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IPC Programming

Faculty of Computer Science and Engineering
Ho chi Minh city University of Technology

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Introduction
 Inter-Process Communication
 Sending messages between processes
 Sharing information between processes
 Synchronization

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/>Faculty of Computer Science and Engineering - HCMUT


IPC
 Communication
Transferring message
Sharing information
Mechanisms:
Pipe
Signal
Message queue
Shared memory
Socket
RPC/RMI

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Synchronization
Solving confliction
Processing order
Mechanisms:
Lock file
Semaphore
Mutex (pthread)

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IPC Programming
 Pipe
 Signal

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/>Faculty of Computer Science and Engineering - HCMUT


Pipe
 Processes communicate to each other using pipe through

FIFO mechanism

P0

write

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read

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P1


Pipe operations
 Write:
#include <unistd.h>
ssize_t write(int fd, const void *buf, size_t count)

 Read:
#include <unistd.h>
ssize_t read(int fd, const void *buf, size_t count)

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Pipe types
 Unnamed pipe
 Local
 Used in processes having parent-child relation
 Named pipe (FIFO)

 Global
 Used by any processes

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Unnamed pipe
#include <unistd.h>
int pipe(int filedes[2]);

 Return value:
 0: if successful, two file descriptors filedes[0], filedes[1]
will be stored in filedes
 -1: if error and error code is stored in external errno
variable

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Unnamed pipe
filedes[1]

P0

P1

filedes[0]

 Unidirectional/half-duplex
 filedes[0] is only used to read
 filedes[1] is only used to write
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#include
#include
#include
#include

<stdio.h>
<stdlib.h>
<unistd.h>
<string.h>

int main() {
int fp[2];
char s1[BUFSIZ], s2[BUFSIZ];

pipe(fp);

Compile and execute
$gcc unpipe.c -o unpipe
$./unpipe
Input: I Love Penguin

From pipe> I Love
Penguin
$

if (fork()==0) { /* Child Write */
printf("\nInput: ");
fgets(s1,BUFSIZ,stdin);
s1[strlen(s1)]=0;

close(fp[0]);
write(fp[1],s1,strlen(s1)+1);
} else {

/*Parent Read*/

close(fp[1]);
read(fp[0],s2,BUFSIZ);
printf("\nFrom pipe> %s\n", s2);
}
return 0;
}

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File redirecting
 dup()
 dup2()

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dup()
#include <unistd.h>
int dup(int oldfd);
stdin

0

stdout
stderr
available

stdin

0

1

stdout

1

2

stderr

2

dup(1)

3
4

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3
available

4

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dup2()
#include <unistd.h>
int dup2(int oldfd, int newfd);
stdin

0

stdout
stderr
available

stdin

0

1

stdout

1

2

stderr

2

3
4

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dup2(1,4)

available

3
4

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Example
// ls -l > output.txt
#include <unistd.h>
int main() {
int fd=open(“output.txt”);
close(1);
dup(fd);
system(“ls –l”);
close (fd);
return 0;
}
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Redirecting with pipe
 Using file redirecting technique to implement pipe
$ ps -ef | grep a01 | sort

ps -ef

cmd1

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|

grep a01

|

cmd2

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...
. . . cmdN


#include <unistd.h>
int main() {
// ps -ef | grep a01 | sort
int pipe1[2], pipe2[2];
pipe(pipe1);
if (fork()) { /* Parent */
pipe(pipe2);
if(fork()) { /* Parent */
close(0);
// Close standard input
dup(pipe2[0]); // standard input -> Read Pipe2
close(pipe1[0]);
close(pipe1[1]);
close(pipe2[0]);
close(pipe2[1]);
execlp("/bin/sort", "sort", NULL);
}
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else { /* Child 2 */
close(0);
// Close standard input
dup(pipe1[0]); // standard input -> Read Pipe1
close(1);
// Close standard output
dup(pipe2[1]); // standard output -> Write Pipe2
close(pipe1[0]); close(pipe1[1]);
close(pipe2[0]); close(pipe2[1]);
execlp("/bin/grep", "grep", “a01”,NULL);
}
} else { /* Child 1 */
close(1);
// Close standard output
dup(pipe1[1]); // standard output -> Write Pipe1
close(pipe1[0]); close(pipe1[1]);
execlp("/bin/ps", "ps", "-ef", NULL);
}
exit(0);
}
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Named pipe
 Similar to unnamed pipe
 Notice:

 Similar to a file on file system (directory entry, file
permission)
 Can be used on any processes
 Can be created from a command on shell (using mknod
command)

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Create named pipe
 System call
#include <sys/types.h>
#include <sys/stat.h>
int mknod(const char *path,mode_t mode,dev_t dev);

 C/C++ library call
#include <sys/types.h>
#include <sys/stat.h>
int mkfifo(const char *pathname, mode_t mode);

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#include <stdlib.h>
#include <stdio.h>
#include <sys/types.h>

#include <sys/stat.h>
#include <sys/errno.h>
extern int errno;

Compile and execute
$gcc fifo.c -o fifo
$./fifo
Parent writes to FIFO1: Test1
Child reads from FIFO1: Test1
Child feedbacks on FIFO2: Test2
Feedback data from FIFO2: Test2
$

#define FIFO1 "/tmp/fifo.1"
#define FIFO2 "/tmp/fifo.2"
#define PERMS 0666
int main(){
char s1[BUFSIZ], s2[BUFSIZ];
int childpid, readfd, writefd;

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if ((mknod(FIFO1, S_IFIFO | PERMS,
(errno!=EEXIST)) {
printf("can't create fifo1: %s",
exit(1);
}

if ((mknod(FIFO2, S_IFIFO | PERMS,
(errno!=EEXIST)) {
unlink(FIFO1);
printf("can't create fifo2: %s",
exit(1);
}
if ((childpid=fork())<0) {
printf("can't fork");
exit(1);
}

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0)<0) &&

FIFO1);

0)<0) &&

FIFO2);

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else if (childpid>0) { /* parent */
if ((writefd=open(FIFO1,1))<0)
perror("parent: can't open writefifo");
if ((readfd=open(FIFO2,0))<0)
perror("parent: can't open readfifo");
printf("\nParent writes to FIFO1: ");

gets(s1);
s1[strlen(s1)]=0;
write(writefd,s1,strlen(s1)+1);
read(readfd,s2,BUFSIZ);
printf("\nFeedback data from FIFO2: %s\n",s2);
while (wait((int*)0)!=childpid);
close(readfd);
close(writefd);
if (unlink(FIFO1)<0) perror("Can't unlink FIFO1");
if (unlink(FIFO2)<0) perror("Can't unlink FIFO2");
exit(0);
}
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else { /* child */
if ((readfd=open(FIFO1,0))<0)
perror("child: can't open readfifo");
if ((writefd=open(FIFO2,1))<0)
perror("child: can't open writefifo");
read(readfd,s2,BUFSIZ);
printf("\nChild read from FIFO1: %s\n",s2);
printf("\nInput string from child to feedback: ");
gets(s1);
s1[strlen(s1)]=0;
write(writefd,s1,strlen(s1)+1);
close(readfd);
close(writefd);

exit(0);
}
}
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IPC Programming
 Pipe
 Signal

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/>Faculty of Computer Science and Engineering - HCMUT


Signal
signals
(events)
Process

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