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What are Named Pipes or FIFO in Linux/Unix systems?
Pipes were meant for communication between related processes. We Cannot use pipes for unrelated process communication. Then to achieve unrelated processes communication, the simple answer is Named Pipes. Even though this works for related processes, it gives no meaning to use the named pipes for related process communication.
Unlike pipe, we can use single named pipe that can be used for two-way communication (communication between the server and the client, plus the client and the server at the same time) as Named Pipe supports bi-directional communication.
Another name for named pipe is FIFO (First-In-First-Out). Let us see the system call (mknod()) to create a named pipe, which is a kind of a special file.
#include <sys/types.h> #include <sys/stat.h> #include <fcntl.h> #include <unistd.h> int mknod(const char *pathname, mode_t mode, dev_t dev);
This system call would create a special file or file system node such as ordinary file, device file, or FIFO. The arguments to the system call are pathname, mode and dev. The pathname along with the attributes of mode and device information. The pathname is relative, if the directory is not specified it would be created in the current directory. The mode specified is the mode of file which specifies the file type such as the type of file and the file mode as mentioned in the following tables. The dev field is to specify device information such as major and minor device numbers.
| File Type | Description | File Type | Description |
|---|---|---|---|
| S_IFBLK | block special | S_IFREG | Regular file |
| S_IFCHR | character special | S_IFDIR | Directory |
| S_IFIFO | FIFO special | S_IFLNK | Symbolic Link |
| File Mode | Description | File Mode | Description |
|---|---|---|---|
| S_IRWXU | Read, write, execute/search by owner | S_IWGRP | Write permission, group |
| S_IRUSR | Read permission,owler | S_IXGRP | Execute/search permission, group |
| S_IWUSR | Write permission, owner | S_IRWXO | Execute/search permission, group |
| S_IXUSR | Execute/search permission, owner | S_IROTH | Read permission, others |
| S_IRWXG | Read, write, execute/search by group | S_IWOTH | Write permission, others |
| S_IRGRP | Read permission, group | S_IXOTH | Execute/search permission, others |
File mode can also be represented in octal notation such as 0XYZ, where X represents owner, Y represents group, and Z represents others. The value of X, Y or Z can range from 0 to 7. The values for read, write and execute are 4, 2, 1 respectively. If needed in combination of read, write and execute, then add the values accordingly.
Say, if, 0640, then this means read and write (4 + 2 = 6) for owner, read (4) for group and no permissions (0) for others.
This call would return zero on success and -1 in case of failure. To know the cause of failure, check with errno variable or perror() function.
#include <sys/types.h> #include <sys/stat.h> int mkfifo(const char *pathname, mode_t mode)
This library function creates a FIFO special file, which is used for named pipe. The arguments to this function is file name and mode. The file name can be either absolute path or relative path. If full path name (or absolute path) is not given, the file would be created in the current folder of the executing process. The file mode information is as described in mknod() system call.
This call would return zero on success and -1 in case of failure. To know the cause of failure, check with errno variable or perror() function.
Let us consider a program of running the server on one terminal and running the client on another terminal. The program would only perform one-way communication. The client accepts the user input and sends the message to the server, the server prints the message on the output. The process is continued until the user enters the string “end”.
Let us understand this with an example −
Step 1 − Create two processes, one is fifoserver and another one is fifoclient.
Step 2 − Server process performs the following −
Creates a named pipe (using system call mknod()) with name “MYFIFO”, if not created.
Opens the named pipe for read only purposes.
Here, created FIFO with permissions of read and write for Owner. Read for Group and no permissions for Others.
Waits infinitely for message from the Client.
If the message received from the client is not “end”, prints the message. If the message is “end”, closes the fifo and ends the process.
Step 3 − Client process performs the following −
1) Opens the named pipe for write only purposes.
2) Accepts the string from the user.
3) Checks, if the user enters “end” or other than “end”. Either way, it sends a message to the server. However, if the string is “end”, this closes the FIFO and also ends the process.
4) Repeats infinitely until the user enters string “end”.
Two-way Communication Using Named Pipes
The communication between pipes are meant to be unidirectional. Pipes were restricted to one-way communication in general and need at least two pipes for two-way communication. Pipes are meant for inter-related processes only. Pipes can’t be used for unrelated processes communication, say, if we want to execute one process from one terminal and another process from another terminal, it is not possible with pipes. Named pipe is meant for communication between two or more unrelated processes and can also have bi-directional communication.
Already, we have seen the one-directional communication between named pipes, i.e., the messages from the client to the server. Now, let us take a look at the bi-directional communication i.e., the client sending message to the server and the server receiving the message and sending back another message to the client using the same named pipe.
Following is an example −
Step 1 − Create two processes, one is fifoserver_twoway and another one is fifoclient_twoway.
Step 2 − Server process performs the following −
Creates a named pipe (using library function mkfifo()) with name “fifo_twoway” in /tmp directory, if not created.
Opens the named pipe for read and write purposes.
Here, created FIFO with permissions of read and write for Owner. Read for Group and no permissions for Others.
Waits infinitely for a message from the client.
If the message received from the client is not “end”, prints the message and reverses the string. The reversed string is sent back to the client. If the message is “end”, closes the fifo and ends the process.
Step 3 − Client process performs the following −
Opens the named pipe for read and write purposes.
Accepts string from the user.
Checks, if the user enters “end” or other than “end”. Either way, it sends a message to the server. However, if the string is “end”, this closes the FIFO and also ends the process.
If the message is sent as not “end”, it waits for the message (reversed string) from the client and prints the reversed string.
Repeats infinitely until the user enters the string “end”.
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