KIERAN C WOOT WOOT 1

transcript

04/12/23 Course material created by D. Woit 1

CPS 393Introduction to Unix and C

START OF WEEK 10 (C-4)

Command line Processing • main is a function--so OS can pass arguments to it using

– Argument Vector, and – Argument Count

• int main(int argc, char *argv[]) • suppose • -make executable cla from cla.c: gcc -o cla cla.c • -and execute as: cla abc "de f" 74• -then inside cla.c:• argc is 4 // # args + 1 (#elts in argv)• argv[0] is "cla" // program name• argv[1] is "abc" // argument 1• argv[2] is "de f" // argument 2• argv[3] is "74" // argument 3

Command line Processing • Program could print its COMMAND LINE

ARGUMENTS as follows:• int main (int argc, char *argv[]) {• int i; • for (i=1; i<argc; i++) • printf("command line argument %d is: %s\n", i,

argv[i]);• printf("Name of program is: %s\n", argv[0]);• exit(0); • }

• /*Source: suma.c• Purpose: to sum the integers supplied• on the command line• Input: any number of integers in command line• Output: the sum of the inputted numbers• */• #include <stdio.h>• #include <stdlib.h>• int main (int argc, char *argv[]) {• int i, /*for loop index*/• sum=0; /*the sum of the arguments*/• • for (i=1; i<argc; i++) {• sum = sum + atoi(argv[i]);• }• printf("Sum of the %d integers is %d\n", argc-1, sum);• exit(0); • }• HMWK: Fix suma.c so that it checks for good integer input

• /*Source: cml2.c• Purpose: print max of 2 int arguments• */• #define GoodInput 0• #define BadInput 1• #include <stdio.h>• int main (int argc, char *argv[]) {• • if (argc != 3 ) {• fprintf(stderr, "Usage: %s int1 int2\n", argv[0]);• exit(BadInput);• }• if (atoi (argv[1]) > atoi (argv[2]) )• fprintf(stdout, "%s\n", argv[1]);• else fprintf(stdout, "%s\n", argv[2]);• exit(GoodInput);• }• Note that functions strtof/strtod etc convert string to float/double etc• Also as: printf("%d\n", ( atoi(argv[2])>atoi(argv[1]) ) ? atoi(argv[2]):

atoi(argv[1]));

File I/O • already used fprintf/fputs to print to FILE stdout, stderr• fprintf(stderr, "bad input: %d\n", x);• Others: • fscanf (fp, str, length) • fgetc (fp); • fputc (ch, fp)• fgets (str, length, fp); • fputs (str, fp);

– fp is a pointer to structure of type FILE (we still did not learn this)– FILE *fp, *fopen();– It is returned after fopen() system call to open the file, see next

slide.

File I/O• Open file named myfile for

reading:• FILE *fp; // FILE in

stdio.h• if ( (fp =

fopen("myfile","r")) == NULL ) {

• fprintf(stderr,"fopen: Error opening file\n");

• exit(1); • }• fscanf(fp, "%d", &i);

• Mode• ----• r open text file for read• w open text file for write • a append to text file• r+ open text file for r/w• w+ create t.f. for r/w • a+ append or create for

r/w• Note: w & w+: if file not

exist, created• if it exists, it is destroyed

& new created

Closing a file

• fclose(fp); 0 if successful

• EOF if not• (exit *should* close all files before pgm termination).• int fgetc(fp); returns: char read if succ (cast to int)• EOF otherwise• int fputc(ch,fp); returns: char written if succ (cast to int)• EOF otherwise

Example• /*Source cio.c• Purpose: write a string to file myfile• */• #include <stdio.h>• #include <stdlib.h>• #define GOOD 0• #define BAD 1• int main(void) {• char str[40] = "String to write onto disk";• FILE *fp;• char ch, *p;• if ((fp = fopen("myfile","w"))==NULL) {• fprintf(stderr,"fopen: Cannot open file\n");• exit(BAD); • }

• p=str;• while (*p) // why contents of the location pointed by p is not ‘\0’• if (fputc(*p++,fp)==EOF) {• fprintf(stderr,"fputc: Error writing file\n");• fclose(fp); /* close the file*/• exit(BAD);• }• fclose(fp); /*good form*/• exit(GOOD);• }

Example • /*Source: cio2.c• Purpose: display contents

of myfile on stdout• */• #include <stdio.h>• #include <stdlib.h>• #define GOOD 0• #define BAD 1• int main(void) {• FILE *fp;• int ch;

• • if ((fp=fopen("myfile",

"r"))==NULL) {• fprintf(stderr,"fopen:

Cannot open file\n");• exit(BAD);• }• while (( ch = fgetc(fp)) !=

EOF)• fputc(ch,stdout);• fclose(fp);• exit(GOOD);• }

• HMWK:

• Modify cio2.c to turn it into a simple version of unix utility "cat". If no cmd-line input, prints stdin on stdout. If cmd-line inputs (files), print each on stdout

• /*Source: copy.c• Purpose: copy file f1 to file f2• Usage: copy f1 f2• */• #include <stdio.h>• #include <stdlib.h>• #include <string.h>• #define GOOD 0• #define BAD 1• int main (int argc, char *argv[]) {• FILE *f1, *f2;• int ch;

• if (argc != 3) {• fprintf(stderr, "Usage: %s <source> <destination>\n",argv[0]);• exit(BAD);• }

• if ((f1=fopen(argv[1],"r"))==NULL) {• fprintf(stderr, "Cannot open %s\n",argv[1]);• exit(BAD);• }• if ((f2=fopen(argv[2],"w"))==NULL) {• fprintf(stderr, "Cannot open %s\n",argv[2]);• exit(BAD);• }• while ((ch=fgetc(f1)) != EOF) • fputc(ch,f2);• fclose(f1); fclose(f2);• exit(GOOD);• }

file i/o• fgetc() returns EOF if• 1) error in reading• 2) hits EndOfFile• can test for these separately• feof(fp) non-0 if at EOF• 0 otherwise• ferror(fp) non-0 if error reading file• 0 otherwise

• while (!feof(f1)) {• ch=fgetc(f1);• if (ferror(f1)) {• fprintf(stderr, "Error

reading ...• exit(BAD);• }• if (!feof(f1)) fputc(ch,f2);• if (ferror(f2)) {• fprintf(stderr, "Error

writing ...• exit(BAD);• }

• fgets(str, length, fp);• gets chars from FILE fp

and stores them• in array str, until• 1. '\n' read (and

transfered to s)• 2. (length-1) chars have

been read, or• 3. EOF• returns ptr to str if

successful• NULL ptr otws

• Note: fp could be stdin • the file read pointer

advanced "length-1" chars in the file. e.g., • fgets(X,3,f1); • fgets(Y,3,f1); for file with• 12345• abcde• will put "12" in X and "34" in

Y•• fputs(str,fp)• writes str to "file" ptd to by

fp• returns EOF if error;

positive int otws

HMWK • 1. Rewrite copy.c using fgets, fputs, feof and ferror• assuming max line length in file is 128 chars.

• HMWK:• 1. Write a program named prt.c that reads text from• stdin until EOF and prints the number of lines and/or• characters that were read in. The program takes• one command line argument, which is l or c or b.• If the argument is l, the program prints the• number of lines, if c, the number of characters,• and if b, it prints the number of both lines and chars.

HMWK • 2. Re-write program prt.c above• except this time, the program takes 2 arguments, • the second argument being the name of a file from • which to read the text. Perform *full* error checking.• 3. Re-write the above prt.c so that any number of• files are processed (number of chars/lines printed• for EACH, in order processed.)• 4. Use fscanf and fprintf to read a file with 2 columns of • integers, such as:• 2 4• 7 9• 10 6• and write the sum of each line to another file, e.g,• 6• 16• 16• The file names are given on the command line

perror • You could also use function perror (print error) instead• of fprintf(stderr...);• When any system or library function is called *and fails*, its

return code (integer) is put in a system variable that perror can access. (clobbered with each failure).

• perror translates the integer to an (human understandable) phrase and prints on stderr, prefixed by its string argument

• e.g.,• FILE *fp;• if ( (fp = fopen("myfile","r")) == NULL ) {• perror("fopen"); • exit(1); • }

perror • if "myfile" did not exist, stderr gets:• fopen: No such file or directory• (most of the built-in C functions use perror)• FYI: All the error codes are defined in /usr/include/asm-

generic/errno.h• This C function perror in section 3 of man pages (man

3 perror)•

Structures• Just as we can group items of same type into arrays,• We can group items of dissimilar types into: structures• struct s-name { • type item1;• type item2;• . . .• type itemN;• } ;

Structures• e.g., a struct to describe cars (in struct1.c)• struct car {• char make[40];• char model[40];• unsigned int year;• float price;• } ;• Then we can declare various variables of type struct car.

• struct car woit, chan1, chan2; //3 cars

• Assign woit's car:• strcpy(woit.make, "Ferrari");• strcpy(woit.model, "F149");• woit.year = 2012;• woit.price = 155000.00;

• Read in chan1's car:• scanf("%s", chan1.make); //Porsche • gets(chan1.model); //Carrera • scanf("%u %f", &chan1.year, &chan1.price); //2013

440000

• Print woit's car's make:• printf("%s",woit.make); //prints Ferrari• printf("%c",woit.make[4]); //prints what?

Typedefs

• give your own name to any data type

• typedef short int Sint;• int i;• Sint j,k;

Typedefs for structures

• struct car {• char make[40];• char model[40];• unsigned int year;• float price;• } chan1 ;

//chan1 is a variable like woit and chan2

• struct car woit;• struct car chan2;

• typedef struct {• char make[40];• char model[40];• unsigned int year;• float price;• } car; //car is a

new datatype (such as Sint above)

• car woit;• car chan1, chan2;• see example cars.c

Arrays of structures • car (struct car) is now a datatype, therefore, we can create

an array of cars• struct car chan[4]; /*if no typedef used*/• or• car chan[4]; /*if typedef used*/• How do we use arrays of structures?• chan's 0th car: chan[0].year = 1932;• chan's 3rd car: chan[3].price = 52399.99;• sizeof(struct car); /*no typedef*/• or• sizeof(car); /*typedef*/ returns size in bytes• See example arraycars.c

HMWK: • Write a program that reads an input file such as:• 1 5• 2 -4• -3 9• 8 2• Each line of the file contains a "complex number".• Assume at most MAX=100 numbers are given. • The program must create a structure for a complex number, and then

store all the complex numbers in an array. (You must have an array of structures.)

• Then your program should loop through the array from end to front, and print out the complex numbers in it. E.g., your output should

• look like• 8 + 2i• -3 + 9i• 2 + -4i• 1 + 5i• on stdout.

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