C unistd.h write（）命令写入额外字符

Question

I've been trying to do c programming implementation of cp command in unix/linux by using system calls (read(), write(), open(), close()).

But when I run my program through terminal by copying my source code of this program to the same directory with name change (the source code is about 300 lines)

and when I open that output file , it has more character than the original file.

The extra line is the same as 200ish line. Where does it came from?

here's the screenshot when compile

argp.c

123.c

This is the source code near the end of the original file(argp.c). You will see how I use the read write method.

 while (count - ind > 1) {

        strcpy(cdir, argv[argc-1]);

        if (!isFile(cdir)) {
            strcat(cdir, basename(arguments.argv[ind]));
        }

        if (arguments.update) {
            stat(cdir,&stDest);
            stat(arguments.argv[ind],&stSrc);
            if (difftime(stDest.st_mtim.tv_sec, stSrc.st_mtim.tv_sec) > 0 ) {
                printf("Destination file is newer\n");
                exit(EXIT_FAILURE);
            }
        }

        //open source file
        src = open(arguments.argv[ind],O_RDONLY);
        //if source file can't be opened
        if (src == -1) {
            printf("\nError opening file %s errno = %d\n",arguments.argv[ind],errno);
            exit(EXIT_FAILURE);
        }

        //open target file
        if (arguments.force) {
            //with -f option(default)
            tgt = open(cdir, O_WRONLY | O_CREAT | O_TRUNC , S_IRUSR | S_IRGRP | S_IROTH | S_IWUSR | S_IWGRP | S_IWOTH);
        } else {
            //with -n option
            tgt = open(cdir, O_WRONLY | O_CREAT | O_EXCL , S_IRUSR | S_IRGRP | S_IROTH | S_IWUSR | S_IWGRP | S_IWOTH);
        }
        //if the target file cannot be read or already exist(with -n option)
        if (tgt == -1) {
            printf("File exist or it's not a file.\nCan't copy.\n");
            exit(EXIT_FAILURE);
        }

        //read source file


        //write target file
        while ((pos = read(src, buffer, BUFFERSIZE)) > 0) {
           if (write(tgt, buffer, BUFFERSIZE) != pos) {
               exit(EXIT_FAILURE);
           }
        }

        //if the source file cannot be read
        if (pos == -1) {
            printf("\nError in reading data from %s\n",arguments.argv[ind]);
        }

        //close source file
        if (close(src) == -1) {
            printf("\nError in closing file %s\n",arguments.argv[ind]);
        }

        //close target file
        if (close(tgt) == -1) {
            printf("\nError in closing file %s\n",cdir);
        }

        ind++;
    }

    if (arguments.verbose) {
        printf("Copy successfully!\n");
    }

    exit(EXIT_SUCCESS);

}

This is the source code near the end of the copy file(123.c)

        while (count - ind > 1) {

        strcpy(cdir, argv[argc-1]);

        if (!isFile(cdir)) {
            strcat(cdir, basename(arguments.argv[ind]));
        }

        if (arguments.update) {
            stat(cdir,&stDest);
            stat(arguments.argv[ind],&stSrc);
            if (difftime(stDest.st_mtim.tv_sec, stSrc.st_mtim.tv_sec) > 0 ) {
                printf("Destination file is newer\n");
                exit(EXIT_FAILURE);
            }
        }

        //open source file
        src = open(arguments.argv[ind],O_RDONLY);
        //if source file can't be opened
        if (src == -1) {
            printf("\nError opening file %s errno = %d\n",arguments.argv[ind],errno);
            exit(EXIT_FAILURE);
        }

        //open target file
        if (arguments.force) {
            //with -f option(default)
            tgt = open(cdir, O_WRONLY | O_CREAT | O_TRUNC , S_IRUSR | S_IRGRP | S_IROTH | S_IWUSR | S_IWGRP | S_IWOTH);
        } else {
            //with -n option
            tgt = open(cdir, O_WRONLY | O_CREAT | O_EXCL , S_IRUSR | S_IRGRP | S_IROTH | S_IWUSR | S_IWGRP | S_IWOTH);
        }
        //if the target file cannot be read or already exist(with -n option)
        if (tgt == -1) {
            printf("File exist or it's not a file.\nCan't copy.\n");
            exit(EXIT_FAILURE);
        }

        //read source file


        //write target file
        while ((pos = read(src, buffer, BUFFERSIZE)) > 0) {
           if (write(tgt, buffer, BUFFERSIZE) != pos) {
               exit(EXIT_FAILURE);
           }
        }

        //if the source file cannot be read
        if (pos == -1) {
            printf("\nError in reading data from %s\n",arguments.argv[ind]);
        }

        //close source file
        if (close(src) == -1) {
            printf("\nError in closing file %s\n",arguments.argv[ind]);
        }

        //close target file
        if (close(tgt) == -1) {
            printf("\nError in closing file %s\n",cdir);
        }

        ind++;
    }

if (arguments.verbose) {
    printf("Copy successfully!\n");
}

exit(EXIT_SUCCESS);

}
it(EXIT_FAILURE);
        }

        //read source file


        //write target file
        while ((pos = read(src, buffer, BUFFERSIZE)) > 0) {
           if (write(tgt, buffer, BUFFERSIZE) != pos) {
               exit(EXIT_FAILURE);
           }
        }

        //if the source file cannot be read
        if (pos == -1) {
            printf("\nError in reading data from %s\n",arguments.argv[ind]);
        }

        //close source file
        if (close(src) == -1) {
            printf("\nError in closing file %s\n",arguments.argv[ind]);
        }

        //close target file
        if (close(tgt) == -1) {
            printf("\nError in closing file %s\n",cdir);
        }

        ind++;
    }

if (arguments.verbose) {
    printf("Copy successfully!\n");
}

exit(EXIT_SUCCESS);

}

Answer 1

You unconditionally write a full buffer, regardless of how much was read:

 //write target file
        while ((pos = read(src, buffer, BUFFERSIZE)) > 0) {
           if (write(tgt, buffer, BUFFERSIZE) != pos) {
               exit(EXIT_FAILURE);
           }
        }

should be:

 //write target file
        while ((pos = read(src, buffer, BUFFERSIZE)) > 0) {
           if (write(tgt, buffer, pos) != pos) {
               exit(EXIT_FAILURE);
           }
        }

Answer 2

You always try to read/write BUFFERSIZE bytes. But what happens if the file you want to copy has a size multiple of BUFFERSIZE ? You write what have been read last time.

read return the number of bytes read, so each write should try to write this number of bytes:

Instead of:

        pos = read(src, buffer, BUFFERSIZE);

        //write target file
        while (pos > 0) {
            write(tgt, buffer, BUFFERSIZE);
            pos = read(src, buffer, BUFFERSIZE);
        }

Use:

        pos = read(src, buffer, BUFFERSIZE);

        //write target file
        while (pos > 0) {
            write(tgt, buffer, pos);
            pos = read(src, buffer, BUFFERSIZE);
        }

Same here, instead of:

        //write target file
        while ((pos = read(src, buffer, BUFFERSIZE)) > 0) {
           if (write(tgt, buffer, BUFFERSIZE) != pos) {
               exit(EXIT_FAILURE);
           }
        }

Use:

        //write target file
        while ((pos = read(src, buffer, BUFFERSIZE)) > 0) {
           if (write(tgt, buffer, pos) != pos) {
               exit(EXIT_FAILURE);
           }
        }