POSIX fjles /pipe 1 last time creating new threads for swtch - PowerPoint PPT Presentation

POSIX fjles /pipe 1

last time creating new threads for swtch trick: write what would be on stack during call to swtch POSIX and Unix fork — copy process to create child process exec — replace program being run by current process waitpid — wait for child process typical pattern: fork; if (child) exec; else wait 2

exercise (1) A and C A and B B. L1 (newline) L2 (newline) L2 E. C. (newline) L2 L2 (newline) L1 F. all of the above G. something else D. L1 int main() { A. for ( int i = 0; i < 2; ++i) { pids[i] = fork(); if (pids[i] == 0) { args[1] = extra[i]; execv("/bin/echo", args); } } for ( int i = 0; i < 2; ++i) { waitpid(pids[i], NULL, 0); } } Assuming fork and execv do not fail, which are possible outputs? 3 pid_t pids[2]; const char *args[] = {"echo", "ARG", NULL}; const char *extra[] = {"L1", "L2"};

exercise (2) (newline) 0 (newline) 1 (newline) 0 (newline) 2 F. C and D C. 1 (newline) 0 (newline) 2 B. G. all of the above D. 1 (newline) 0 (newline) 2 (newline) 0 H. something else 0 A, B, and C int main() { for ( int i = 0; i < 2; ++i) { pid_t pids[2]; for ( int i = 0; i < 2; ++i) { pids[i] = fork(); if (pids[i] == 0) { execv("/bin/echo", args); } } printf("1\n"); fflush(stdout); waitpid(pids[i], NULL, 0); E. } printf("2\n"); fflush(stdout); } Assuming fork and execv do not fail, which are possible outputs? A. 0 (newline) 0 (newline) 1 (newline) 2 4 const char *args[] = {"echo", "0", NULL};

shell allow user (= person at keyboard) to run applications user’s wrapper around process-management functions upcoming homework — make a simple shell 5

aside: shell forms POSIX: command line you have used before also: graphical shells e.g. OS X Finder, Windows explorer other types of command lines? completely difgerent interfaces? 6

some POSIX command-line features searching for programs (not in assignment) running in background (not in assignment) ./someprogram & redirection: ./someprogram >output.txt ./someprogram <input.txt pipelines: ./someprogram | ./somefilter 7 ls -l ≈ /bin/ls -l make ≈ /usr/bin/make

some POSIX command-line features searching for programs (not in assignment) running in background (not in assignment) ./someprogram & redirection: ./someprogram >output.txt ./someprogram <input.txt pipelines: ./someprogram | ./somefilter 8 ls -l ≈ /bin/ls -l make ≈ /usr/bin/make

searching for programs POSIX convention: PATH environment variable example: /home/cr4bd/bin:/usr/bin:/bin list of directories to check in order environment variables = key/value pairs stored with process by default, left unchanged on execve, fork, etc. one way to implement: [pseudocode] for (directory in path) { } 9 execv(directory + "/" + program_name, argv);

some POSIX command-line features searching for programs (not in assignment) running in background (not in assignment) ./someprogram & redirection: ./someprogram >output.txt ./someprogram <input.txt pipelines: ./someprogram | ./somefilter 10 ls -l ≈ /bin/ls -l make ≈ /usr/bin/make

some POSIX command-line features searching for programs (not in assignment) running in background (not in assignment) ./someprogram & redirection: ./someprogram >output.txt ./someprogram <input.txt pipelines: ./someprogram | ./somefilter 11 ls -l ≈ /bin/ls -l make ≈ /usr/bin/make

shell assignment implement a simple shell that supports redirection and pipeline (for Linux or another POSIX system — not xv6) …and prints the exit code of program in the pipeline simplifjed parsing: space-seperated: 12 okay: /bin/ls � -1 � > � tmp.txt not okay: /bin/ls � -l � >tmp.txt okay: /bin/ls � -1 � | � /bin/grep � foo � > � tmp.txt not okay: /bin/ls � -1 � |/bin/grep � foo � >tmp.txt

POSIX: everything is a fjle the fjle: one interface for devices (terminals, printers, …) regular fjles on disk networking (sockets) local interprocess communication (pipes, sockets) basic operations: open(), read(), write(), close() 13

the fjle interface open before use setup, access control happens here byte-oriented real device isn’t? operating system needs to hide that explicit close 14

the fjle interface open before use setup, access control happens here byte-oriented explicit close 14 real device isn’t? operating system needs to hide that

1 or 2 or …via bufger 3 …via bufger data from disk bufger: recently read 2 read block of data from disk 1 from fjle read char 3 kernel bufgering (reads) program 2 from terminal read char waiting for program bufger: keyboard input 1 keypress happens, read disk keyboard operating system 15

1 or 2 or …via bufger 3 …via bufger data from disk bufger: recently read 2 read block of data from disk 1 from fjle read char 3 kernel bufgering (reads) program 2 from terminal read char waiting for program bufger: keyboard input 1 keypress happens, read disk keyboard operating system 15

1 or 2 or …via bufger 3 …via bufger data from disk bufger: recently read 2 read block of data from disk 1 from fjle read char 3 kernel bufgering (reads) program 2 from terminal read char waiting for program bufger: keyboard input 1 keypress happens, read disk keyboard operating system 15

kernel bufgering (reads) …via bufger 3 …via bufger data from disk bufger: recently read 2 read block of data from disk 1 from fjle read char 3 15 program 2 from terminal read char waiting for program bufger: keyboard input 1 keypress happens, read disk keyboard operating system 1 or 2 or

kernel bufgering (reads) …via bufger 3 …via bufger data from disk bufger: recently read 2 read block of data from disk 1 from fjle read char 3 15 program 2 from terminal read char waiting for program bufger: keyboard input 1 keypress happens, read disk keyboard operating system 1 or 2 or

kernel bufgering (reads) …via bufger 3 …via bufger data from disk bufger: recently read 2 read block of data from disk 1 from fjle read char 3 15 program 2 from terminal read char waiting for program bufger: keyboard input 1 keypress happens, read disk keyboard operating system 1 or 2 or

kernel bufgering (writes) to remote machine to be written on disk bufger: data waiting write block of data from disk (when ready) to fjle write char print char program waiting for network bufger: output send data (when ready) disk network operating system 16

kernel bufgering (writes) to remote machine to be written on disk bufger: data waiting write block of data from disk (when ready) to fjle write char print char program waiting for network bufger: output send data (when ready) disk network operating system 16

kernel bufgering (writes) to remote machine to be written on disk bufger: data waiting write block of data from disk (when ready) to fjle write char print char program waiting for network bufger: output send data (when ready) disk network operating system 16

kernel bufgering (writes) to remote machine to be written on disk bufger: data waiting write block of data from disk (when ready) to fjle write char print char program waiting for network bufger: output send data (when ready) disk network operating system 16

kernel bufgering (writes) to remote machine to be written on disk bufger: data waiting write block of data from disk (when ready) to fjle write char print char program waiting for network bufger: output send data (when ready) disk network operating system 16

read/write operations read()/write(): move data into/out of bufger possibly wait if bufger is empty (read)/full (write) actual I/O operations — wait for device to be ready trigger process to stop waiting if needed 17

layering application standard library system calls kernel’s fjle interface device drivers hardware interfaces kernel’s bufgers read/write cout/printf — and their own bufgers 18

why the extra layer better (but more complex to implement) interface: read line formatted input (scanf, cin into integer, etc.) formatted output less system calls (bigger reads/writes) sometimes faster bufgering can combine multiple in/out library calls into one system call more portable interface cin, printf, etc. defjned by C and C++ standards 19

fjlesystem abstraction regular fjles — named collection of bytes also: size, modifjcation time, owner, access control info, … directories — folders containing fjles and directories hierarchical naming: /net/zf14/cr4bd/fall2018/cs4414 mostly contains regular fjles or directories 20

open ... int read_fd = open("dir/file1", O_RDONLY); int write_fd = open("/other/file2", O_WRONLY | O_CREAT | O_TRUNC, 0666); int rdwr_fd = open("file3", O_RDWR); 21 int open( const char *path, int flags); int open( const char *path, int flags, int mode);

open path = fjlename e.g. "/foo/bar/file.txt" file.txt in directory bar in directory foo in “the root directory” e.g. "quux/other.txt other.txt in directory quux in “the current working directory” (set with chdir() ) 22 int open( const char *path, int flags); int open( const char *path, int flags, int mode);