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![UNIX pipe( ) system call
- pipe( ) takes array of two
FDs as input
- fd[0] is the read end of
the pipe
- fd[1] is the write end of
the pipe
- Implemented as a FIFO
queue in OS
P1
pipe(fd[2])
PCB (P1)
0 1 2 fd[0] fd[1]
Pipe (FIFO Queue)
IN (Read)
OUT (Write)](https://image.slidesharecdn.com/4-file-api-250911162716-8094c8df/75/4-file-api-pdf4-file-api-pdf4-file-api-pdf4-file-api-pdf-25-2048.jpg)
![UNIX pipe( ) with fork( )
- fork( ) duplicates the file
descriptors
- At this point,both the
parent and the child
processes can read/write to
the pipe
Parent
pipe(fd[2])
PCB (Parent)
0 1 2 fd[0] fd[1]
Pipe (FIFO Queue)
IN (Read)
OUT (Write)
PCB (Child)
0 1 2 fd[0] fd[1]](https://image.slidesharecdn.com/4-file-api-250911162716-8094c8df/75/4-file-api-pdf4-file-api-pdf4-file-api-pdf4-file-api-pdf-26-2048.jpg)
![UNIX pipe( ) with fork( )
Parent
pipe(fd[2])
PCB (Parent)
0 1 2 fd[0] fd[1]
Pipe (FIFO Queue)
IN (Read)
OUT (Write)
PCB (Child)
0 1 2 fd[0] fd[1]
- fork( ) duplicates the file
descriptors
- close( ) one end of the pipe,
both in child and parent
- Result
- A queue between
parent and child](https://image.slidesharecdn.com/4-file-api-250911162716-8094c8df/75/4-file-api-pdf4-file-api-pdf4-file-api-pdf4-file-api-pdf-27-2048.jpg)
![Shell piping : ls | wc -l
- pipe( ) followed by fork( )
- Parent: exec( “ls”) after
making STDOUT → out fd
of the pipe (using dup)
Parent
pipe(fd[2])
PCB (Parent)
0 1 2 fd[0] fd[1]
Pipe (FIFO Queue)
IN (Read)
OUT (Write)
PCB (Child)
0 1 2 fd[0] fd[1]](https://image.slidesharecdn.com/4-file-api-250911162716-8094c8df/75/4-file-api-pdf4-file-api-pdf4-file-api-pdf4-file-api-pdf-28-2048.jpg)
![Shell piping : ls | wc -l
- pipe( ) followed by fork( )
- Parent: exec( “ls”) after
making STDOUT → out fd
of the pipe (using dup)
- Child: exec(“wc”) after
closing STDIN and duping
in fd of pipe
- Result: input of “wc”is
connected to output of “ls”
Parent
pipe(fd[2])
PCB (Parent)
0 1 2 fd[0] fd[1]
Pipe (FIFO Queue)
IN (Read)
OUT (Write)
PCB (Child)
0 1 2 fd[0] fd[1]](https://image.slidesharecdn.com/4-file-api-250911162716-8094c8df/75/4-file-api-pdf4-file-api-pdf4-file-api-pdf4-file-api-pdf-29-2048.jpg)
![UNIX pipe( ) system call
- pipe( ) takes array of two
FDs as input
- fd[0] is the read end of
the pipe
- fd[1] is the write end of
the pipe
- Implemented as a FIFO
queue in OS
P1
pipe(fd[2])
PCB (P1)
0 1 2 fd[0] fd[1]
Pipe (FIFO Queue)
IN (Read)
OUT (Write)](https://crownmelresort.com/image.slidesharecdn.com/4-file-api-250911162716-8094c8df/75/4-file-api-pdf4-file-api-pdf4-file-api-pdf4-file-api-pdf-25-2048.jpg)
![UNIX pipe( ) with fork( )
- fork( ) duplicates the file
descriptors
- At this point,both the
parent and the child
processes can read/write to
the pipe
Parent
pipe(fd[2])
PCB (Parent)
0 1 2 fd[0] fd[1]
Pipe (FIFO Queue)
IN (Read)
OUT (Write)
PCB (Child)
0 1 2 fd[0] fd[1]](https://crownmelresort.com/image.slidesharecdn.com/4-file-api-250911162716-8094c8df/75/4-file-api-pdf4-file-api-pdf4-file-api-pdf4-file-api-pdf-26-2048.jpg)
![UNIX pipe( ) with fork( )
Parent
pipe(fd[2])
PCB (Parent)
0 1 2 fd[0] fd[1]
Pipe (FIFO Queue)
IN (Read)
OUT (Write)
PCB (Child)
0 1 2 fd[0] fd[1]
- fork( ) duplicates the file
descriptors
- close( ) one end of the pipe,
both in child and parent
- Result
- A queue between
parent and child](https://crownmelresort.com/image.slidesharecdn.com/4-file-api-250911162716-8094c8df/75/4-file-api-pdf4-file-api-pdf4-file-api-pdf4-file-api-pdf-27-2048.jpg)
![Shell piping : ls | wc -l
- pipe( ) followed by fork( )
- Parent: exec( “ls”) after
making STDOUT → out fd
of the pipe (using dup)
Parent
pipe(fd[2])
PCB (Parent)
0 1 2 fd[0] fd[1]
Pipe (FIFO Queue)
IN (Read)
OUT (Write)
PCB (Child)
0 1 2 fd[0] fd[1]](https://crownmelresort.com/image.slidesharecdn.com/4-file-api-250911162716-8094c8df/75/4-file-api-pdf4-file-api-pdf4-file-api-pdf4-file-api-pdf-28-2048.jpg)
![Shell piping : ls | wc -l
- pipe( ) followed by fork( )
- Parent: exec( “ls”) after
making STDOUT → out fd
of the pipe (using dup)
- Child: exec(“wc”) after
closing STDIN and duping
in fd of pipe
- Result: input of “wc”is
connected to output of “ls”
Parent
pipe(fd[2])
PCB (Parent)
0 1 2 fd[0] fd[1]
Pipe (FIFO Queue)
IN (Read)
OUT (Write)
PCB (Child)
0 1 2 fd[0] fd[1]](https://crownmelresort.com/image.slidesharecdn.com/4-file-api-250911162716-8094c8df/75/4-file-api-pdf4-file-api-pdf4-file-api-pdf4-file-api-pdf-29-2048.jpg)
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- 1.
- 2. The file system / etcbin sbin home lib code file.txt USER Storage devices Hard disk drive SSD Others OS End-user wants see a nice tree view.Let me enable it through a simple system call APIs.
- 3. The file system / etcbin sbin home lib code file.txt USER OS File system layer Storage devices Hard disk drive SSD Others - File system is an important OS subsystem - Provides abstractions like files and directories - Hides the complexity of underlying storage devices
- 4. File system interfacing Input/OutputLibrary (fopen, fclose, fread, fprintf …) - Processes identify files through a file handle a.k.a.file descriptors - In UNIX,the POSIX file API is used to access files,devices, sockets etc. - What is the mapping between library functions and system calls? System call API (open, close, read, write …) Files Devices Sockets
- 5. open: getting ahandle int open (char *path, int flags, mode_t mode)
- 6. open: getting ahandle int open (char *path, int flags, mode_t mode) - Access mode specified in flags : O_RDONLY,O_RDWR,O_WRONLY - Access permissions check performed by the OS - On success,a file descriptor (integer) is returned - If flags contain O_CREAT,mode specifies the file creation mode - Refer man page (“man 2 open")
- 7. Process view offile - Per-process file descriptor table with pointer to a “file”object - file object → inode is many-to-one P1 fd1 =open(“file1”) P2 fd1 = open(“file1”) fd2 = open(“file2”) file 1 file 1 file 2 Inode 1 Inode 2 PCB (P1) 0 1 2 3 PCB (P2) 0 1 2 3 4
- 8. Process view offile - Per-process file descriptor table with pointer to a “file”object P1 fd1 =open(“file1”) P2 fd1 = open(“file1”) fd2 = open(“file2”) file 1 file 1 file 2 Inode 1 Inode 2 PCB (P1) 0 1 2 3 PCB (P2) 0 1 2 3 4 - What do file descriptors 0,1 and 2 represent? - What happens to the FD table and the file objects across fork( )? - What happens in exec( )? - Can multiple FDs point to the same file object?
- 9. Read and Write ssize_tread (int fd, void *buf, size_t count); - fd → file handle - buf → user buffer as read destination - count → #of bytes to read - read ( ) returns #of bytes actually read,can be smaller than count
- 10. Read and Write ssize_tread (int fd, void *buf, size_t count); - fd → file handle - buf → user buffer as read destination - count → #of bytes to read - read ( ) returns #of bytes actually read,can be smaller than count ssize_t write (int fd, void *buf, size_t count); - Similar to read
- 11. Process view offile - Per-process file descriptor table with pointer to a “file”object P1 fd1 =open(“file1”) P2 fd1 = open(“file1”) fd2 = open(“file2”) file 1 file 1 file 2 Inode 1 Inode 2 PCB (P1) 0 1 2 3 PCB (P2) 0 1 2 3 4 - What do file descriptors 0,1 and 2 represent? - 0 → STDIN,1 → STDOUT and 2 → STDERR - What happens to the FD table and the file objects across fork( )? - What happens in exec( )? - Can multiple FDs point to the same file object?
- 12. lseek off_t lseek(int fd,off_t offset, int whence); - fd → file handle - offset → target offset - whence → SEEK_SET,SEEK_CUR,SEEK_END - On success,returns offset from the starting of the file
- 13. lseek off_t lseek(int fd,off_t offset, int whence); - fd → file handle - offset → target offset - whence → SEEK_SET,SEEK_CUR,SEEK_END - On success,returns offset from the starting of the file - Examples - lseek(fd,100,SEEK_CUR) → forwards the file position by 100 bytes - lseek(fd,0,SEEK_END) → file pos at EOF,returns the file size - lseek(fd,0,SEEK_SET) → file pos at beginning of file
- 14. File information (stat,fstat) int stat(const char *path, struct stat *sbuf); - Returns the information about file/dir in the argument path - The information is filled up in structure called stat
- 15. File information (stat,fstat) int stat(const char *path, struct stat *sbuf); - Returns the information about file/dir in the argument path - The information is filled up in structure called stat struct stat sbuf; stat(“/home/user/tmp.txt”, &sbuf); printf(“inode = %d size = %ldn”, sbuf.st_ino, sbuf.st_size); - Other useful fields in struct stat : st_uid,st_mode (Refer stat man page)
- 16. Process view offile - Per-process file descriptor table with pointer to a “file”object P1 fd1 =open(“file1”) P2 fd1 = open(“file1”) fd2 = open(“file2”) file 1 file 1 file 2 Inode 1 Inode 2 PCB (P1) 0 1 2 3 PCB (P2) 0 1 2 3 4 - What do file descriptors 0,1 and 2 represent? - 0 → STDIN,1 → STDOUT and 2 → STDERR - What happens to the FD table and the file objects across fork( )? - What happens in exec( )? - The FD table is copied across fork( ) ⇒ File objects are shared - On exec,open files remain shared by default - Can multiple FDs point to the same file object?
- 17. Duplicate file handles(dup and dup2) - The dup() system call creates a “copy”of the file descriptor oldfd - Returns the lowest-numbered unused descriptor as the new descriptor - The old and new file descriptors represent the same file int dup(int oldfd);
- 18. Duplicate file handles(dup and dup2) int fd, dupfd; fd = open(“tmp.txt”); close(1); dupfd = dup(fd); //What will be the value of dupfd? printf(“Hello worldn”); // Where will be the output?
- 19. Duplicate file handles(dup and dup2) int fd, dupfd; fd = open(“tmp.txt”); close(1); dupfd = dup(fd); //What will be the value of dupfd? printf(“Hello worldn”); // Where will be the output? - Value of dupfd = 1 (assuming STDIN is open) - “Hello world”will be written to tmp.txt file
- 20. Duplicate file handles(dup and dup2) - Close newfd before duping the file descriptor oldfd - dup2 (fd,1) equivalent to - close(1); - dup(fd); int dup2(int oldfd, int newfd);
- 21. Duplicate file handles(dup and dup2) - Lowest numbered unused fd (i.e.,1) is used (Assume STDOUT is closed before) - Duplicate descriptors share the same file state - Closing one file descriptor does not close the file P1 fd1 =open(“file1”) file 1 PCB (P1) 0 1 2 3 + dup(fd1) Before dup( ) file 1 PCB (P1) 0 1 2 3 After dup( )
- 22. Use of dup:shell redirection - Example: ls > tmp.txt - How implemented?
- 23. Use of dup:shell redirection - Example: ls > tmp.txt - How implemented? fd = open (“tmp.txt”) close( 1); close(2); // close STDOUT and STDERR dup(fd); dup(fd) // 1→ fd, 2 → fd exec(ls )
- 24. Process view offile - Per-process file descriptor table with pointer to a “file”object P1 fd1 =open(“file1”) P2 fd1 = open(“file1”) fd2 = open(“file2”) file 1 file 1 file 2 Inode 1 Inode 2 PCB (P1) 0 1 2 3 PCB (P2) 0 1 2 3 4 - What do file descriptors 0,1 and 2 represent? - 0 → STDIN,1 → STDOUT and 2 → STDERR - What happens to the FD table and the file objects across fork( )? - What happens in exec( )? - The FD table is copied across fork( ) ⇒ File objects are shared - On exec,open files remain shared by default - Can multiple FDs point to the same file object? - Yes,duped FDs share the same file object (within a process)
- 25. UNIX pipe( )system call - pipe( ) takes array of two FDs as input - fd[0] is the read end of the pipe - fd[1] is the write end of the pipe - Implemented as a FIFO queue in OS P1 pipe(fd[2]) PCB (P1) 0 1 2 fd[0] fd[1] Pipe (FIFO Queue) IN (Read) OUT (Write)
- 26. UNIX pipe( )with fork( ) - fork( ) duplicates the file descriptors - At this point,both the parent and the child processes can read/write to the pipe Parent pipe(fd[2]) PCB (Parent) 0 1 2 fd[0] fd[1] Pipe (FIFO Queue) IN (Read) OUT (Write) PCB (Child) 0 1 2 fd[0] fd[1]
- 27. UNIX pipe( )with fork( ) Parent pipe(fd[2]) PCB (Parent) 0 1 2 fd[0] fd[1] Pipe (FIFO Queue) IN (Read) OUT (Write) PCB (Child) 0 1 2 fd[0] fd[1] - fork( ) duplicates the file descriptors - close( ) one end of the pipe, both in child and parent - Result - A queue between parent and child
- 28. Shell piping :ls | wc -l - pipe( ) followed by fork( ) - Parent: exec( “ls”) after making STDOUT → out fd of the pipe (using dup) Parent pipe(fd[2]) PCB (Parent) 0 1 2 fd[0] fd[1] Pipe (FIFO Queue) IN (Read) OUT (Write) PCB (Child) 0 1 2 fd[0] fd[1]
- 29. Shell piping :ls | wc -l - pipe( ) followed by fork( ) - Parent: exec( “ls”) after making STDOUT → out fd of the pipe (using dup) - Child: exec(“wc”) after closing STDIN and duping in fd of pipe - Result: input of “wc”is connected to output of “ls” Parent pipe(fd[2]) PCB (Parent) 0 1 2 fd[0] fd[1] Pipe (FIFO Queue) IN (Read) OUT (Write) PCB (Child) 0 1 2 fd[0] fd[1]