Reference code for the DSA Discussion Forum on Moodle

gistfile1.txt

#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
int main() {
    int fd1, fd2;
    char a[16] = "ABCDEFGHIJKLMNOP", ch;
    fd1 = open("test1", O_RDWR | O_CREAT, 0666);
    write(fd1, a, sizeof(a));
    close(fd1); //closing and opening fd1 will reset the file handle to start of the file
    fd1 = open("test1", O_RDWR, 0666);
    // Now create a new file in which you will store new data.
    fd2 = open("test2", O_RDWR | O_CREAT | O_TRUNC, 0666);
    while(read(fd1, &ch, sizeof(char))) {
        switch(ch) {
            case 'D' :
            case 'E' :
            case 'F' :
            case 'G' : break;
            default  : write(fd2, &ch, sizeof(char));
        }
    }
    close(fd2);
    //Now we have to delete test1 and rename test2 to test1
    fd1 = open("test1", O_RDWR | O_CREAT | O_TRUNC, 0666);
    fd2 = open("test2", O_RDWR, 0666);
    while(read(fd2, &ch, sizeof(char)))
        write(fd1, &ch, sizeof(char));
    //Copying ends here.
    //Now we need to delete test2
    close(fd2);
    fd2 = open("test2", O_RDWR | O_TRUNC, 0666);
    close(fd2);
    close(fd1);
    return 0;
}
/* The only drawback of this method is that , it will also create an empty file called test2 along with modified test1.
 * If anyone finds a way to work around this using only read(), write(), open() and close() please let me know
 */