UNIX System Programming UNIX System Programming 1. What is a Process? 1. What is a Process? Processes 2. fork() 2. fork() 3. Example: 3. Example: talkto.c talkto.c 4. exec() 4. exec() 5. wait() 5. wait() 6. Process Data 6. Process Data � Objectives Objectives � 7. Special Exit Cases 7. Special Exit Cases – look at how to program UNIX processes – look at how to program UNIX processes 8. Process IDs 8. Process IDs – fork() and exec() – fork() and exec() 1 2 1730 UNIX System Programming: Processes Maria Hybinette 1730 UNIX System Programming: Processes Maria Hybinette Overview Overview 1. What is a Process? 1. What is a Process? 1. What is a Process? � A process is an executing program. A process is an executing program. � 2. fork() 3. Example: talkto.c � A process: A process: � 4. exec() $ cat file1 file2 & 5. wait() � Two processes: Two processes: 6. Process Data � $ ls | wc - l $ 7. File Descriptors across Processes 8. Special Exit Cases � Each user can run many processes at once Each user can run many processes at once � 9. IO Redirection (e.g. using & (e.g. using & ) ) 10. User/Group ID real and effective continued 3 4 1730 UNIX System Programming: Processes Maria Hybinette 1730 UNIX System Programming: Processes Maria Hybinette A More Precise Definition What makes up a Process? A More Precise Definition What makes up a Process? � program code program code � � A process is the A process is the context context (the (the � information/data) maintained for an information/data) maintained for an � data variables data variables � executing program. executing program. � open files (file descriptors) open files (file descriptors) � � an Environment (environment variables; an Environment (environment variables; � credentials for security) credentials for security) 5 6 1730 UNIX System Programming: Processes Maria Hybinette 1730 UNIX System Programming: Processes Maria Hybinette 1 UNIX: processes Maria Hybinette
Some of the Context Some of the Context Information Information – Process ID ( Process ID ( pid ) unique integer – pid ) unique integer – – Pointer to program code Pointer to program code – Parent process ID ( – Parent process ID ( ppid ppid ) ) – Pointer to data – Pointer to data Memory for global vars Memory for global vars – Real User ID – Real User ID ID of user/process which ID of user/process which – – Pointer to stack Pointer to stack Memory for local Memory for local vars vars started this process started this process – Pointer to heap Pointer to heap Malloc’d memory memory – Malloc’d – Effective User ID – Effective User ID ID of user who wrote ID of user who wrote the process’ program the process’ program – Execution priority Execution priority – – Current directory – Current directory – – Signal information Signal information – File descriptor table – File descriptor table – Environment – Environment VAR=VALUE pairs pairs VAR=VALUE continued 7 8 1730 UNIX System Programming: Processes Maria Hybinette 1730 UNIX System Programming: Processes Maria Hybinette Important System Unix Start Up Processes Diagram Important System Unix Start Up Processes Diagram Processes Processes OS kernel Process 0 � init init – – Mother of all processes. init is started Mother of all processes. init is started � (sched) at boot time and is responsible for starting at boot time and is responsible for starting Process 1 other processes. other processes. (init) – init uses file – init uses file inittab inittab & directories: /etc/ & directories: /etc/rc?.d rc?.d � getty getty – – login process that manages login login process that manages login � getty getty getty sessions. sessions. login login csh bash 9 10 1730 UNIX System Programming: Processes Maria Hybinette 1730 UNIX System Programming: Processes Maria Hybinette Pid and Parentage and Parentage 2. fork() Pid 2. fork() � A process ID or A process ID or pid pid is a positive integer that uniquely is a positive integer that uniquely � � #include <sys/types.h> identifies a running process, and is stored in a variable of identifies a running process, and is stored in a variable of #include <unistd.h> type type pid_t pid_t . . pid_t fork( void ); � You can get the You can get the process process pid pid or or parent’s parent’s pid pid � � Creates a child process by making a copy of Creates a child process by making a copy of � #include <sys/types> the parent process. the parent process. main() { pid_t pid, ppid; � Both the child Both the child and and the parent continue the parent continue printf( "My PID is:%d\n\n",(pid = getpid()) ); � printf( "Par PID is:%d\n\n",(ppid = getppid()) ); running. running. } 11 12 1730 UNIX System Programming: Processes Maria Hybinette 1730 UNIX System Programming: Processes Maria Hybinette 2 UNIX: processes Maria Hybinette
fork() as a diagram fork() as a diagram Process IDs (pids Process IDs ( pids revisited) revisited) Parent � pid pid = fork(); = fork(); � pid = fork() Child � In the child: In the child: pid == 0 ; ; � pid == 0 In the parent: pid In the parent: == the process ID of the the process ID of the Returns a new PID: pid == pid == 0 e.g. pid == 5 child. child. Shared Program Data Data Copied � A program can use this A program can use this pid pid difference to do difference to do � different things in the parent and child. different things in the parent and child. 13 14 1730 UNIX System Programming: Processes Maria Hybinette 1730 UNIX System Programming: Processes Maria Hybinette fork() Example (parchld.c parchld.c) ) fork() Example ( #include <stdio.h> #include <sys/types.h> #include <unistd.h> int main() else { { pid_t pid; /* could be int */ /* child */ int i; for( i=0; I < 1000; i++ ) pid = fork(); printf( “CHILD %d\n”, i ); if( pid > 0 ) } { return 0; /* parent */ } for( i=0; i < 1000; i++ ) printf(“\t\t\tPARENT %d\n”, i); } 15 16 1730 UNIX System Programming: Processes Maria Hybinette 1730 UNIX System Programming: Processes Maria Hybinette Possible Output Things to Note Things to Note Possible Output i is copied between parent and child. is copied between parent and child. � i � CHILD 0 CHILD 1 CHILD 2 � The switching between the parent and child The switching between the parent and child PARENT 0 � PARENT 1 depends on many factors: depends on many factors: PARENT 2 – – machine load, system process scheduling machine load, system process scheduling PARENT 3 CHILD 3 CHILD 4 � I/O buffering effects amount of output shown. I/O buffering effects amount of output shown. � PARENT 4 : � Output interleaving is Output interleaving is nondeterministic nondeterministic � – cannot determine output by looking at code – cannot determine output by looking at code 17 18 1730 UNIX System Programming: Processes Maria Hybinette 1730 UNIX System Programming: Processes Maria Hybinette 3 UNIX: processes Maria Hybinette
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