C 系统调用 — 使用 lseek 和 read 不断出错

Question

这是我正在为类做的练习，我不明白为什么它不会运行...

我在尝试从变量 (num2) 分配长度的 char 数组(缓冲区)时遇到问题。

你可以像这样执行文件:

./file.c offset numOfChars filename.txt

./file.c 4 10 somefile.txt

如果某个文件包含文本:

Why isn't this c program working. I can't figure it out

程序应该打印

isn't this

代码如下:

#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>

main(int ac, char *av[]){
    // Save the command line variables
    int num1 = av[1];
    int num2 = av[2];

    long numbyte1 = av[1];
    long numbyte2 = av[2];

    int fd = open(av[3], O_RDONLY);

    // Try to open the file
    if( fd < 0 )
        perror(fd + " - Could not open file!");

    // use stat to get file size
    struct stat sb;
    if(fstat(fd,&sb) < 0)    
        return 1;

    // Check to see if the file is big enough for the provided offset
    if(sb.st_size < num1+num2){
        perror(fd + " - Size of file is not large enough for provided offset!" + fd);
    }

    char buffer[num2];

    if(lseek(fd, numbyte1 ,SEEK_SET) < 0) return 1;

    if(read(fd, buffer, numbyte2) != numbyte2) return 1;

    printf("%s\n", buffer);

    return 0;
}

Answer 1

我看到的问题:

1. ./file.c 不是运行程序的正确方法。您需要编译程序并创建可执行文件。然后，您可以运行它。

   如果您有 gcc，请使用:

   gcc -o file -Wall file.c
   ./file 4 10 somefile.txt
   

2. 这些行

   int num1 = av[1];
   int num2 = av[2];
   

   不对。编译器应该报告警告。使用 gcc，我收到这两行的以下警告:

   soc.c: In function ‘main’:
   soc.c:4:15: warning: initialization makes integer from pointer without a cast [enabled by default]
   int num1 = av[1];
              ^
   soc.c:5:15: warning: initialization makes integer from pointer without a cast [enabled by default]
   int num2 = av[2];
   

   av[1] and av[2] are of type char*. If the contain integers, you can extract the integers from them by using one of several functions from the standard library. E.g.

   int num1 = atoi(av[1]);
   int num2 = atoi(av[2]);
   

3. 线条

   long numbyte1 = av[1];
   long numbyte2 = av[2];
   

   遇到同样的问题。您可以使用已经提取的数字来初始化 numbypte1 和 numbypte2

   long numbyte1 = num1;
   long numbyte2 = num2;
   

4. 你有

   char buffer[num2];
   

   这将不足以容纳具有 num2 个字符的字符串。您需要数组中的另一个元素来保存终止空字符。使用:

   char buffer[num2+1];
   

5. 从文件中读取数据后，将终止空字符添加到 buffer。

   if(read(fd, buffer, numbyte2) != numbyte2) return 1;
   buffer[num2] = '\0';