程序代写案例-2C

  • February 8, 2021

1Unit 1: C Programming and Intro to Linux A crash course for C++/Windows Users 2C vs. C++: Differences • C does not have classes/objects! – all code is in functions (subroutines). • C structures can not have methods • C I/O is based on library functions: – printf, scanf, fopen, fclose, fread, fwrite, … 3C vs. C++: Differences (cont.) • C does not support any function overloading (you can’t have 2 functions with the same name). • C does not have new or delete, you use malloc() and free() library functions to handle dynamic memory allocation/deallocation. • C does not have reference variables 4C vs. C++: Similarities • Built-in data types: int, double, char, etc. • Preprocessor (handles #include, #define, etc.) • Control structures: if, while, do, for, etc. • Operators: + – * / = == != < > += ++ etc. 5C vs. C++: Similarities (cont.) • There must be a function named main(). • function definitions are done the same way. • Can split code in to files (object modules) and link modules together. 6Evolution of C • Traditional C: 1978 by K&R • Standard C: 1989 (aka ANSI C) • Standard C: 1995, amendments to C89 standard • Standard C: 1999, is the new definitive C standard replace all the others. • GCC is a C99 compliant compiler (mostly, I think :-)). 7Standard C (C89) • The addition of truly standard library – libc.a, libm.a, etc. • New processor commands and features • Function prototypes — argument types specified in the function declaration • New keywords: const, volatile, signed • Wide chars, wide strings and multibyte characters. • Clarifications to conversion rules, declarations and type checking 8Standard C (C95) • 3 new std lib headers: iso646.h, wctype.h and wchar.h • New formatting codes for printf/scanf • A large number of new functions. 9Standard C (C99) • Complex arithmetic • Extensions to integer types, including the longer standard type (long long, long double) • Boolean type (stdbool.h) • Improved support for floating-point types, including math functions for all types • C++ style comments (//) • For-loop variable initialization – for (int i = 0; …; … ) {} 10 Simple C Program #include int main(void) { printf(“Hello World\n”); return(0); } 11 Another Program #include void printhello(int n) { int i; for (i=0;iprintf(“Hello World\n”); } void main() { printhello(5); } Valid C Program?! • Is the following program a valid C file? 12 int foo(int & x) { return x * x; } int main() { int y = 4; return foo(y); } 13 Typical C Program includes #include #include #define MAX 1000 typedef char bigstring[MAX]; char *reverse( char *s) { char buf[MAX]; int i,len; len = strlen(s); printf(“reversing % s\n”,s); for (i=0;ibuf[i] = s[len-i-1]; buf[i]=’\0′; strcpy(s,buf); return(s); } void main(int argc,char **argv) { if (argc<2) { printf("Invalid usage - must supply a string\n"); exit(0); } printf("% s\n",reverse(argv[1])); } defines, data type definitions, global variable declarations function definitions main() 14 A Real C Example Program • Program that accepts one command line argument. • Treats the command line argument as a string, and reverses the letters in the string. • Prints out the result (the reversed string). 15 reverse.c - part 1 #include /* printf */ #include /* malloc,free */ /* MAX is the size of the largest string we can handle */ #define MAX 1000 /* bigstring is a new data type */ typedef char bigstring[MAX]; 16 reverse.c - part 2 /* reverses a string in place returns a pointer to the string */ char *reverse( char *s ) { bigstring buf; int i,len; len = strlen(s); /* find the length */ for (i=0;ibuf[i] = s[len-i-1]; buf[i]='\0'; /* null terminate!*/ strcpy(s,buf); /* put back in to s */ return(s); } 17 reverse.c - part 3 void main(int argc,char **argv) { if (argc<2) { printf("Invalid usage - must supply a string\n"); exit(0); } printf("%s\n",reverse(argv[1])); } 18 Using dynamic allocation char *reverse( char *s) { char *buf; int i,len; len = strlen(s); /* allocate memory len + 1 for null term */ buf = (char *)malloc(len+1); for (i=0;ibuf[i] = s[len-i-1]; buf[i]='\0'; strcpy(s,buf); free(buf); return(s); } 19 Compiling on Unix Traditionally the name of the C compiler that comes with Unix is “cc”. We can use the Gnu compiler named “gcc”. gcc –Wall –o reverse reverse.c tells the compiler to create executable file with the name reverse tells the compiler the name of the input file. 20 Running the program >./reverse Hello olleH >./reverse This is a long string sihT >./reverse “This is a long string” gnirts gnol a si sihT 21 C Libraries • Standard I/O: printf, scanf, fopen, fread, … • String functions: strcpy, strspn, strtok, … • Math: sin, cos, sqrt, exp, abs, pow, log,… • System Calls: fork, exec, signal, kill, … 22 Quick I/O Primer – printf int printf( const char *, . . . ); . . . means “variable number of arguments”. The first argument is required (a string). Given a simple string, printf just prints the string (to standard output). 23 Simple printf printf(“Hi Dr. J., I’m Dr. G!\n”); printf(“I\thave\ttabs\n”); char s[100]; strcpy(s,”printf is fun!\n”); printf(s); 24 arguing with printf You can tell printf to embed some values in the string – these values are determined at run-time. printf(“here is an integer: %d\n”,i); the %d is replaced by the value of the argument following the string (in this case i). 25 More integer arguments printf(“%d + %d = %d\n”,x,y,x+y); for (j=10;j>=0;j–) { printf(“%d\n”, j); /* countdown */ } printf(“%d is my favorite number\n”,13); 26 printf is dumb • %d is replaced by the value of the parameter when treated as a integer. • If you give printf something that is not an integer – it doesn’t know! printf(“Print an int %d\n”,”Hi Dr. J.”); Print an int 134513884 27 Other formats • %d is a format – it means “treat the parameter as a signed integer” • %u means unsigned integer • %x means print as hexadecimal • %s means “treat it as a string” • %c is for characters (char) • %f is for floating point numbers • %% means print a single ‘%’ 28 Fun with printf char *s = “Hi Dr. J”; printf(“The string \”%s\” is %d characters long\n”,s,strlen(s)); printf(“The square root of 10 is %f\n”,sqrt(10)); 29 Controlling the output • There are formatting options that you can use to control field width, precision, etc. printf(“The square root of 10 is %20.15f\n”,sqrt(10)); The square root of 10 is 3.162277660168380 30 Lining things up int i; for (i=1;i<5;i++) printf("%2d %f %20.15f\n", i,sqrt(i),sqrt(i)); 1 1.000000 1.000000000000000 2 1.414214 1.414213562373095 3 1.732051 1.732050807568877 4 2.000000 2.000000000000000 31 Alas, we must move on • There are more formats and format options for printf. • The man page for printf includes a complete description (any decent C book will also). • NOTE: to view the man page for printf you should type the following: man 3 printf (in linux) 32 Input - scanf • scanf provides input from standard input. • scanf is every bit as fun as printf! • scanf is a little scary, you need to use pointers • Actually, you don’t really need pointers, just addresses. 33 Remember Memory? • Every C variable is stored in memory. • Every memory location has an address. • In C you can use variables called pointers to refer to variables by their address in memory. 34 scanf int scanf(const char *format, ...); • Remember “. . .” means “variable number of arguments” • Looks kinda like printf 35 What scanf does • Uses format string to determine what kind of variable(s) it should read. • The arguments are the addresses of the variables. • The & operator here means “Take the address of”: int x, y; scanf(“%d %d”,&x,&y); 36 A simple example of scanf float x; printf("Enter a number\n"); scanf("%f",&x); printf("The square root of %f is %f\n“,x,sqrt(x)); 37 scanf and strings Using %s in a scanf string tells scanf to read the next word from input – NOT a line of input: char s[100]; // ALLOC SPACE!! printf("Type in your name\n"); scanf("%s",s); // note: s is a char * printf("Your name is %s\n",s); 38 man scanf • Check out the man page for more details. 39 Reading a line • You can use the function fgets to read an entire line: char *fgets(char *s, int size, FILE *stream); size is the maximum # chars FILE is a file handle 40 Using fgets to read from stdin char s[101]; printf("Type in your name\n"); fgets(s,100,stdin); printf("Your name is %s\n",s); 41 Other I/O stuff • fopen,fclose • fscanf, fprintf, fgets • fread, fwrite • Check the man pages for the details. 42 String functions char *strcpy(char *dest, const char *src); size_t strlen(const char *s); char *strtok(char *s, const char *delim); 43 Math library • The math library is often provided as a external library (not as part of the standard C library). • You must tell the compiler you want the math library: gcc –o myprog myprog.c -lm means “add in the math library” 44 Useful Predefined MACROS • __LINE__ : line # in source code file (%d) • __FILE__ : name of current source code file (%s) • __DATE__ : date “Mmm dd yyy” (%s) • __TIME__ : time of day, “hh:mm:ss” (%s) • __STDC_ : 1 if compiler is ISO compliant • __STDC_VERSION__ : integer (%s) 45 Editors: Vi(m), Emacs or Pico/Nano • Emacs – it more than an editor, it’s a full- on environment. Steep learning curve, but yields the greatest productivity for most developers. • Vi – the grandfather of editors. Small memory foot print. • Pico/Nano – easy to use but lacks a lot of advanced features developers want. Pop Quiz (earn an A or an F) • The best editor is: – A: vi (or derivatives) – B: emacs (or derivatives) – C: Notepad – D: Sublime Text – E: Vendor specific (Visual Studio / Xcode) 46 47 Review: C is a “subset” of C++ • Primary difference is the C has nothing to do with classes. – no constructors – no destructors – no inheritance – no operator overloading – no new or delete operator – memory allocated via system calls – no templates (no STL) – no String or Boolean (C89) types only base types: char, short, int, float, and double 48 O-O still possible with C • Keyword struct is used to declare a record or “methodless” class, like struct Student { char first_name[32]; char last_name[32]; unsigned int id; double gpa; }; 49 Using a struct int main() { struct Student Suzy; /* init data members by hand */ /* strcpy is a standard libC function */ strcpy( Suzy.last_name, “Chapstick”); Suzy.id = 12345; } 50 Variable Declarations (pre-C99: for most modern C compilers, this won't be an issue, but it's good to know/recognize when reading older code) int main() { struct Student Suzy; strcpy( Suzy.last_name, “Chapstick”); int i; /* WRONG!! */ struct Student Sam; /* WRONG !*/ Suszy.id = 12345; } All vars must be declared before the first executable statment in a function or block. This has a significant impact on your “for” loops! 51 “for” Loops in C • In C++ you will typically do: – for( int i=0; i < num; i++ ) • In C you MUST do: – int i; /* top of scope */ – for( i = 0; i < num; i++ ) – note, “i” exists outside of the for loop scope! – NO LONGER TRUE IN C99!!! 52 Comments in C • The GNU C/C++ compiler (gcc) will support.. /* this is a comment */ // this is a comment as well /* this is a comment also */ /* this is a comment too */ • nested comments are not allowed in GNU C/C++ compiler 53 Memory Allocation • Use C system calls: – void *malloc(size_t size) returns a pointer to a chunk of memory which is the size in bytes requested – void *calloc(size_t nmemb, size_t size) same as malloc but puts zeros in all bytes and asks for the number of elements and size of an element. – void free(void *ptr) deallocates a chunk of memory. Acts much like the delete operator. – void *realloc(void *ptr, size_t size) changes the size of the memory chunk point to by ptr to size bytes. – prototypes found in the stdlib.h header file. 54 Example Memory Allocation void main() { char *cptr; double *dblptr; struct Student *stuptr; /* equiv to cptr = new char[100]; */ cptr = (char *) malloc(100); /* equiv to dlbptr = new double[100]; */ dblptr = (double *) malloc(sizeof(double) * 100); /* equiv to stuptr = new Student[100]; */ stuptr = (struct Student *) malloc( sizeof( struct Student) * 100); } 55 Input & Output • Cannot use << , >>, cout, or cin in C. • Instead use scanf for input and printf output. • These system calls take a variable number of arguments. – first argument is format string – remaining args are variables to write from in printf or read into for scanf. – printf format string, all characters are printed themselves except those following a %, which means substitute the value of the next argument here and treat as a particular type as determined by the characters following the %. 56 Special Format Characters • printf – %d: signed integer – %lld: signed long long integer (64 bits) – %u: unsigned integer – %x: integer in hexadecimal format – %f: double – %Lf: long double – %s – a string • scanf is the same except that %f is for float and %lf is a double • clang will ”help” you if you use the wrong format 57 I/O Examples • printf: – printf(“Hello World! \n”); – printf(“X = %u \n”, x); – printf(“X = %u, Y = %f and Z = %s \n”, x,y,z); • scanf – scanf(“%d”, &i); – scanf(“%d %d %d”, &i, &j, &k); – scanf(“%lf”, &my_double); • prototypes found in stdio.h header file. 58 Well Used Header Files (based on Linux) • stdio.h – printf/scanf/ FILE type • stdlib.h – convert routines like string-to-XX, (strtol, strtod, stro), rand num gen, calloc and malloc • unistd.h – system calls like fork, exec, read, write • math.h/float.h – math routines • errno.h – standard error numbers for return values and error handling routines like perror, system call numbers (in Linux). 59 C only passes arguments by value! • In C++ you can do the following: void square_it( int &x ) { x = x * x; // in calling scope x is now x^2 } • In C you MUST do: void square_it( int *x ) { *x = (*x ) * (*x); // caller must pass a pointer to var!! } • Thus, you must be GOOD at POINTERS • You will get even better when you write assembly language code!! 60 Pointers, Arrays & Memory Addresses • What is the difference between: char array[4][4]; • and char **array; int i; array = (char **)malloc(4 * sizeof(char *)); for( i = 0; i < 4; i++ ) array[i] = (char *)malloc(4); • Is &array[2][2] the same in both cases?? • Note, & operator is only to take the address of a variable (i.e., symbol in a program). Pop Quiz • What is the difference between: char array[4][4]; and char **array; int i; array = (char **)malloc(4 * sizeof(char *)); for( i = 0; i < 4; i++ ) array[i] = (char *)malloc(4); • Is &array[2][2] the same in both cases?? 61 62 Memory • a linear series of address starting a zero • addressed in 32 or 64 bit chunks called words word N byte 0 word N byte 1 word N byte 2 word N byte 3 word 1 byte 0 word 1 byte 1 word 1 byte 2 word 1 byte 3 word 0 byte 0 word 0 byte 1 word 0 byte 2 word 0 byte 3 •byte order is Big-endian •could be Little- endian (i.e., 3,2,1,0) 63 Arrays: char a[4][4] a[3][0] addr 12 a[3][1] addr 13 a[3][2] addr 14 a[3][3] addr 15 a[2][0] addr 8 a[2][1] addr 9 a[2][2] addr 10 a[2][3] addr 11 a[1][0] addr 4 a[1][1] addr 5 a[1][2] addr 6 a[1][3] addr 7 a[0][0] addr 0 a[0][1] addr 1 a[0][2] addr 2 a[0][3] addr 3 symbol “a” or a[0][0] starts here 64 Pointers: char **a • here the first layer is an array of pointers • each pointer then points to a string or char * a[3] a[2] a[1] a[0] a[3][0] a[3][1] a[3][2] a[3][3] a[2][0] a[2][1] a[2][2] a[2][3] a[1][0] a[1][1] a[1][2] a[1][3] a[0][0] a[0][1] a[0][2] a[0][3] 65 Dynamic Arrays a[3] a[2] a[1] a[0] a[3][0] a[2][0] a[2][1] a[2][2] a[1][0] a[1][0] a[1][2] a[1][3] a[0][0] a[0][1] • each “row” can have an independent number of elments from the other rows. • implemented a an char ** data structure Pop Quiz • What is the difference between: char array[4][4]; and char **array; int i; array = (char **)malloc(4 * sizeof(char *)); for( i = 0; i < 4; i++ ) array[i] = (char *)malloc(4); • Is &array[2][2] the same in both cases?? 66 67 Function Pointers! • Guess what “code” is really just “data”! – Jon Von Neumann’s idea of the Stored Program Computer is still with us today...50 years later • A computer program is divided largely into 3 areas or segments. – text or code segment – heap (static/global and dynamic memory) – stack (where your local variables are stored • C/C++ supports the ability to have a pointer that points to a function that is in the text segment of a program. 68 Typical Program Segments TEXT/CODE •Stack will typically grow down (in memory address) towards the Heap •Heap will grow up towards Stack •Data contains initialized and uninitialized program variables •Text generally does not change except when dynamic classes or libs being loaded DATA HEAP STACK 69 Function Pointer Syntax /* declare a function pointer type */ typedef int (*function_pointer_t)( int ); int square_it( int x) {return x*x;} int main() { function_pointer_t sq_it_ptr=NULL; sq_it_ptr = (function_ptr_t) &square_it; return sq_it_ptr( 2 ); } 70 Hacking your Stack! • If “code” is really “data”, then can you copy it? • Can you modify or “write” to your text segment? – memory protection in Unix/Linux prevents this. – will talk more about later. • But nothing prevents us from running code off the stack! 71 Unix Accounts • To access a Unix system you need to have an account. • Unix account includes: – username and password – userid and groupid – home directory – shell Crash Course in Unix For more info check out the Unix man pages -or- Unix in a Nutshell (an O’Reilly book) -or- Linux User’s Guide from class webpage -or- Google 72 73 username • A username is (typically) a sequence of alphanumeric characters of length no more than 8. • username is the primary identifying attribute of your account. • username is (usually) used as an email address • the name of your home directory is usually related to your username. 74 password • a password is a secret string that only the user knows (not even the system knows!) • When you enter your password the system encrypts it and compares to a stored string. • passwords are (usually) no more than 8 characters long. • It's a good idea to include numbers and/or special characters (don't use an english word!) 75 userid • a userid is a number (an integer) that identifies a Unix account. Each userid is unique. • It's easier (and more efficient) for the system to use a number than a string like the username. • You don't need to know your userid! 76 Unix Groups and groupid • Unix includes the notion of a "group" of users. • A Unix group can share files and active processes. • Each account is assigned a "primary" group. • The groupid is a number that corresponds to this primary group. • A single account can belong to many groups (but has only one primary group). Pop Quiz • To log on to a Unix system you need your: – A: userid – B: username – C: primary group – D: username and primary group – E: userid and primary group 77 78 Home Directory • A home directory is a place in the file system where files related to an account are stored. • A directory is like a Windows folder (more on this later). • Many unix commands and applications make use of the account home directory (as a place to look for customization files). 79 Shell • A Shell is a unix program that provides an interactive session - a text-based user interface. • When you log in to a Unix system, the program you initially interact with is your shell. • There are a number of popular shells that are available. 80 Logging In • To log in to a Unix machine you can either: – sit at the console (the computer itself) – access via the net (using telnet, rsh, ssh, kermit, or some other remote access client). • The system prompts you for your username and password. • Usernames and passwords are case sensitive! 81 Session Startup • Once you log in, your shell will be started and it will display a prompt. • When the shell is started it looks in your home directory for some customization files. – You can change the shell prompt, your PATH, and a bunch of other things by creating customization files. 82 Your Home Directory • Every Unix process* has a notion of the “current working directory”. • Your shell (which is a process) starts with the current working directory set to your home directory. *A process is an instance of a program that is currently running. 83 Interacting with the Shell • The shell prints a prompt and waits for you to type in a command. • The shell can deal with a couple of types of commands: – shell internals - commands that the shell handles directly. – External programs - the shell runs a program for you. 84 Files and File Names • A file is a basic unit of storage (usually storage on a disk). • Every file has a name. • Unix file names can contain any characters (although some make it difficult to access the file). • Unix file names can be long! – how long depends on your specific flavor of Unix 85 File Contents • Each file can hold some raw data. • Unix does not impose any structure on files – files can hold any sequence of bytes. • Many programs interpret the contents of a file as having some special structure – text file, sequence of integers, database records, etc. 86 Directories • A directory is a special kind of file - Unix uses a directory to hold information about other files. • We often think of a directory as a container that holds other files (or directories). • Mac and Windows users: A directory is the same idea as a folder. • Folders are used as a GUI interface to directories and not unique to Unix/Linux/FreeBSD 87 More about File Names • Review: every file has a name. • Each file in the same directory must have a unique name. • Files that are in different directories can have the same name. 88 The Filesystem / bin etc home tmp usr slotag scully bin etc comporg pads X ls who 89 Unix Filesystem • The filesystem is a hierarchical system of organizing files and directories. • The top level in the hierarchy is called the "root" and holds all files and directories. • The name of the root directory is / 90 Pathnames • The pathname of a file includes the file name and the name of the directory that holds the file, and the name of the directory that holds the directory that holds the file, and the name of the … up to the root • The pathname of every file in a Unix filesystem is unique. 91 Pathnames (cont.) • To create a pathname you start at the root (so you start with "/"), then follow the path down the hierarchy (including each directory name) and you end with the filename. • In between every directory name you put a "/". 92 Pathname Examples / bin etc home tmp usr slotag scully bin etc pads comporg X ls who /usr/bin/ls syllabus /home/slotag/comporg/syllabus 93 Absolute Pathnames • The pathnames described in the previous slides start at the root. • These pathnames are called "absolute pathnames". • We can also talk about the pathname of a file relative to a directory. 94 Relative Pathnames • If we are in the directory /home/slotag, the relative pathname of the file syllabus in the directory /home/slotag/comporg/ is: comporg/syllabus • Most Unix commands deal with pathnames! • We will usually use relative pathnames when specifying files. 95 Example: The ls command • Exercise: login to a unix account and type the command "ls". • The names of the files are shown (displayed) as relative pathnames. • Try this: ls /usr • ls should display the name of each file in the directory /usr. 96 Disk vs. Filesystem • The entire hierarchy can actually include many disk drives. – some directories can be on other computers / bin etc users tmp usr laprej scully 97 The current directory and parent directory • There is a special relative pathname for the current directory: . • There is a special relative pathname for the parent directory: .. 98 Your home directory • There is also a special relative pathname for the current user’s home directory: ~ • Try this: touch /home/yourusername/afile ls –l /home/yourusername touch –l ~/anotherfile ls ~ Pop Quiz • Which of these is an absolute path: – A: usr/bin/matlab – B: ./usr/bin/matlab – C: ~/matlab – D: /home/slotag/matlab 99 100 Some Simple Commands • Here are some simple commands to get you started: – ls lists file names (like DOS dir command). – who lists users currently logged in. – date shows the current time and date. – pwd print working directory 101 The ls command • The ls command displays the names of some files. • If you give it the name of a directory as a command line parameter it will list all the files in the named directory. 102 ls Command Line Options • We can modify the output format of the ls program with a command line option. • The ls command support a bunch of options: –l long format (include file times, owner and permissions) –a all (shows hidden* files as well as regular files) –F include special char to indicate file types. *hidden files have names that start with "." 103 Moving Around in the Filesystem • The cd command can change the current working directory: cd change directory • The general form is: cd [directoryname] 104 cd • With no parameter, the cd command changes the current directory to your home directory. • You can also give cd a relative or absolute pathname: cd /usr cd .. 105 Some more commands and command line options • ls -R will list everything in a directory and in all the subdirectories recursively (the entire hierarchy). – you might want to know that Ctrl-C will cancel a command (stop the command)! • pwd: print working directory • df: shows what disk holds a directory. 106 Copying Files • The cp command copies files: cp [options] source dest • The source is the name of the file you want to copy. • dest is the name of the new file. • source and dest can be relative or absolute. 107 Another form of cp • If you specify a dest that is a directory, cp will put a copy of the source in the directory. • The filename will be the same as the filename of the source file. cp [options] source destdir 108 Deleting (removing) Files • The rm command deletes files: rm [options] names... • rm stands for "remove". • You can remove many files at once: rm foo /tmp/blah /users/clinton/intern 109 File attributes • Every file has some attributes: – Access Times: • when the file was created • when the file was last changed • when the file was last read – Size – Owners (user and group) – Permissions 110 File Time Attributes • Time Attributes: – when the file was last changed: ls -l – when the file was created*: ls -lc – when the file was last accessed: ls -ul *actually it’s the time the file status last changed (ctime). This can come from chmod. Depends on OS! Check your man page! 111 File Owners • Each file is owned by a user. • You can find out the username of the file's owner with the -l option to ls, • Each file is also owned by a Unix group. • ls -lg also shows the group that owns the file. 112 File Permissions • Each file has a set of permissions that control who can mess with the file. • There are three kinds of permissions: – read abbreviated r – write abbreviated w – execute abbreviated x • There are separate permissions for the file owner, group owner and everyone else. 113 ls -l > ls -l foo -rw-rw—- 1 slotag grads 13 Jan 10 23:05 foo permissions owner group size time name 114 ls -l and permissions -rwxrwxrwx Owner Group Others Type of file: – means plain file d means directory 115 rwx • Files: – r: allowed to read. – w: allowed to write. – x: allowed to execute • Directories: – r: allowed to see the names of the files. – w: allowed to add and remove files. – x: allowed to enter the directory 116 Changing Permissions • The chmod command changes the permissions associated with a file or directory. • There are a number of forms of chmod, this is the simplest: chmod mode file 117 chmod mode file • Mode has the following form*: [ugoa][+-=][rwx] u=user g=group o=other a=all + add permission – remove permission = set permission *The form is really more complicated, but this simple version will do enough for now. 118 chmod examples > ls -al foo rwxrwx–x 1 laprej grads … > chmod g-x foo > ls -al foo -rwxrw—x 1 laprej grads >chmod u-r . >ls -al foo ls: .: Permission denied 119 Other filesystem and file commands • mkdir make directory • rmdir remove directory • touch change file timestamp (can also create a blank file) • cat concatenate files and print out to terminal. Pop Quiz • test/some_file has rwxrwxrwx Both the file and directory are yours. What are the minimum permissions are needed to see it with ls test? – A: dr——- – B: d–x—— – C: dr-x—— – D: drwx—— 120 121 Shells Also known as: Unix Command Interpreter 122 Shell as a user interface • A shell is a command interpreter that turns text that you type (at the command line) in to actions: – runs a program, perhaps the ls program. – allows you to edit a command line. – can establish alternative sources of input and destinations for output for programs. 123 Running a Program • You type in the name of a program and some command line options: – The shell reads this line, finds the program and runs it, feeding it the options you specified. – The shell establishes 3 I/O channels: • Standard Input • Standard Output • Standard Error 124 Programs and Standard I/O Program Standard Input (STDIN) Standard Output (STDOUT) Standard Error (STDERR) 125 Unix Commands • Most Unix commands (programs): – read something from standard input. – send something to standard output (typically depends on what the input is!). – send error messages to standard error. 126 Defaults for I/O • When a shell runs a program for you: – standard input is your keyboard. – standard output is your screen/window. – standard error is your screen/window. 127 Terminating Standard Input • If standard input is your keyboard, you can type stuff in that goes to a program. • To end the input you press Ctrl-D (^D) on a line by itself, this ends the input stream. • The shell is a program that reads from standard input. • What happens when you give the shell ^D? 128 Popular Shells sh Bourne Shell ksh Korn Shell csh C Shell bash Bourne-Again Shell fish Friendly Interfactive Shell zsh Z Shell 129 Customization • Each shell supports some customization. – User prompt – Where to find mail – Shortcuts • The customization takes place in startup files – files that are read by the shell when it starts up 130 Startup files sh,ksh: /etc/profile (system defaults) ~/.profile bash: ~/.bash_profile ~/.bashrc ~/.bash_logout csh: ~/.cshrc ~/.login ~/.logout 131 Wildcards (metacharacters) for filename abbreviation • When you type in a command line the shell treats some characters as special. • These special characters make it easy to specify filenames. • The shell processes what you give it, using the special characters to replace your command line with one that includes a bunch of file names. 132 The special character * • * matches anything. • If you give the shell * by itself (as a command line argument) the shell will remove the * and replace it with all the filenames in the current directory. • “a*b” matches all files in the current directory that start with a and end with b. 133 Understanding * • The echo command prints out whatever you give it: > echo hi hi • Try this: > echo * 134 * and ls • Things to try: ls * ls –al * ls a* ls *b 135 Input Redirection • The shell can attach things other than your keyboard to standard input. – A file (the contents of the file are fed to a program as if you typed it). – A pipe (the output of another program is fed as input as if you typed it). 136 Output Redirection • The shell can attach things other than your screen to standard output (or stderr). – A file (the output of a program is stored in file). – A pipe (the output of a program is fed as input to another program). 137 How to tell the shell to redirect things • To tell the shell to store the output of your program in a file, follow the command line for the program with the “>” character followed by the filename: ls > lsout the command above will create a file named lsout and put the output of the ls command in the file. 138 Input redirection • To tell the shell to get standard input from a file, use the “sort < nums • The command above would sort the lines in the file nums and send the result to stdout. 139 You can do both! sort < nums > sortednums tr a-z A-Z < letter > rudeletter Note: “tr” command is translate. Here it replaces all letters “a-z” with “A-Z” Pop Quiz • Running gcc main.c > compile.log puts all output into compile.log instead of printing it to the terminal – A: True – B: False 140 141 More Output redirection • To tell the shell to print standard error to a file, use the “2>“ phrase: gcc buggy_file.c 2> compile.log • The command above would send any error messages during the compile to compile.log 142 Even More Output redirection • To tell the shell to print standard error AND standard out to the same file, use the “&>“ phrase: ./kind_of_works.out &> run.log • The command above would send any output and errors to run.log 143 Pipes • A pipe is a holder for a stream of data. • A pipe can be used to hold the output of one program and feed it to the input of another. prog1 prog2 STDOUT STDIN 144 Asking for a pipe • Separate 2 commands with the “|” character. • The shell does all the work! ls | sort ls | sort > sortedls 145 Shell Variables • The shell keeps track of a set of parameter names and values. • Some of these parameters determine the behavior of the shell. • We can access these variables: – set new values for some to customize the shell. – find out the value of some to help accomplish a task. 146 Example Shell Variables sh / ksh / bash PWD current working directory PATH list of places to look for commands HOME home directory of user MAIL where your email is stored TERM what kind of terminal you have HISTFILE where your command history is saved 147 Displaying Shell Variables • Prefix the name of a shell variable with “$”. • The echo command will do: echo $HOME echo $PATH • You can use these variables on any command line: ls -al $HOME 148 Setting Shell Variables • You can change the value of a shell variable with an assignment command (this is a shell builtin command): HOME=/etc PATH=/usr/bin:/usr/etc:/sbin NEWVAR=”blah blah blah” PATH=/usr/bin/foo:$PATH 149 set command (shell builtin) • The set command with no parameters will print out a list of all the shell varibles. • You’ll probably get a pretty long list… • Depending on your shell, you might get other stuff as well… 150 The PATH • Each time you give the shell a command line it does the following: – Checks to see if the command is a shell built- in. – If not – tries to find a program whose name (the filename) is the same as the command. • The PATH variable tells the shell where to look for programs (non built-in commands). 151 echo $PATH ======= [foo.cs.rpi.edu] – 22:43:17 ======= /home/laprej/comporg echo $PATH /home/laprej/bin:/usr/bin:/bin:/usr/local/bin :/usr/sbin:/usr/bin/X11:/usr/games:/usr/loc al/packages/netscape • The PATH is a list of “:” delimited directories. • The PATH is a list and a search order. • You can add stuff to your PATH by changing the shell startup file (~/.bashrc) Pop Quiz • The current directory “.” is not included by default in $PATH because… – A: They forgot to make it a default – B: OS developers love seeing you get frustrated when you forget to type ./ – C: It’s a security risk – D: It’s assumed you want to the shell to look in the current directory without checking $PATH 152 153 Job Control • The shell allows you to manage jobs – place jobs in the background – move a job to the foreground – suspend a job – kill a job 154 Background jobs • If you follow a command line with “&”, the shell will run the job in the background. – you don’t need to wait for the job to complete, you can type in a new command right away. – you can have a bunch of jobs running at once. – you can do all this with a single terminal (window). ls -lR > saved_ls & 155 Listing jobs • The command jobs will list all background jobs: > jobs [1] Running ls -lR > saved_ls & > • The shell assigns a number to each job (this one is job number 1). 156 Suspending and Killing the Foreground Job • You can suspend the foreground job by pressing ^ Z (Ctrl-Z). – Suspend means the job is stopped, but not dead. – The job will show up in the jobs output. • You can kill the foreground job by pressing ^C (Ctrl-C). • If ^ C does not work, use ^ Z to get back to your terminal prompt and issue: $> kill -9 %1 157 Quoting – the problem • We’ve already seen that some characters mean something special when typed on the command line: * (also ?, []) • What if we don’t want the shell to treat these as special – we really mean *, not all the files in the current directory: echo here is a star * 158 Quoting – the solution • To turn off special meaning – surround a string with double quotes: ➢echo here is a star “*” ➢here is a star * 159 Quoting Exceptions • Some special characters are not ignored even if inside double quotes: • $ (prefix for variable names) • ” the quote character itself • \ slash is always something special (\n) – you can use \$ to mean $ or \” to mean ” >echo “This is a quote \” ” >This is a ” 160 Single quotes • You can use single quotes just like double quotes. – Nothing (except ‘) is treated special. > echo ‘This is a quote \” ‘ > This is a quote \” 161 Backquotes are different! • If you surround a string with backquotes the string is replaced with the result of running the command in backquotes: > echo `ls` foo fee file? > PS1=`date` Tue Jan 25 00:32:04 EST 2000 162 Programming tools • Text editors – Nano – Vi (vim) – GNU emacs, Xemacs • Compilers: gcc / clang • Debuggers: gdb, lldb, ddd => data display debugger • Build tools: Make, autoconf, libtool 163 What are stdin, stdout, stderr? • File descriptors…or more precisely a pointer to type FILE. • These FILE descriptors are setup when your program is run. • So, then what about regular user files… 164 File I/O Operations • fopen — opens a file • fclose — close a file • fprintf — “printf” to a file. • fscanf — read input from a file. • …and many other routines.. 165 fopen #include void main() { FILE *myfile; myfile = fopen( “myfile.txt”, “w”); } • 2nd arg is mode: – w — create/truncate file for writing – w+ — create/truncate for writing and reading – r — open for reading – r+ — open for reading and writing 166 fclose #include #include void main() { FILE *myfile; if( NULL == (myfile = fopen( “myfile.txt”, “w”))) { perror(“fopen failed in main”); exit(-1); } fclose( myfile ); /* could check for error here, but usually not needed */ } 167 fscanf #include #include void main() { FILE *myfile; int i, j, k; char buffer[80]; if( NULL == (myfile = fopen( “myfile.txt”, “r”))) { perror(“fopen failed in main”); exit(-1); } fscanf( myfile, “%d %d %d %s”, &i, &j, &k, buffer); fclose( myfile ); /* could check for error here, but usually not needed */ } 168 sscanf #include #include void main() { FILE *myfile; int i, j, k; char buffer[1024]; char name[80]; if( NULL == (myfile = fopen( “myfile.txt”, “w”))) { perror(“fopen failed in main”); exit(-1); } fgets( buffer, 1024, myfile ); sscanf( buffer, “%d %d %d %s”, &i, &j, &k, name); fclose( myfile ); /* could check for error here, but usually not needed */ } 169 fprintf #include #include void main() { FILE *myfile; int i, j, k; char buffer[80]; if( NULL == (myfile = fopen( “myfile.txt”, “w+”))) { perror(“fopen failed in main”); exit(-1); } fscanf( myfile, “%d %d %d %s”, &i, &j, &k, buffer); fprintf( myfile, “%d %d %d %s, i, j, k, buffer ); fclose( myfile ); /* could check for error here, but usually not needed */ } 170 Pipes • They to are realized as a file descriptor which links either ouput to input or input to output. – recall doing shell commands of the form: – > ls -al | grep “Jan 1” | more – “|” is implemented as a libc call to “popen” 171 Operating Systems: Unix/Linux 172 O.S. Responsibilities • Manages Resources: – I/O devices (disk, keyboard, mouse, terminal) – Memory • Manages Processes: – process creation, termination – inter-process communication – multi-tasking (scheduling processes) 173 Posix – Portable Operating System Interface • Posix is a popular standard for Unix-like operating systems. • Posix is actually a collection of standards that cover system calls, libraries, applications and more… • Posix 1003.1 defines the C language interface to a Unix-like kernel. 174 Posix and Unix • Most current Unix-like operating systems are Posix compliant (or nearly so). Linux, BSD, Mac OS X • We won’t do anything fancy enough that we need to worry about specific versions/flavors of Unix (any Unix will do). 175 Posix 1003.1 • process primitives – creating and managing processes • managing process environment – user ids, groups, process ids, etc. • file and directory I/O • terminal I/O • system databases (passwords, etc) 176 System Calls • A system call is an interface to the kernel that makes some request for a service. • The actual implementation (how a program actually contacts the operating system) depends on the specific version of Unix and the processor. • The C interface to system calls is standard (so we can write an program and it will work anywhere). 177 Unix Processes • Every process has the following attributes: – a process id (a small integer) – a user id (a small integer) – a group id (a small integer) – a current working directory. – a chunk of memory that hold name/value pairs as text strings (the environment variables). – a bunch of other things… 178 Creating a Process • The only way to create a new process is to issue the fork() system call. • fork() splits the current process in to 2 processes, one is called the parent and the other is called the child. 179 Parent and Child Processes • The child process is a copy of the parent process. • Same program. • Same place in the program (almost – we’ll see in a second). • The child process gets a new process ID. 180 Process Inheritence • The child process inherits many attributes from the parent, including: – current working directory – user id – group id 181 The fork() system call #include pid_t fork(void); fork() returns a process id (a small integer). fork() returns twice! In the parent – fork returns the id of the child process. In the child – fork returns a 0. 182 Example #include #include void main(void) { if (fork()) printf(“I am the parent\n”); else printf(“I am the child\n”); printf(“I am the walrus\n”); } 183 Bad Example (don’t try this!) #include #include void main(void) { while (fork()) { printf(“I am the parent %d\n“ ,getpid()); } printf(“I am the child %d\n“ ,getpid()); } 184 I told you so… • Try pressing Ctrl-C to stop the program. • It might be too late. • If this is your own machine – try rebooting. • If this is a campus machine – run for your life. If they catch you – deny everything. 185 Switching Programs • fork() is the only way to create a new process. • This would be almost useless if there was not a way to switch what program is associated with a process. • The exec() system call is used to start a new program. 186 exec • There are actually a number of exec functions: execlp execl execle execvp execv execve • The difference between functions is the parameters… (how the new program is identified and some attributes that should be set). 187 The exec family • When you call a member of the exec family you give it the pathname of the executable file that you want to run. • If all goes well, exec will never return! • The process becomes the new program. 188 execl() int execl(char *path, char *arg0, char *arg1, …, char *argn, (char *) 0); execl(“/home/laprej/reverse”, “reverse”, “Hello!”,NULL); 189 A complete execl example #include /* exec, getcwd */ #include /* printf */ /* Exec example code */ /* This program simply execs “/bin/ls” */ void main(void) { char buf[1000]; printf(“Here are the files in %s:\n”, getcwd(buf,1000)); execl(“/bin/ls”,”ls”,”-al”,NULL); printf(“If exec works, this line won’t be printed\n”); } 190 fork() and exec() together • Program does the following: – fork() – results in 2 processes – parent prints out it’s PID and waits for child process to finish (to exit). – child prints out it’s PID and then execs “ls” and exits. 191 execandfork.c part 1 #include /* exec, getcwd */ #include /* printf */ #include /* need for wait */ #include /* wait() */ 192 execandfork.c part 2 void child(void) { int pid = getpid(); printf(“Child process PID is %d\n”,pid); printf(“Child now ready to exec ls\n”); execl(“/bin/ls”,”ls”,NULL); } 193 execandfork.c part 3 void parent(void) { int pid = getpid(); int stat; printf(“Parent process PID is %d\n”,pid); printf(“Parent waiting for child\n”); wait(&stat); printf(“Child is done. Parent now transporting to the surface\n”); } 194 execandfork.c part 4 void main(void) { printf(“In main – starting things with a fork()\n”); if (fork()) { parent(); } else { child(); } printf(“Done in main()\n”); } 195 execandfork.c output > ./execandfork In main – starting things with a fork() Parent process PID is 759 Parent process is waiting for child Child process PID is 760 Child now ready to exec ls exec execandfork fork exec.c execandfork.c fork.c Child is done. Parent now transporting to the surface Done in main() > 欢迎咨询51作业君

LATEST POSTS
MOST POPULAR

ezAce多年来为广大留学生提供定制写作、留学文书定制、语法润色以及网课代修等服务,超过200位指导老师为您提供24小时不间断地服务。

Close Bitnami banner
Bitnami