Question

我正在创建一个函数来将一个字符数组复制到C中的文件中。我收到有关指针的警告，所以我很确定这个问题。在我的主程序中，我进行了系统调用：

int main(int argc, char *argv[])
{
    int *out_fd = open("test.txt", O_RDWR, 0644); //Yes, I will fix the 3rd argument
    char arr[] = "hello";
    copyCharArr(arr, out_fd);
}

copyCharArr（）函数在这里：

int copyCharArr(char *arr[], int fd)
{
    int num_written = write(fd, arr, sizeof(arr));
    if(num_written == -1){
            printf("Unable to write file\n");
            return -1;
    }
    if(num_written != sizeof(arr)){
            printf("Unable to write entire file\n");
            return -1;
    }
return 0;
}

以下是警告：

options.c: In function ‘addFile’:
options.c:22: warning: initialization makes pointer from integer without a cast
options.c:24: warning: passing argument 1 of ‘copyCharArr’ from incompatible pointer type
copies.h:5: note: expected ‘char **’ but argument is of type ‘char *’

任何帮助都表示赞赏，因为我对指针有点生疏！我甚至不确定是否可以将文件传递给这样的函数。

Answer 1

你有几个错误：

int copyCharArr(char *arr[], int fd)
//                   ^^^^^^
//         Use  char arr[]
//         or
//              char* arr  
{
    // sizeof(arr) does not give you the length of the string.
    // it gives you the size of a pointer. You need strlen(arr)
    //                write(fd, arr, strlen(arr));  
    int num_written = write(fd, arr, sizeof(arr));
    if(num_written == -1){
            printf("Unable to write file\n");
            return -1;
    }

    // Once again, sizeof(arr) is not appropriate.
    // Use strlen(arr)
    if(num_written != sizeof(arr)){
            printf("Unable to write entire file\n");
            return -1;
    }
    return 0;
}

更新版本。

int copyCharArr(char *arr, int fd)
{
    int len = strlen(arr);
    int num_written = write(fd, arr, len);
    if(num_written == -1){
            printf("Unable to write file\n");
            return -1;
    }

    if(num_written != len){
            printf("Unable to write entire file\n");
            return -1;
    }
    return 0;
}

Answer 2

下面的代码中修复了一些错误和不准确之处。请参阅有关详细信息的评论：

#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>

int copyCharArr(char *, const size_t, int); /* A prototype is mandatory as the 
                                         compiler had not seen copyCharArr() yet. */

/* int main(int argc, char *argv[]) */
int main(void) /* If argc and argv aren't used to not define them. */
{
  int out_fd = open("test.txt", O_RDWR, 0644); /* open() returns int so declare 
                                                  out_fd as such. */
  char arr[] = "hello";
  copyCharArr(arr, sizeof arr, out_fd); /* As the size of arr gets lost when passing 
                                           it to a function pass it explicitly. 
                                           Pass sizeof arr - 1 if the "string"'s 
                                           0-terminator shall not be written to file.*/

  /* Do not forget to close the file. */
}


int copyCharArr(char * arr, const size_t s, int fd) /* Passing a char[] to a function 
                                                 let's it decay to a pointer to its 
                                                 1st element, that is char* here. 
                                                 That's also why we need to pass the 
                                                 size explcitly as s. */
{
  ssize_t result = write(fd, arr, s); /* write() returns ssize_t not int. */
  if (result == -1) {
      perror("write() failed"); /* Using this library function gives more details on 
                                   failure. Also the error goes to stderr, where error
                                   messages shall go. */
      return -1;
  }

  {
    size_t num_written = result;
    if (num_written != s) {
        fprintf(stderr, "Unable to write entire file.\n"); /* Prints the error message to 
                                                             stderr, but to standard output. */
        return -1;
    }
  }

  return 0;
}

Answer 3

open返回一个低级整数文件描述符。使用int指针无效

int fd = open(...)   // correct

不

int *fd = open(...)  // error

将从系统调用open（）打开的文件传递给函数

3 个答案: