The document discusses signals and daemon processes in Unix/Linux systems. It covers key concepts like process structure, environment variables, resource limits, wait/fork functions, exec functions, and the system() function. It provides code examples to demonstrate using these functions and concepts for interprocess communication and launching new processes.

2. Stack Heap Uninitialized data (bss) Initialized data Text Command line args and environment variables Initialized to zero by exec Read from program by exec

3. Variable Standards- POSIX.1 Standards- XPG3 Implementation: SVR4 Implementation-4.3+BSD Description HOME     Home directory LANG    Name of locale LC_ALL    Name of locale LC_COLLATE    Name of locale for collation LC_CTYPE    Name of locale for character classification LC_MONETARY    Name of locale for monetary editing LC_NUMERIC    Name of locale for numeric editing LC_TIME    Name of locale for date/time formatting LOGNAME     Login name NLSPATH   Sequence of templates for message catalogs PATH     List of path prefixes to search for executable files TERM     Terminal type TZ     Time zone information

4. Function Std- ANSI C Std: POSIX.1 Std: XPG3 Impl: SVR4 Impl: 4.3BSD getenv      putenv (maybe)    setenv  unsetenv  clearenv (maybe)

5. #include <sys/time.h> #include <sys/resource.h> int getrlimit (int resource, struct rlimit *rlptr); int setrlimit (int resource, const struct rlimit *rlptr); struct rlimit{ rlim_t rlim_cur; /*soft limit: current limit */ rlim_t rlim_max; /* hard limit: maximum value for rlim_cur */ }; getrlimit and setrlimit functions

7. RLIMIT_CORE The maximum size in bytes of a core file RLIMIT_CPU The maximum amount of CPU time in seconds (SIGXCPU) RLIMIT_DATA The maximum size in bytes of the data segment RLIMIT_FSIZE The maximum size in bytes of a file that may be created (SIGFSZ) RLIMIT_MEMLOCK Locked-in memory address space RLIMIT_NOFILE The maximum number of open files per process RLIMIT_NPROC The maximum number of child processes per user id RLIMIT_OFILE Same as RLIMIT_NOFILE RLIMIT_RSS Maximum resident set size in bytes RLIMIT_STACK The maximum size in bytes of the stack RLIMIT_VMEM The maximum size in bytes of the mapped address space

8. #include <sys/types.h> #include <sys/time.h> #include <sys/resource.h> #include &quot;ourhdr.h&quot; #define doit(name) prlimits(#name, name) static void prlimits(char *, int); int main (void) { doit(RLIMIT_CORE); doit(RLIMIT_CPU); doit(RLIMIT_DATA); doit(RLIMIT_FSIZE); #ifdef RLIMIT MEMLOCK doit(RLIMIT_MEMLOCK); #endif #ifdef RLIMIT_NOFILE /* SVR4 name */ doit(RLIMIT_NOFILE); #endif #ifdef RLIMIT_OFILE /* 4.3+BSD name */ doit(RLIMIT_OFILE); #endif #ifdef RLIMIT_NPROC doit(RLIMIT_NPROC); #endif #ifdef RLIMIT_RSS doit(RLIMIT RSS); #endif doit(RLIMIT_STACK); #ifdef RLIMIT_VMEM doit(RLIMIT_VMEM); #endif exit(0); }

9. static void pr_limits (char *name, int resource) { struct rlimit limit ; if (getrlimit(resource, &limit) < 0) err_sys(&quot;getrlimit error for %S&quot;, name); printf (&quot;%-14s &quot;, name); if (limit.rlim_cur == RLIM_INFINITY) printf(&quot; (infinite) &quot;); else printf(&quot;%10ld &quot;, limit.rlim_cur); if (limit.rlim_max == RLIM_INFINITY) printf(&quot; (infinite)&quot;); else printf(&quot;%10ld&quot;, limit.rlim_max); }

10. Output – SVR4 RLIMIT_CORE 1048576 1048576 RLIMIT_CPU (infinite) (infinite) RLIMIT_DATA 16777216 16777216 RLIMIT_FSIZE 2097152 2097152 RLIMIT_NOFILE 64 1024 RLIMIT_STACK 16777216 16777216 RLIMIT_VMEM 16777216 16777216

12. File descriptor table current directory root … . Text Data Stack Process U-area A Process Process Table Per-process region table A UNIX process data structure Kernel region Table

14. Process Table Data Structure of Parent and child after fork Kernel region Table Text Data Stack Stack Data File desc table File desc table File Table Parent Child

19. int main (void) { pid_t pid; if ( (pid = fork() <0) err_sys(“fork error&quot;); else if (pid == 0) /* first child */ { if ( (pid = fork () < 0) err_sys(&quot;fork error&quot;); else if (pid > 0) exit (0); /* parent from second fork == first child */ /* We're the second child; our parent becomes init as soon as our real parent calls exit () in the statement above. Here's where we'd continue executing, knowing that when we're done, init will reap our status. */ sleep(2); printf (&quot; second child, parent pid = %d It, getppid () ) ; exit(0); } if (waitpid(pid, NULL, 0) != pid) /* wait for first child */ err_sys(&quot;waitpid error&quot;); /* We're the parent (the original process); we continue executing, knowing that we're not the parent of the second child. */ exit (0) ; }

20. Wait3 and Wait4 functions #include <sys/types.h> #include <sys/wait.h> #include <sys/time.h> #include <sys/resource.h> pid_t wait3(int *statloc, int options, struct rusage *rusage); pid_t wait4(pid_t pid, int *statloc, int options, struct rusage *rusage); Function Pid Options Rusage POSIX.1 SVR4 4.3+BSD wait waitpid wait3 wait 4

22. #include &quot;ourhdr.h&quot; TELL_WAIT () ; /* set things up for TELL xxx & WAIT xxx */ if ( (pid = fork () < 0) err_sys(&quot;fork error&quot;); else if (pid == 0) { /* child */ /* child does whatever is necessary... */ TELL_PARENT(getppid()); /* tell parent we're done */ WAIT_PARENT () ; /* and wait for parent */ /* and the child continues on its way... */ exit(0); } /* parent does whatever is necessary... */ TELL_CHILD (pid) ; /* tell child we're done */ WAIT_CHILD () ; /* and wait for child */ /* and the parent continues on its way... */ exit (0) ;

23. #include <sys/types. h> #include &quot;ourhdr.h&quot; static void charatatime (char *); int main (void) { pid_t pid; if ( (pid = fork () < 0) err_sys (&quot;fork error&quot;); else if (pid == 0) { charatatime (&quot;output from child&quot;); ) else { charatatime(.&quot;output from parent&quot;); ) exit(0) ; } static void charatatime (char *str) { char *ptr; int c; setbuf (stdout, NULL); /* set unbuffred */ for (ptr = str; c = *ptr++; ) putc(c, stdout); }

24. Output $ a.out output from child output from parent $ a.out oouuttppuutt ffrroomm cphairledn t $ a.out oouuttppuutt ffrroomm pcahrielndt $ a.out ooutput from parent utput from child

25. int main (void) { pid_t pid; + TELL_WAIT () ; + if ( (pid = fork() < 0) err_sys(&quot;fork error&quot;); else if (pid == 0) { + WAIT_PARENT(); /* parent goes first */ charatatime(&quot;output from child&quot;); } else { charatatime(&quot;output from parent&quot;); + TELL_CHILD(pid); } exit (0) ; } static void charatatime(char *str) { char *ptr; int c; setbuf(stdout, NULL); /* set unbuffered */ for (ptr = str; c = *ptr++;) putc(c, stdout); }

27. Differences Limit on the total size of the argument list : ARG_MAX 4096 on a POSIX.1 system. Eg: grep _POSIX_SOURCE /usr/include/*/*.h Function pathname filename arg list argv[ ] environ emp[ ] execl execlp execle execv execvp execve (Letter in name) P L V e

29. Relationship execlp execl execle execvp execv execve (system call) build argv try each PATH prefix use environ build argv build argv

30. char *env_init[] = { &quot;USER=unknown&quot;, &quot;PATH=/tmp&quot;, NULL }; int main (void) { pid_t pid; if ( (pid = fork () < 0) err_sys (&quot; fork error&quot;); else if (pid == 0) { /* specify pathname, specify environment */ if (execle (&quot; /home/stevens/bin/echoall&quot;, &quot;echoall&quot;, &quot;myarg1&quot;, &quot;MY ARG2&quot;, (char *) 0, env_init < 0) err_sys(&quot;execle error&quot;); } if (waitpid(pid, NULL, 0) < 0) err_sys (&quot;wait error&quot;); if ( (pid = fork () < 0) err_sys (&quot;fork error&quot;); else if (pid == 0) { /* specify filename, inherit environment */ if (execlp(&quot;echoall“,&quot;echoall&quot;, &quot;only 1 arg&quot;, (char *) 0) < 0) err_sys(&quot;execlp error&quot;); } exit (0) ; }

31. #include &quot;ourhdr.h&quot; int main(int argc, char *argv[]) { int i; char **ptr; extern char **environ; for (i = 0; i < argc; i++) /* echo all command-line args */ printf(&quot;argv[%d]: %s&quot;, i, argv[i]); for (ptr = environ; *ptr != 0; ptr++) printf(&quot;%s&quot;, *ptr); /* and all env strings *f exit (0) ; }

32. $ a.out argv[0]: echoall argv[l]: myarg1 argv[2]: MY ARG2 USER=unknown PATH=/tmp $ argv[0]: echoall argv[1]: only 1 arg USER=stevens HOME=/home/stevens LOGNAME=stevens 31 more lines……. EDITOR=/usr/ucb/vi

33. int system(const char *cmdstring) { pid_t pid; int status; if (cmdstring == NULL) return(1); /* always a command processor with UNIX */ if ((pid = fork()) < 0) { status = -1; /* probably out of processes */ } else if (pid == 0) { /* child */ execl(&quot;/bin/sh&quot;, &quot;sh&quot;, &quot;-c&quot;, cmdstring, (char *)0); _exit(127); /* execl error */ } else { /* parent */ while (waitpid(pid, &status, 0) < 0) { if (errno != EINTR) { status = -1; /* error other than EINTR from waitpid() */ break; } } } return(status); }

34. Exit Code Number Meaning Example Comments 1 catchall for general errors let &quot;var1 = 1/0&quot; miscellaneous errors, such as &quot;divide by zero&quot; 2 misuse of shell builtins, according to Bash documentation Seldom seen, usually defaults to exit code 1 126 command invoked cannot execute permission problem or command is not an executable 127 &quot;command not found&quot; possible problem with $PATH or a typo 128 invalid argument to exit exit 3.14159 exit takes only integer args in the range 0 - 255 128+n fatal error signal &quot;n&quot; kill -9 $PPIDof script $? returns 137 (128 + 9) 130 script terminated by Control-C Control-C is fatal error signal 2, (130 = 128 + 2, see above) 255 exit status out of range exit -1 exit takes only integer args in the range 0 - 255

47. int main (void) { pid_t pid; if ( (pid = fork ()) < 0) err_sys(&quot;fork error&quot;); else if (pid != 0) { /* parent */ sleep(2); exit(2); /* terminate with exit status 2 */ } /* first. child */ if ( (pid = fork()) < 0) err_sys(&quot;fork error&quot;); else if (pid != 0) { sleep(4); abort (); /* terminate with core dump */ } /* second child */ if ( (pid = fork()) < 0) err_sys(&quot;fork error&quot;); else if (pid != 0) { execl (&quot; /usr/bin/dd&quot;, &quot;dd&quot;, &quot;if=/boot&quot;, &quot;of=/dev/null&quot;,NULL) exit(7); /* shouldn't get here */ /* third child */ if ( (pid = fork ()) < 0) err_sys(&quot;fork error&quot;); else if (pid != 0) { sleep(8); exit(0); /* normal exit */ } /* fourth child */ sleep(6); kill(getpid(), SIGKILL); /* terminate with signal, no core dump */ exit(6); /* shouldn't get here */ }

52. Network Login init inetd inetd telnetd Login shell Pseudo-terminal device driver User at a terminal TCP connection request From TELNET client fork exec fd 0, 1, 2 Executes /etc/rc When connection request arrives Network connection through telnetd server and telnet client telnet hostname

57. Controlling Terminal Login shell proc1 proc2 proc3 proc4 proc5 Background Process group session leader = controlling prcess Background Process group Foreground Process group Controlling terminal Terminal input and terminal generated signals Modem disconnect (hangup signal)