo("MyPipes", 0666);

        /* open named pipe */
        fd = open ("MyPipes",O_WRONLY);

        write (fd, ptr, strlen(ptr));
        close (fd);
}

@paxdiablo already given command line method of creating named pipe.

edited Jan 27 '14 at 06:43

answered Jan 27 '14 at 05:51

sujin

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@paxdiablo Thanks for your valid info. How about using `mknod` before `open` to create fifo?. – sujin Jan 27 '14 at 06:12

Named pipes Example in C

4 Answers4