Parent,child wait,signal

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# 8  
Old 06-06-2011
Originally Posted by Cuervo
Corona688 i think you mistook FIFO with pipes...right?
Nope. Look at what I said more closely. I never said anything about writing to the fifo, but what happens when you open it.

Lemme write you a little example.

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

int main(void)
        pid_t pid;
        int fd;
        mkfifo("./myfifo", 0660);

        if(pid == 0)    // child code
                fprintf(stderr, "C\tWaiting 3 seconds\n");
                fprintf(stderr, "C\tOpening fifo\n");
                fd=open("./myfifo", O_RDONLY);
                fprintf(stderr, "C\tOpening fifo\n");
                fd=open("./myfifo", O_RDONLY);
        else if(pid > 0)        // parent code
                fprintf(stderr, "P\tOpening fifo\n");
                fd=open("./myfifo", O_WRONLY);
                fprintf(stderr, "P\tFIFO opened\n");

        perror("Couldn't fork");

The parent creates the fifo, then creates the child, then tries to immediately open the fifo.

The child waits 3 seconds before opening the fifo, forcing the parent to wait 3 seconds as well. The fifo will finish opening only when there's something trying to open both ends.
This User Gave Thanks to Corona688 For This Post:
# 9  
Old 06-06-2011
mmm i see...thank you!! i ll find a way to use this.Thank you very much!!

---------- Post updated at 05:27 PM ---------- Previous update was at 05:18 PM ----------

Still...the waiting part doesn't work when it is done by two different programs right?

---------- Post updated at 05:35 PM ---------- Previous update was at 05:27 PM ----------

Let me explain what i mean.I want to make the parent process wait but through a separate program.So...
        if (pid1 == 0) 
            sprintf(ppidstr,"%d", ppid);
            if(execl("b.exe","b.exe",ppidstr) == -1)
                fprintf(stderr,"execl Error!\n");
                fd2 = open(myfifo2, O_RDONLY);
                read(fd2, buf, sizeof(MAX_BUF));
                   printf("PReceived: %s\n", buf);
                printf("My child ended ");

And this is a separate program
int main(int argc, char **argv)
    char * myfifo1 = "/cygdrive/c/a/progsys/tmp1",buf[MAX_BUF];
    char * myfifo2 = "/cygdrive/c/a/progsys/tmp2";
    mkfifo(myfifo1, 0666);
    mkfifo(myfifo2, 0666);

    printf("Doing stuff");
    int fd2 = open(myfifo2, O_WRONLY);
    write(fd2, "Hid", sizeof("Hid"));
    int ppid=atoi(argv[1]),fd;
    int fd1 = open(myfifo1, O_RDONLY);

To be more specific, i want the "Doing stuff" to appear first...but it doesn't
# 10  
Old 06-06-2011
there's a reason I was using stderr, printf() is buffered. That printf(), without a newline, doesn't print until your program quits! Especially weird things can happen to printf statements in a fork, too -- if you have text waiting in a buffer like that, it will be printed twice. Try printing to stderr. It prints immediately with no buffer.

Also, try writing hid\n instead of hid. I think fifos and pipes may line-buffer.

Last edited by Corona688; 06-06-2011 at 09:01 PM..
# 11  
Old 06-06-2011
OK, i did everything like you said. It worked fine and I can manage two different programs like this. The only program i have is that when i write on the FIFO the other program can't read it immediatelly. I think that the first program doesn't have enough time to write on the FIFO so that the other can read.I had to place a sleep(5) so that it can collect the data properly. Is there a way to lose the sleep?

---------- Post updated at 07:44 PM ---------- Previous update was at 07:07 PM ----------

Also i can only send 3-4 characters through the FIFO, is this normal?
# 12  
Old 06-06-2011
You've only posted extremely incomplete parts of your program so I have no idea what's happening, let alone what's going wrong.
# 13  
Old 06-06-2011
I understand, i will post the important stuff. When i try to execute this fprintf(stderr,"\n!%s!",buf); on the child i only get 3-4 chars. argv is 3 5 6 asdf and i only get 3 5 on the child.
I am grateful for you helping me, i don't want to impose. If you have time please just take a look. Thank you!!

if (pid == 0) 
            if(execl("b.exe","b.exe",NULL) == -1)
                fprintf(stderr,"execl Error!\n");
                fd = open(myfifo, O_WRONLY);
                   write(fd, data, MAX_BUF);
                printf("Parent ended ");
                fd = open(myfifo, O_RDONLY);


#define MAX_BUF 5000

int main(int argc, char **argv)
    int fd,i,j,iteration=1,lowerBound,upperBound,numPoints;
    char * myfifo = "/cygdrive/c/a/progsys/tmp";
    char buf[MAX_BUF]="",permbuf[MAX_BUF]="",sourcefile[MAX_BUF]="",finalsourcefile[MAX_BUF]="";
    fd = open(myfifo, O_RDONLY);
    read(fd, buf, sizeof(MAX_BUF));

# 14  
Old 06-06-2011
read() isn't a string function. It doesn't give you a null terminator. You have to check its return value to see how much you read, and print it with write() fwrite() or the like.

Your program is still extremely incomplete, so I have no idea what you're writing to the fifo.

Also, I see you're using cygwin, not UNIX, so the behavior of this program may be very suspect in any case. Windows doesn't actually have named pipe files.

Last edited by Corona688; 06-06-2011 at 11:50 PM..
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