This document provides an overview of Linux including: - Its origins from Unix developed at Bell Labs in the late 1960s - Why Linux was created with its entire source code being free and open - Popular Linux distributions like Debian, RedHat, SUSE, and others - The GNU/Linux architecture including the kernel, shells, and applications - Key components like the GNU compiler collection (GCC) and makefiles - File handling and process APIs in Linux

1. Linux 12 January 2010 Knowx innovation 1

2. Unix Developed at Bell Labs in late 1960s Owned by AT&T at the time, which meant you had to buy it once AT&T figured out it was valuable Written in C (a high level language) Needs to be compiled to run Can be easily moved to a different system by re-compiling for new hardware 12 January 2010 Knowx innovation 2

3. Why Linux Entire OS source code is free Full freedom to study, modify, redistribute. No payments or restrictions. Kernel and many packages follow GNU GPL, LGPL or BSD style license. Powerful command line (shells) Multitasking, SMP, NUMA 12 January 2010 Knowx innovation 3

4. Protected memory True multiuser capabilities Filesystem choices:ext3, JFS, XFS, reiserFS Highly modular & scalable: From embedded systems to IBM mainframes 12 January 2010 Knowx innovation 4

6. RedHat (related: Fedora, CentOS)

7. SUSE (related: OpenSUSE)

8. Mandriva

9. Slackware

10. Embedded Linuxes12 January 2010 Knowx innovation 5

11. GNU/Linux architecture 12 January 2010 Knowx innovation 6

12. Linux kernel Mediates access to the hardware and peripherals Shells Provides user access to the kernel Applications Provide the useful function for the operating system 12 January 2010 Knowx innovation 7

13. Kernel architecture 12 January 2010 Knowx innovation 8

14. Gnu compiler collection(gcc) Compiler and set of utilities to build binaries from high level source code. Standard for embedded systems development Since its supports so many different target architectures Supports a number of languages C, C++, ada, java, FORTRAN, Pascal 12 January 2010 Knowx innovation 9

15. Stage of compilation 12 January 2010 Knowx innovation 10

16. 12 January 2010 Knowx innovation 11

17. makefile 12 January 2010 Knowx innovation 12

18. make utility uses a developer-created input file to describe the project built GNU uses the name Makefile as the default name for its input file. 12 January 2010 Knowx innovation 13

19. appexp : main.o app.o bar.o lib.o gcc –o appexp main.o app.o bar.o lib.o main.o : main.c lib.h app.h gcc –c -o main.o main.c app.o: app.c lib.h app.h gcc –c -o app.o app.c bar.o : bar.c lib.h gcc –c -o bar.o bar.c lib.o : lib.c lib.h gcc –c -o lib.o lib.c Line 1: is the rule The portion of the rule before the colon is called targetand after the colon is called dependencies. 12 January 2010 Knowx innovation 14

20. File handling Accomplished through the standard C library We can create and manipulate ASCII text or binary files with the same API. API fopen, fclose, fwrite , fread, fseek, and rewind. 12 January 2010 Knowx innovation 15

21. fopen Opening a file can also be the mechanism to create a file prototype: FILE * fopen(const char *filename,const char *mode) filename - file we wish to access or create mode – mode we wish to use FILE * - fopen returns the FILE pointer (FILE *) 12 January 2010 Knowx innovation 16

22. Example FILE *fin; fin = fopen("inpfile.txt", "r"); 12 January 2010 Knowx innovation 17

23. Program #include <stdio.h> #define MYFILE "missing.txt" main() { FILE *fin; fin = fopen( MYFILE, "r" ); /* Try to open the file for read */ if (fin == (FILE *)0) /* Check for failure to open */ { printf(“error in opening file”); /* Emit an error message and exit */ exit(-1); } fclose( fin ); /* All was well, close the file */ } 12 January 2010 Knowx innovation 18

24. fread() and fwrite() Prototypes: size_t fread(void *ptr, size_t size, size_t nmemb,FILE *stream); size_t fwrite(const void *ptr, size_t size, size_t nmemb,FILE *stream); fseek() Prototype: int fseek (FILE * stream, long offset, int whence); Function allows to the new position given an index whence argument defines 12 January 2010 Knowx innovation 19

25. SEEK_SET - moves the file position to the position defined by offset SEEK_CUR -moves the file position the number of bytes defined by offset from the current file position SEEK_END - moves the file position to the number of bytes defined by offset from the end of the file rewind () Resets the file read pointer back to the start of the file Prototype void rewind (FILE * stream); 12 January 2010 Knowx innovation 20

26. Program #include <stdio.h> #define MAX_LINE 40 #define FILENAME "myfile.txt“ typedef struct { int id; float x_coord; float y_coord; char name[MAX_LINE+1]; } MY_TYPE_T; MY_TYPE_T object; int main() { int i; FILE *fin; fin = fopen( FILENAME, "r" ); /* Open the input file */ 12 January 2010 Knowx innovation 21

27. if (fin == (FILE *)0) exit(-1); fseek( fin, (2 * sizeof(MY_TYPE_T)), SEEK_SET ); /* Get the last entry */ fread( &object, sizeof(MY_TYPE_T), 1, fin ); printf("%d %f %f %s", object.id, object.x_coord, object.y_coord, object.name ); rewind( fin ); /* Get the second to last entry */ fseek( fin, (1 * sizeof(MY_TYPE_T)), SEEK_SET ); fread( &object, sizeof(MY_TYPE_T), 1, fin ); printf("%d %f %f %s", object.id, object.x_coord, object.y_coord, object.name ); 12 January 2010 Knowx innovation 22

28. /* Get the first entry */ rewind( fin ); fread( &object, sizeof(MY_TYPE_T), 1, fin ); printf("%d %f %f %s", object.id, object.x_coord, object.y_coord, object.name ); fclose( fin ); return 0; } 12 January 2010 Knowx innovation 23

29. getpid() - to get the current process ID. getppid() - to get the parent process ID. getuid() - to get the user ID. getgid() - to get the group ID. 12 January 2010 Knowx innovation 24

30. Program #include <stdio.h> #include <unistd.h> #include <sys/types.h> int main() { pid_t myPid; pid_t myParentPid; gid_t myGid; uid_t myUid; myPid = getpid(); myParentPid = getppid(); myGid = getgid(); 12 January 2010 Knowx innovation 25

31. myUid = getuid(); printf( "my process id is %d", myPid ); printf( "my parent's process id is %d", myParentPid ); printf( "my group id is %d", myGid ); printf( "my user id is %d", myUid ); return 0; } Output my process id is 10932. my parents process id is 10795 My group id is 500 My user id is 500 12 January 2010 Knowx innovation 26

32. Process api Fork Create a new child process Wait Suspend execution until a child processes exits Signal Install a new signal handler Exec Replace the current process image with a new process image 12 January 2010 Knowx innovation 27

33. Fork Creating new processes with in a given process. return value of fork is greater than zero then in the parent context. Equal to zero then in the child context. less than zero then error occurred. 12 January 2010 Knowx innovation 28

34. Example pid_t pid; pid = fork(); if(pid>0) {/*parent context*/} else if (pid ==0) {/*child context*/} else {/*error occurred no child created*/} 12 January 2010 Knowx innovation 29

35. 12 January 2010 Knowx innovation 30

36. Program #include <sys/types.h> #include <sys/wait.h> #include <unistd.h> #include <stdio.h> #include <errno.h> int main() { pid_t ret; int status, I, role = -1; 12 January 2010 Knowx innovation 31

37. ret = fork(); if (ret > 0) { printf("Parent: This is the parent process (pid %d)", getpid()); for (i = 0 ; i < 6 ; i++) { printf("Parent: At count %d", i); sleep(1); } ret = wait( &status ); role = 0; } else if (ret == 0) { printf("Child: This is the child process (pid %d)", getpid()); 12 January 2010 Knowx innovation 32

38. for (i = 0 ; i < 6 ; i++) { printf("Child: At count %d", i); sleep(1); } role = 1; } else { printf("Parent: Error trying to fork() (%d)", errno); } printf("%s: Exiting...", ((role == 0) ? "Parent" : "Child")); return 0; } 12 January 2010 Knowx innovation 33

39. wait Suspend the calling process until a child process(created by this process) exits or until a signal is delivered. Prototype pid_t wait (int *status); 12 January 2010 Knowx innovation 34

40. Signal Install a signal handler for a process. Prototype sighandler_t signal (int signum, sighandler_t handler); SIGHUP - hang up –commonly used to restart a task. SIGKILL - kill signal SIGINT - interrupt from the keyboard SIGSTOP – stop process SIGQUIT – quit signal from keyboard 12 January 2010 Knowx innovation 35

41. Program #include <stdio.h> #include <sys/types.h> #include <sys/wait.h> #include <unistd.h> #include <signal.h> #include <errno.h> void usr1_handler( int sig_num ) { printf( "Process (%d) got the SIGUSR1", getpid() ); } int main() { pid_t ret; int status role = -1; signal( SIGUSR1, usr1_handler ); 12 January 2010 Knowx innovation 36

42. ret = fork(); if (ret > 0) { /* Parent Context */ printf( "Parent: This is the parent process (pid %d)", getpid() ); role = 0; pause(); printf( "Parent: Awaiting child exit" ); ret = wait( &status ); } 12 January 2010 Knowx innovation 37

43. else if (ret == 0) { /* Child Context */ printf( "Child: This is the child process (pid %d)", getpid() ); role = 1; pause(); } else { /* Parent Context -- Error */ printf( "Parent: Error trying to fork() (%d)", errno ); } printf( "%s: Exiting...", ((role == 0) ? "Parent" : "Child") ); return 0; } 12 January 2010 Knowx innovation 38

44. exec Replaces the current process image altogether. Once the exec function replaces the current process, its pid is the same as the creating process Permits the current process context to be replace with the program specifies as the first argument. 12 January 2010 Knowx innovation 39

45. Example execl(“/bin/ls”,”ls”,”-la”,null); This command replaces the current process with the ls (list directory) Prototypes for the variants of exec int execl (const char *path,const char * arg, …,..) int execlp(const char *path,const char *arg,……) int execv(const char *path, char *const argv[]) int execvp(const char *file, char *const argv[]) 12 January 2010 Knowx innovation 40

46. Program for shell interpreter #include <sys/types.h> #include <sys/wait.h> #include <unistd.h> #include <stdio.h> #include <stdlib.h> #include <string.h> #define MAX_LINE 80 int main() { int status; pid_t childpid; char cmd[MAX_LINE+1]; char *sret; 12 January 2010 Knowx innovation 41

47. while (1) { printf("mysh>"); sret = fgets( cmd, sizeof(cmd), stdin ); if (sret == NULL) exit(-1); cmd[ strlen(cmd)-1] = 0; if (!strncmp(cmd, "bye", 3)) exit(0); childpid = fork(); if (childpid == 0) { execlp( cmd, cmd, 0 ); } else if (childpid > 0) { waitpid( childpid, &status, 0 ); } printf(""); } return 0; } 12 January 2010 Knowx innovation 42

48. Message queues Messages are small collections of data (400 bytes, for example) that can be passed between cooperating programs through a message queue. Messages within a queue can be of different types, and any process with proper permissions can receive the messages. 12 January 2010 Knowx innovation 43

50. Program #include <stdio.h> #include <sys/msg.h> #define MY_MQ_ID 111 int main() { int msgid; /* Create the message queue with the id MY_MQ_ID */ msgid = msgget( MY_MQ_ID, 0666 | IPC_CREAT ); if (msgid >= 0) { printf( "Created a Message Queue %d", msgid ); } return 0; } 12 January 2010 Knowx innovation 45

52. MSGSZ is the length of the message sent in bytes. MSGFLG specifies the action to be taken if one or more of the following are true. The number of bytes already in the queue is equal to msg_qbytes. The total number of messages on all queues on the system has reached a maximum limit Action to be taken If (msgflg & IPC_NOWAIT) is non-zero, the message will not be sent and the calling process will return immediately If (msgflg & IPC_NOWAIT) is 0, the calling process will suspend execution until one of the following occurs: 12 January 2010 Knowx innovation 47

53. Program #include <sys/types.h> #include <sys/ipc.h> #include <sys/msg.h> #include <stdio.h> #include <string.h> #define MSGSZ 128 typedef struct msgbuf /* will hold the message to be put in the queue */ { long mtype; /* priority of message */ char mtext[MSGSZ]; /* the message that is stored */ } message_buf; main() { int msqid; int msgflg = IPC_CREAT | 0666; key_t key; message_buf sbuf; size_t buf_length; key = 10; printf("Calling msgget with key %#lx and flag %#o",key,msgflg); 12 January 2010 Knowx innovation 48

54. /*A queue is created using the msget function with a key value 10 and the flag parameter being IPC_CREAT|06668 */ if ((msqid = msgget(key, msgflg )) < 0) { perror("msgget"); exit(1); } else printf("msgget: msgget succeeded: msqid = %d", msqid); sbuf.mtype = 1; /*setting the priority as 1 */ printf("msgget: msgget succeeded: msqid = %d", msqid); (void) strcpy(sbuf.mtext, "I am in the queue?"); /* copy the text "I am in the queue" into the array mtext which is message array */ printf("msgget: msgget succeeded: msqid = %d", msqid); 12 January 2010 Knowx innovation 49

55. buf_length = strlen(sbuf.mtext) + 1 ; /*sending the message with option IPC_NOWAIT */ if (msgsnd(msqid, &sbuf, buf_length, IPC_NOWAIT) < 0) { printf ("%d, %d, %s, %d", msqid, sbuf.mtype, sbuf.mtext, buf_length); perror("msgsnd"); exit(1); } else printf("Message: amp;quot;%samp;quot; Sent", sbuf.mtext); exit(0); } 12 January 2010 Knowx innovation 50

57. program #include <sys/types.h> #include <sys/ipc.h> #include <sys/msg.h> #include <stdio.h> #define MSGSZ 128 typedef struct msgbuf /* structure that will hold the message obtained from the queue*/ { long mtype; /*priority of message */ char mtext[MSGSZ]; /*message that stored */ } message_buf; main() { 12 January 2010 Knowx innovation 52

58. int msqid; key_t key; message_buf rbuf; key = 10; /*creating a queue with the key value 10, */ if ((msqid = msgget(key, 0666)) < 0) { perror("msgget"); exit(1); } /*acquires the message from the queue into rbuf*/ if (msgrcv(msqid, &rbuf, MSGSZ, 1, 0) < 0) { perror("msgrcv"); exit(1); } printf("%s", rbuf.mtext); exit(0); } 12 January 2010 Knowx innovation 53

60. 12 January 2010 Knowx innovation 55

61. Variable that is protected. It provides a means to restrict access to a resource that is shared amongst two or more processes Two operations are permitted , called acquire and release. 2 types of semaphore. Binary and Counting 12 January 2010 Knowx innovation 56 Semaphore

62. The signal notations P(semaphore variable) for wait V(semaphore variable) for signal 12 January 2010 Knowx innovation 57

63. A binary semaphore is a variable that can take only the values 0 and 1. Represents a single resource and therefore when one process has acquired it,others are blocked until it is released. P(sv) if sv greater than 0 , decrement sv.ifsvis zero suspend execution of the process V(sv) if other process has been suspended waiting for sv, make it resume.if no waiting, increment sv 12 January 2010 Knowx innovation 58 Binary semaphore

64. 12 January 2010 Knowx innovation 59

65. Represent shared resources in quantities greater than one. It could represent the entire set of buffers by setting its value to the number of buffers available. Whenever the process acquires the semaphore, the value decrement . When the semaphore value reaches zero, process are blocked until it becomes a non zero. 12 January 2010 Knowx innovation 60 Counting semaphore

66. 12 January 2010 Knowx innovation 61

68. sem_id, is the semaphore identifier, as returned from semget. sem_ops, is a pointer to an array of structures, each of which will have at least the following members: Struct sembuf {short sem_num; short sem_op; short sem_flg; } 12 January 2010 Knowx innovation 63

69. Program to acquire the semaphore. #define MY_SEM_ID 111 #define MY_SEMARRAY_ID 112 #define NUM_SEMAPHORES 10 #include <stdio.h> #include <sys/sem.h> #include <stdlib.h> #include "common.h" int main() { int semid; struct sembufsb; /* Get the semaphore with the id MY_SEM_ID */ semid = semget( MY_SEM_ID, 1, 0 ); 12 January 2010 Knowx innovation 64

70. if (semid >= 0) { sb.sem_num = 0; sb.sem_op= -1; sb.sem_flg= 0; printf( "semacq: Attempting to acquire semaphore %d", semid ); /* Acquire the semaphore */ if (semop( semid, &sb, 1 ) == -1) { printf("semacq: semop failed."); exit(-1); } printf( "semacq: Semaphore acquired %d", semid); } return 0; } 12 January 2010 Knowx innovation 65

71. Program to release the semaphore. #define MY_SEM_ID 111 #define MY_SEMARRAY_ID 112 #define NUM_SEMAPHORES 10 #include <stdio.h> #include <sys/sem.h> #include <stdlib.h> #include "common.h" int main() { int semid; struct sembufsb; /* Get the semaphore with the id MY_SEM_ID */ semid = semget( MY_SEM_ID, 1, 0 ); 12 January 2010 Knowx innovation 66

72. if (semid >= 0) { printf( "semrel: Releasing semaphore %d", semid); sb.sem_num = 0; sb.sem_op= 1; sb.sem_flg= 0; /* Release the semaphore */ if (semop( semid, &sb, 1 ) == -1) { printf("semrel: semop failed."); exit(-1); } printf( "semrel: Semaphore released %d", semid ); } return 0; } 12 January 2010 Knowx innovation 67

73. 12 January 2010 Knowx innovation 68