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plurals

PO
primarykey
Id
2962483
data
AcceptedAnswerId
0
AnswerCount
0
ClosedDate
CommentCount
0
CommunityOwnedDate
CreationDate
2010-06-03T00:00:38.023
FavoriteCount
0
LastActivityDate
2010-06-03T09:36:12.367
LastEditDate
2017-05-23T11:45:16.630
LastEditorUserId
-1
OwnerUserId
330982
ParentId
2939065
PostTypeId
2
Score
4
ViewCount
0
LastEditorDisplayName
text
Body
Given <a href="https://stackoverflow.com/questions/2939065/unix-fifo-how-to-allow-only-one-writer-reader-pair-to-use-a-fifo/2942937#2942937">the code you posted in a separate answer</a>, here is a modified version that fixes the problems you were having. See the comments for details, but in a nutshell: <ul> <li>The writer checks the return value of <code>mkfifo</code> is checked to see if another writer already created the pipe.</li> <li>The reader gets an exclusive advisory lock on the pipe (via <code>flock</code>) after opening it, to avoid the race condition where a second reader could have opened the pipe before the first reader deleted it.</li> </ul> Writer: <pre><code>#include <stdio.h> #include <stdlib.h> #include <unistd.h> #include <sys/stat.h> /* needed for mkfifo */ #include <sys/types.h> #include <sys/wait.h> #include <fcntl.h> #define BUFFERSIZE 50 #define CHMOD 0777 int main (int argc, char **argv) { char outbuf[BUFFERSIZE]; int fifo, j, anzahl; if (argc != 2) { printf("Ungültiger Parameter! Bsp.: ./fifow 10\n"); return 1; } anzahl=atoi(argv[1]); /* mkfifo fails if the file already exists, which means there's a * writer waiting for a reader. This assures that only one writer * will write to the pipe, since it only opens the pipe if it was * the one who created it. */ if (mkfifo("namedpipe4", CHMOD) == -1) { printf("namedpipe4 already exists\n"); return 1; } fifo = open("namedpipe4", O_WRONLY); for (j = 0; j < anzahl; j++) { printf("Writer PID: %d writes record nr. %6d\n", getpid(), j + 1); sprintf(outbuf, "Writer PID: %d writes record nr. %6d\n", getpid(), j + 1); write(fifo, outbuf, BUFFERSIZE); remove("namedpipe4"); sleep(1); } close(fifo); exit(0); } </code></pre> Reader: <pre><code>#include <stdio.h> #include <stdlib.h> #include <unistd.h> #include <sys/file.h> /* for flock */ #include <sys/types.h> #include <sys/wait.h> #include <fcntl.h> #define BUFFERSIZE 50 int main (int argc, char **argv) { char inbuf[BUFFERSIZE]; int fifo, var; printf("\n Waiting for a Pipe....\n"); /* There are *two* ways the open can fail: the pipe doesn't exist * yet, *or* it succeeded, but a different writer already opened * it but didn't yet remove it. */ while (1) { while ((fifo = open("namedpipe4", O_RDONLY)) == -1) { /* Since you didn't specify O_CREAT in the call to open, there * is no way that namedpipe4 would have been created by the * reader. If there *is* now a namedpipe4, a remove here * would delete the one the writer created! */ sleep(1); } /* Get an exclusive lock on the file, failing if we can't get * it immediately. Only one reader will succeed. */ if (flock (fifo, LOCK_EX | LOCK_NB) == 0) break; /* We lost the race to another reader. Give up and wait for * the next writer. */ close (fifo); } /* We are definitely the only reader. */ /* *Here* we delete the pipe, now that we've locked it and thus * know that we "own" the pipe. If we delete before locking, * there's a race where after we opened the pipe, a different * reader also opened, deleted, and locked the file, and a new * writer created a new pipe; in that case, we'd be deleting the * wrong pipe. */ remove("namedpipe4"); while ((var = read(fifo, inbuf, BUFFERSIZE)) > 0) { printf("Reader PID: %d reads record: %s\n", getpid(), inbuf); /* No need to sleep; we'll consume input as it becomes * available. */ } close(fifo); printf("\n EOF..\n"); exit(0); } </code></pre>
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1. This table or related slice is empty.
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1. POUNIX FIFO: How to allow only one writer/reader pair to use a FIFO?
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PostTypePostTypeId
1. PTAnswer
UserLastEditorUserId
1. USCommunity
UserOwnerUserId
1. USPaul Kuliniewicz
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