Question

我一直在尝试使用系统调用（read（），write（），open（），close（））在unix / linux中进行cp命令的c编程实现。

但是当我通过终端运行我的程序时，将我的程序源代码复制到名称更改的同一目录（源代码大约是300行）

当我打开该输出文件时，它的字符数比原始文件多。

额外的线与200ish线相同。它来自哪里？

这是屏幕截图when compile

这是原始文件（argp.c）末尾附近的源代码。您将看到我如何使用读写方法。

 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);

}

这是复制文件末尾附近的源代码（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

无论读取多少，您都无条件地写入完整缓冲区：

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

应该是：

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

Answer 2

您总是尝试读/写BUFFERSIZE个字节。但是，如果要复制的文件的大小为BUFFERSIZE，会发生什么？你写的是上次读过的内容。

read返回读取的字节数，因此每个write应该尝试写入这个字节数：

代替：

        pos = read(src, buffer, BUFFERSIZE);

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

使用：

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

此处相同，而不是：

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

使用：

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

C unistd.h write（）命令写入额外字符

2 个答案: