LAB 1: C PRIMER CS444/544 WENJIN HU JAN 16TH, 2009

Transcription

1 LAB 1: C PRIMER CS444/544 WENJIN HU JAN 16TH, 2009

2 SIMPLE C PROGRAM helloworld.c #include <stdio.h> int main() printf("hello world!"); return 0; csguest:~$ gcc helloworld.c -o helloworld Notice: what's the difference from C++ program?

3 DIFFERENCE FROM C++ PROGRAM helloworld.c using namespace std; #include <stdio.h> X int main() int c=5; X cout<<"hello world!"; printf("hello world!"); return 0; csguest:~$ gcc helloworld.c -o helloworld Notice: variables MUST be declared at the very beginning of the code No bool in C, using 0/1 for T/F

4 STDIN/STDOUT

5 STDOUT printf instead of cout #include <stdio.h> int main() int num = 3; char c = 'c'; char str[20]="abcde"; printf("the No. %d character in \"%s\" is %c.\n", num, str, c); return 0; output: The No. 3 character in "abcde" is c.

6 STDIN readline instead of cin #include <stdio.h> #include<readline/readline.h> int main() char * str; str = readline ("csguest:~> ") printf("the input command is %s\n", command); return 0; output: csguest@:~$ gcc -lreadline realine.c -o readline csguest@:~$./realine csguest:~> ls The input command is ls csguest@:~$

7 CHARACTER ARRAY OR STRING

8 CHARACTER ARRAY VS STRING string = array of characters ended with \0 char cmd[100]; char cmd[100] = 'W','\n', ' ', 'J', '\0'; cmd[0] = 'W'; cmd[1] = '.' cmd[2] = 'J'; cmd[3] = '.' cmd[4] = '\0' printf("%s", cmd); Output: W.J. Notice: A string MUST end with '\0' in an array, indicating the end of the string.

9 ARRAY OF STRINGS char history[10][100]; #include <stdio.h> int main() char cmd[10][100] = "aaaa", "bbb",; printf("%s->%s", cmd[0], cmd[1]); return 0;

10 STRING LIBRARY FUNCTIONS #include <string.h> char a[20] = "abcdefg" char b[10] = "bbbbbb" char c[8] strlen i = strlen(a) // i =7 strncpy strncpy(c, a, 7) //c[] = "abcdefg" strncmp strncmp(a, b, 20) 0: a==b

11 POINTER Pointer: a variable storing the address of another variable Define a character array's pointer: char * ptr; char **pptr; //*ptr[] char cmd[100]; char history[10][100]; ptr = cmd; ptr = history; ptr = history[0]; ptr = history[9]; An array's name stands for the address of that array

12 REFERENCE AND DEREFERENCE Reference: the address of a variable int i =5, j; int *iptr; iptr= &i; Dereference: the content/value of a memory address j=*iptr; printf("%d =? %d =? %d", i, *iptr, j); More pointers: char *cptr; float *fptr; etc.

13 DYNAMIC MEMORY ALLOCATION #include <stdlib.h> sizeof() sizeof(int) sizeof(char) malloc() allocate the memory only when you need int i[100] int * ptr; ptr = (int *) malloc(sizeof(int)*100) free() remember to properly release the memory free(ptr);

14 USER-DEFINED DATATYPE STRUCT

15 USER-DEFINED DATATYPE struct a new user-defined datatype, grouping multiple primitive system-defined datatypes typedef struct int num; char array[20]; foo; int main() foo data; data.num=12; strncpy(data.array,"hello world!", 20); printf("%d, %s", data.num, data.array); return 0;

16 MORE COMPLEX USAGE OF C Combination of struct, pointer, dynamic memory allocation int i; foo * struct_ptr; struct_ptr = (foo*)malloc*(sizeof(foo)); struct_ptr->num =i+1; strcncpy(struct_ptr->array, "Default", 20); free(struct_ptr); struct_ptr = (foo*)malloc*(sizeof(foo)); for(i=0; i++; i<10) struct_ptr[i].num =i+1; strcncpy(struct_ptr[i].array, "Default", 20); free(struct_ptr);

17 LAB1 UNIX SHELL Reminders: Be familiar with system calls Implement one functionality at a time. Don't do two things at the same time. First try to implement a baby program to validate your idea The gdb debugger will be your good friend Course Page:

18 PREREQUISITE: FILE IO OPERATIONS Open a file -> Read/Write -> Close a file FILE * fopen(const char * filename, char *mode) int fclose(file *) int fgetc(file *) int fputc(char c, FILE *) fgets()/fputs() fread()/fwrite() File pointer FILE * fd; //file pointer File descriptor number int fileno(file *) stdin 0 stdout 1

19 FILE OPERATION SAMPLE #include <stdio.h> int main() char c; FILE *infile, *outfile; infile = fopen("1.txt", "r"); if(null=infile) printf("error:can't open file.\n"); outfile = fopen("2.txt", "w"); while(!feof(infile)) c=fgetc(infile); printf("%c", c); fputc(c, outfile); fclose(infile); fclose(outfile); return 0;

20 PREREQUISITE: REDIRECTION Example: ls -a -l <infile >outfile Function: dup2() int main() char *s; FILE *infile, *outfile; infile = fopen("1.txt", "r"); outfile = fopen("2.txt", "w"); dup2(fileno(infile), 0); dup2(fileno(outfile), 1); s = readline("csguest:~> "); printf("the command is %s\n", s); return 0; Try the command "cat 2.txt"

21 PREREQUISITE: COMMAND PARSER parser.h struct commandtype char *command; char *VarList[MAX_VAR_NUM]; int VarNum; ; e.g.: csguest:~$ ls -a -l less csguest:~$ /* parsing information structure */ typedef struct int boolinfile; /* boolean value - infile specified */ int booloutfile; /* boolean value - outfile specified */ int boolbackground; /* run the process in the background? */ struct commandtype CommArray[PIPE_MAX_NUM]; int pipenum; char infile[file_max_size]; /* file to be piped from */ char outfile[file_max_size]; /* file to be piped into */ parseinfo;

22 PREREQUISITE: PROCESS CONTROL Process = running instance of a program pid = a unique Process ID for each process fork() create a new process a copy of the current process How can you tell which is which? wait(int *status) suspend the current process till one child terminates enforce the termination order More complicated systemcall waitpid see

23 PROCESS CONTROL: FORK() SAMPLE #include <stdio.h> #include <sys/types.h> int main() pid_t pid; pid = fork(); if(0 == pid ) printf("i am the child process\n"); else printf("i am the parent processor\n"); return 0; Output: What will the print info be? More details see

24 PREREQUISITE: PROGRAM EXECUTION execvp() call another program within a program int execvp (const char * file, char *const argv[]) #include <unistd.h> int main() char cmd[20]="/bin/ls"; char *args[]="/home/csguest","-al", (char*)0 ; execvp(cmd, args); return 0; What's the difference between execvp and execv?

25 C PREPROCESSOR Use ifdef To separate Windows code from Unix code #ifdef UNIX //Linux-specific Code #endif #ifdef WIN Windows-specific Code #endif In Linux: csguest:~$gcc -DUNIX shell.c