Question

I have an assignment that has asked me to copy a file using buffered i/o. It has multiple requirements:

1. Take one parameter and an optional second
2. Open the first parameter for reading
3. Open the second for writing
4. If there is no second parameter make a new file called prog1.out
5. Use a buffer size of 20 bytes
6. When copying the file, print any buffer starting with the characters “rwxr”
7. close all opened files before exiting.

The problem I’m having is with number six, I’ve looked around and can’t figure this out. I’ve tried memchr but I don’t think I’m on the right track. If anyone can help push me in the right direction I’d be grateful.

This is my code:

# include <stdlib.h>
# include <stdio.h>

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

    FILE *readfile, *writefile;
    char buffer[1024];
    int fileSize;
    int readResult;
    int writeResult;

    // making sure arguments exist
    if (argc < 2|| argc > 3){
            printf("This program takes either 1 or 2 arguments.\n");
            exit(1);
    }

    //Opening file for reading
    readfile = fopen(argv[1], "r");
    if (!readfile) {
            printf("Unable to open file %s.\n", argv[1]);
            exit(1);
    }

    //finding the file size
    fseek (readfile, 0, SEEK_END);
    fileSize = ftell (readfile);
    fseek (readfile, 0, SEEK_SET);

    // read the file
    readResult = fread(buffer, 20, fileSize/20, readfile);
    if (readResult == 0) {
            printf("A read error occured.\n");
            exit(1);
    }

    //check to see if there is a second parameter (argument)
    if (argc == 3) {
            writefile = fopen(argv[2], "w");

            if (!writefile) {
                    printf("Unable to open file  %s.\n", argv[2]);
                    exit(1);
            }

            writeResult = fwrite(buffer, 20, fileSize/20, writefile);
            if (writeResult != readResult) {
                    printf("A write error occured.\n");
                    exit(1);
            }
            printf("File %s successfully copied to %s.\n", argv[1], argv[2]);
    }
    else {
            writefile = fopen("program1.out", "w");

            if (!writefile) {
                    printf("Unable to open file program1.out\n");
                    exit(1);
            }

            writeResult = fwrite(buffer, 20, fileSize/20, writefile);
            if (writeResult != readResult) {
                    printf("A write error occured.\n");
                    exit(1);
            }
            printf("File %s successfully copied to %s.\n", argv[1], "program1.out
    }
    fclose(readfile);
    fclose(writefile);
    exit(0);
}

Answer 1

There’s the naive way:

if(buffer[0] == 'r' && buffer[1] == 'w' 
   && buffer[2] == 'x' && buffer[3] == 'r') {
   //do something
}

But take a look at strncmp(), which you can use for comparing parts of a string.

• remember to first check if you have read at least 4 chars into the buffer. e.g. if the file is 21 bytes long, your 2. fread might only read 1 character, and you shouldn’t compare against the other 3 chars in the buffer.

• If you print out the buffer with e.g. printf or puts or any other function that expects a string, the buffer needs to end with a ‘\0’ byte, otherwise the string functions doesn’t know when to stop.