C unistd.h写()命令写额外字符

Question

我一直试图在unix/linux中使用系统调用(read()、write()、open()、close())来实现cp命令。

但是，当我在终端上运行我的程序时，我会将这个程序的源代码复制到同一个目录中，并进行名称更改(源代码大约是300行)。

当我打开输出文件时，它比原始文件有更多的字符。

额外的线与200的线相同。它是从哪里来的？

这是截图编译时

argp.c

123.c

这是接近原始文件末尾的源代码(argp.c)。您将看到我是如何使用读写方法的。

 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);

}

这是拷贝文件末尾的源代码(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

您总是尝试读取/写入BUFFERSIZE字节。但是，如果要复制的文件大小为BUFFERSIZE的倍数，会发生什么情况？你写上一次读过的东西。

read返回读取的字节数，因此每个write都应该尝试写入这个字节数：

而不是：

        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);
        }

在这里相同，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);
           }
        }

Answer 2

无条件地写入一个完整的缓冲区，不管读取了多少：

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

应：

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