The Environment of Unix Process 2/19/2019 The Environment of Unix Process The environment of a single process How the main function is called How command-line arguments are passed to the new program What the typical memory layout looks like How to allocate additional memory How the process can use environment variables Different ways of process to terminate 2/19/2019 Nittida Nuansri

The Environment of Unix Process 2/19/2019 The Environment of Unix Process main function int main(int argc, char *argv[]); argc -- the number of command-line arguments argv -- an array of pointers to the arguments When a C program is started by the kernel a special start-up routine is called before the main function is called the start-up routine takes values from the kernel the command-line arguments the environment the start-up routine sets things up (and also clear things up) 2/19/2019 Nittida Nuansri

The Environment of Unix Process: Process Termination Normal Termination return from main calling exit() calling _exit() Abnormal Termination calling abort() terminated by a signal The start-up routine calls the exit() function when the main function returns (clear things up) 2/19/2019 Nittida Nuansri

The Environment of Unix Process: Process Termination Process Termination :: exit() and _exit() functions #include <stdlib.h> void exit(int status); #include <unistd.h> void _exit(int status); _exit() returns to the kernel immediately exit() performs certain cleanup processing and then returns to the kernel fclose() is called for all open streams caused all buffered output data to be flushed 2/19/2019 Nittida Nuansri

The Environment of Unix Process: Process Termination atexit Function A process can register up to 32 functions that are automatically called by exit() These are called exit handlers and are registered by calling the atexit() function #include <stdlib.h> int atexit(void (* func)(void); Returns: 0 if ok, nonzero on error The exit() function calls these functions in reverse order of their registration Each function is called as many times as it was registered 2/19/2019 Nittida Nuansri

The Environment of Unix Process: Process Termination 2/19/2019 The Environment of Unix Process: Process Termination user process user function exit handler _exit return call exit return call main function exit handler return exit _exit call call exit function return call call standard I/O cleanup return C start-up routine exit _exit exec kernel 2/19/2019 Nittida Nuansri

The Environment of Unix Process Note: The only way a program is executed by the kernel is when one of the exec functions is called 2/19/2019 Nittida Nuansri

Example of exit handlers $ a.out main is done first exit handler second exit handler 2/19/2019 Nittida Nuansri

The Environment of Unix Process Command-Line Arguments When a program is executed, the process that does the exec can pass command-line arguments to the new program TO DO : Write your own program to demonstrate how to pass command-line arguments…. And display how many arguments are passed each time When is it useful? 2/19/2019 Nittida Nuansri

The Environment of Unix Process Environment List Each program is also passed an environment list which is an array of char pointers, Each pointer containing the address of a null-terminated string The address of the array of pointers is contained in the global variable environ extern char **environ; 2/19/2019 Nittida Nuansri

The Environment of Unix Process 2/19/2019 The Environment of Unix Process Environment List Environment list Environment Strings NULL SHELL=/bin/sh\0 Environment Strings NULL Environment list Environment Strings NULL USER=xx\0 Environment Strings NULL Environment Strings NULL Environment list Environment Strings NULL Environment pointer environ: HOME=/home/xx\0 PATH=:/bin:/sbin\0 extern char **environ; 2/19/2019 Nittida Nuansri

The Environment of Unix Process Environment Variables #include <stdlib.h> char *getenv(char *name);’ Returns: pointer to value associated with name, NULL if not found int putenv(char *str); int setenv(char *name, char *value, int rewrite); Returns: 0 if OK, nonzero on error 2/19/2019 Nittida Nuansri

The Environment of Unix Process Memory Layout of a C Program Historically a C program has been composed of Text segment Initialised data segment Uninitialised data segment Stack Heap 2/19/2019 Nittida Nuansri

The Environment of Unix Process: Memory Layout Text segment machine instructions, executed by the CPU sharable, only a single copy needs to be in the memory for frequently executed program read-only, to prevent a program from being accidently modifying its instructions Initialised data segment (or data segment) contains variables that are specifically initialised in the program e.g. int max = 99; appearing outside any function -- stored in the initialised data segment 2/19/2019 Nittida Nuansri

The Environment of Unix Process: Memory Layout Unintialised data segment often called “bss” segment -- block started by symbol data is initialised by the kernel to be 0 before the program starts executing e.g. long sum[100]; appear outside any function ==> stored in uninitialised data segment Stack stores automatic variables + information saved each time a function is called returned address caller’s environment information 2/19/2019 Nittida Nuansri

The Environment of Unix Process: Memory Layout Heap Dynamic memory allocation usually takes place on the heap Normally located between the uniintialised data segment and the stack Write a Program with all types of variables to be stored in each of these segments 2/19/2019 Nittida Nuansri

The Environment of Unix Process: Memory Layout Typical Memory Management high address command-line arguments and environment variables stack heap initialised to zero by exec uninitialised data uninitialised data read from prog file by exec low address text 2/19/2019 Nittida Nuansri

The Environment of Unix Process: Memory Layout The size(1) command reports size of the text data, bss segments, e.g $ size /bin/cc /bin/sh text data bss dec hex 81920 16384 664 98968 18298 /bin/cc 90112 16384 0 106496 1a000 /bin/sh 2/19/2019 Nittida Nuansri

The Environment of Unix Process: Shared Library A single copy of library routine is used by many processes Reduce the size of each executable file The library function can be replaced with new versions without having to re-link every program that uses the library Disadvantage : may add some run-time overhead, either when the program is first execed, or when the library function is called 2/19/2019 Nittida Nuansri

The Environment of Unix Process: Shared Library TO DO Write a “classic hello.c” program, and compile it normally (cc hello.c) using a shared library option What are the differences, in term or size measures Explain “why?” 2/19/2019 Nittida Nuansri

The Environment of Unix Process: Memory Allcoation Three functions for memory allocation malloc() calloc() realloc() Self-study !!!! 2/19/2019 Nittida Nuansri

The Environment of Unix Process: setjmp and longjmp Functions goto statement Scope? setjmp() and longjmp() useful for handling error conditions that occur in a deeply nested function call #include <setjmp.h> int setjmp(jmp_buf env); void longjmp(jmp_buf env, int val); More 2/19/2019 Nittida Nuansri

Process Control Process Control Provided by UNIX Creation of new processes 2/19/2019 Nittida Nuansri

Process Control Process Identifiers -- Process ID Unique nonnegative integer Some special processes, such as swapper -- process ID 0 part of the kernel known as a system process init -- process ID 1 a normal user process, runs with superuser privileges bringing up a Unix system read the system-dependent initialisation files never dies pagedaemon -- process ID 2 supporting the paging of the virtual memory system 2/19/2019 Nittida Nuansri

Process Control Process Identifiers -- Other IDs parent process ID getppid() real user ID getuid() effective user ID geteuid() real group ID getgid() effective group ID getegid) 2/19/2019 Nittida Nuansri

Process Control fork Function pid_t fork(void) creates a new process -- child process called once, but returns twice the return value in the child is 0 the return value in the parent is the process ID of the the child WHY the return values are 0 and the child PID ? (given all functions related to processes are as from the previous page)’ both the child and parent continue executing with the instruction that follos the call to fork the child is a copy of the parent gets a copy of the parent’s data space, heap, and stack 2/19/2019 Nittida Nuansri

Process Control Write a program to demonstrate the fork function…. when executed, the result should be as follows $ a.out a write to standout out --- > by “write()” before fork --- > by “printf()” pid = XXX, global variable = XX, var = XX When global => external variable in initialised data var => automatic variable on the stack 2/19/2019 Nittida Nuansri

Process Control Run the program several times Then run the program as What is the order of the execution -- from the parent or child process first? Why? -- reasons to support your answer Then run the program as $ a.out > temp.out Study the result Explain your result 2/19/2019 Nittida Nuansri

Process Control File Sharing -- for parent & child processes All descriptors that are opened in the parent are duplicated in the child Consider a process that has three different files opened for standard input, standard output and standard error. Write a diagram of file descriptors arrangement (process table, file table, etc.) on return from fork AND explain your diagram in words. 30 mins. 2/19/2019 Nittida Nuansri

Process Control File offset sharing and Synchronisation issue It is important that the child and parent share the same file offset E.g. --- assume that the parent and child write to stdout a parent forks a child, then wait for a child to complete if the parent has its stdout redirected (by a shell), it is essential that the parent’s file offset must be updated by the child the child can write to stdout while the parent is waiting the parent continue writing on the completing of the child -- its output must be appended to whaever the child wrote 2/19/2019 Nittida Nuansri

Process Control Two normal cases for handling the descriptors after a fork The parent waits for the child to complete parent does not need to do anything with its descriptors when the child terminated, file offsets are updated accordingly The parent and child each go their own way after the fork, the parent closes the descriptors that does not need the child does the same thing -- > not interferes with the other’s open fd often the case with network servers 2/19/2019 Nittida Nuansri

2/19/2019 Process Control Study the process characteristics/properties and answer the following questions What are the properties of the parent that that are inherited by the child What are the differences between the parent and the child Study vfork() function, what is the different between fork() and vfork() 2/19/2019 Nittida Nuansri

Process Control Process Termination By return() exit(), atexit(), _exit() abort() when a process receives certain signals, such as devided by zero.. 2/19/2019 Nittida Nuansri

Process Control Process Termination The same code in the kernel is executed closes all open filederscriptors release memory occupied by the process Notify the parent process, by for nomal termination : passing an exit status (if exit and _exit is used) to the parent for abnormal termination : the kernel generate a termination status to indicate the reason for the abnormal termiation 2/19/2019 Nittida Nuansri

Process Control Process Termination The parent process can obtain this status from wait() or waitpid() If the child terminates (before the parent) and does not send a status to the parent the kernel keeps information about all terminating processes this information is available for the parent process (when wait() or waitpid() is called) 2/19/2019 Nittida Nuansri

Process Control wait() and waitpid() Functions When a process is terminated the kernel sends SIGCHLD signal to the parent it is a synchronous notification signal the parent can choose to ignore, or provide a signal handler function (is called when a signal occurs) 2/19/2019 Nittida Nuansri

Process Control wait() and waitpid() Functions If a process calls wait() or waitpid(), it can block (if all of its child are still running), or return immediately with the termination status of a child (if a child has terminated and is waiting for its status to be fetched), or return immediately with an error (if it doesn’t have any child processes) 2/19/2019 Nittida Nuansri

Process Control wait() and waitpid() Functions #include <sys/types.h> #include <sys/wait.h> pid_t wait(int *statloc); pid_t waitpid(pid_t pid, int *statloc, int options); wait() can block the caller until a child process terminates waitpid() has an option that prevents it from blocking 2/19/2019 Nittida Nuansri