Question

Rewrite the attached mycat.c program in 3 using read, write, open and close (System I/O functions) instead of the standard I/O functions.

#include<stdio.h>


int main(int argc, char* argv[])
{
        FILE *fp;

        void filecopy(FILE *, FILE *);

        if (argc == 1)
        {
                filecopy(stdin, stdout);
        }
        else
        {
                while(--argc >0)
                {
                        if ((fp = fopen(*++argv, "r")) == NULL)
                        {
                                printf("cat: can not open %s\n", *argv);
                                return 1;
                        }
                        else
                        {
                                filecopy(fp, stdout);
                                fclose(fp);
                        }
                }
        }

        return 0;
}

void filecopy(FILE *ifp, FILE *ofp)
{
        int c;

        while ((c = getc(ifp)) != EOF)
        {
                putc(c, ofp);
        }
}

Answer 1

Se here is the changed prohram.

#include<stdio.h>
#include<fcntl.h>
//we need to import fcntl to give the modes of opening the files like O_RDONLY
#include<stdlib.h>
#include<string.h>

//Main driver function
int main(int argc, char *argv[])
{
int fd;
//here in system calls we will definee the file descriptor fd
// here fd has three paramets 0- input,1- output 2- standard error
char c;
if(argc==3)
{
//open the first file in the read only mode and check if file is
// opened successfully or not if not exit from the program
fd = open(argv[1], O_RDONLY, 0);
//it is unable to open the file it will return -1
if((fd)==-1){
//Failed to open it will return the fd as -1
printf("\nError opening the file") ;
exit(1);
}
char buf1[100];
int idx = 0;
while(read(fd,&c,1)!=0){
//priint data read will return the number of items read
//(read takes 3 parameters 1->filedescriptor,2-> buffer to store data,
// 3->number of items to be read)
printf("%c",c);
buf1[idx++] = c;
}
buf1[idx++] = '\0';
int fdw;
fdw = open(argv[2], O_CREAT); // create file if not present
close(fdw);
fdw = open(argv[2], O_WRONLY); // open file in write only mode
//write data to the file(write takes 3 parameters
write(fdw, buf1, strlen(buf1));
close(fdw);
}
//close the file
close(fd);
}

Run the compiled code with two command line arguments a.txt and b.txt

The context of a.txt will be copied in b.txt.