Question

我正在为一个C课程开发一个程序，我到了一个我不知道该怎么做的地方。我们正在实现一个String库类型。

我有我的头文件（MyString.h）

typedef struct {
    char *buffer;
    int length;
    int maxLength;
} String;

String *newString(const char *str);

实现函数的文件（MyString.c）

#include <stdlib.h>
#include <stdio.h>
#include "MyString.h"

String *newString(const char *str) {

// Allocate memory for the String
String *newStr = (String*)malloc(sizeof(String));

if (newStr == NULL) {
    printf("ERROR: Out of memory\n");
    return NULL;
}   

// Count the number of characters
int count;
for (count = 0; *(str + count) != '\0'; count++);
count++;

// Allocate memory for the buffer
newStr->buffer = (char*)malloc(count * sizeof(char));

if (newStr->buffer == NULL) {
    printf("ERROR: Out of memory\n");
    return NULL;
}

// Copy into the buffer
while (*str != '\0')
    *(newStr->buffer++) = *(str++);
*(++newStr->buffer) = '\0';

// Set the length and maximum length
newStr->length = count;
newStr->maxLength = count;

printf("newStr->buffer: %p\n",newStr->buffer); // For testing purposes

return newStr;
}

测试人员（main.c）

#include <stdio.h>
#include "MyString.h"

main() {
char str[] = "Test character array";

String *testString = newString(str);

printf("testString->buffer: %p\n",testString->buffer); // Testing only
}

问题在于，即使testString指向在newString（）中创建的String，它们的缓冲区也指向不同的内存地址。那是为什么？

提前致谢

Answer 1

使用*(++newStr->buffer)和*(newStr->buffer++)，您将newStr->buffer移动到基本上指向字符串的末尾。您需要修改代码：

#include <stdlib.h>
#include <stdio.h>
#include "MyString.h"

String *newString(const char *str) {
    // Allocate memory for the String
    String *newStr = (String*)malloc(sizeof(String));

    if (newStr == NULL) {
        printf("ERROR: Out of memory\n");
        return NULL;
    }

    // Count the number of characters
    int count;
    for (count = 0; *(str + count) != '\0'; count++);
    count++;

    // Allocate memory for the buffer
    newStr->buffer = (char*)malloc(count * sizeof(char));

    if (newStr->buffer == NULL) {
        printf("ERROR: Out of memory\n");
        return NULL;
    }

    char *pBuffer = newStr->buffer; // don't move newStr->buffer, have another pointer for that.

    // Copy into the buffer
    while (*str != '\0')
        *(pBuffer++) = *(str++);
    *pBuffer = '\0';

    // Set the length and maximum length
    newStr->length = count;
    newStr->maxLength = count;

    printf("newStr->buffer: %p\n", newStr->buffer); // For testing purposes

    return newStr;
}

Answer 2

您正在修改新创建的String结构内的缓冲区指针。

你应该这样做：

char *newBuffer = newStr->buffer;
// Copy into the buffer
while (*str != '\0')
    *(newBuffer++) = *(str++);
*(++newBuffer) = '\0';

Answer 3

正如其他同事已经指出的那样，你修改了你的分配指针，这是不行的。在这里你的例子，但翻译成更“专业”的方式。

我会将您的结构更改为：

typedef struct {
   char *buffer;
   size_t length;        /* strings and allocation in C are of type size_t not int */
   size_t alloclength;
} String;

String *newString(const char *str);

该功能将更改为。

#include <stdlib.h>
#include <stdio.h>
#include "MyString.h"

String *newString(const char *str)
{
  // Allocate memory for the String
  String *newStr = malloc(sizeof (String));  /* No typecast of void * in C, it's bad taste. */

  if(!newStr) {
    fprintf(stderr, "ERROR: Out of memory\n");     /* Errors are meant to be printed on stderr, not stdio */
    return NULL;
  }   
  // Count the number of characters
  newStr->length = strlen(str);          /* Learn to use the stdlib, there are a lot of usefull functions */
  newStr->alloclength = newStr->length+1;
  // Allocate memory for the buffer
  newStr->buffer = malloc(newStr->alloclength);   /* sizeof (char) is by definition always 1 */
  if(!newStr->buffer) {
    fprintf(stderr, "ERROR: Out of memory\n");
    return NULL;
  }
  // Copy into the buffer
  strcpy(newStr->buffer, str);   /* Because we already scaned the input with strlen, we can use safely the "unsafe" strcpy function. The strcpy will add the trailing 0 */
  printf("newStr->buffer: %p\n",newStr->buffer); // For testing purposes
  return newStr;
}

Answer 4

解释非常简单：您正在修改newString()函数中的缓冲区指针：

// Copy into the buffer
while (*str != '\0')
    *(newStr->buffer++) = *(str++);
*(++newStr->buffer) = '\0';

您可以在此处使用临时指针（如其他答案中所述），但我建议您使用string.h中提供的标准函数：

// Count the number of characters
int count;
count = strlen(str) + 1;

// Copy into the buffer
memcpy(newString->buffer, str, count)

Answer 5

问题已得到解答，但我认为您应该添加一段代码以避免内存泄漏的微妙来源：

// Allocate memory for the buffer
newStr->buffer = (char*)malloc(count * sizeof(char));

if (newStr->buffer == NULL) {
    printf("ERROR: Out of memory\n");
    free(newStr); // free the memory allocated for newStr
    return NULL;
}

C指针不一致

5 个答案: