error: invalid conversion from 'const char' to 'char' when trying to use pointer arithmetics

Question

我觉得问这个问题很愚蠢，因为解决方案一定很明显。我收到以下错误

error: invalid conversion from 'const char*' to 'char*' [-fpermissive]
     char *subtopic = topic+strlen(mqtt_meta_ctrl_topic);

对于以下代码：

void HandleIncomingMQTTData(const char* topic, const int topic_len, const char* data, const int data_len)
{
    // Determine subtopic
    char *subtopic = topic+strlen(mqtt_meta_ctrl_topic);
    printf("%.*s", topic_len-strlen(mqtt_meta_ctrl_topic), subtopic);
}

如您所见，我尝试在 topic-string 中获取一个“视图”，其中 subtopic 的地址仍在 topic-string 中，但位于更下游的地址。我想我的指针算法有点不对劲，但我不知道为什么，因为我没有更改 const char *topic 字符串。

Answer 1

topic 是 const char *，但是你的 subtopic 是 char*。

 const char *subtopic = topic + whatever;

printf("%.*s", topic_len-strlen(...)

请注意 strlen returns size_t，但 .* 需要 int。你应该在这里做一个转换 printf("%.*s", (int)(topic_len - strlen(...)).

为了提高性能，使用 fwrite 等而不是 printf 可能更好。

Answer 2

查看下面的代码：

#include <stdio.h>
#include <string.h>

char mqtt_meta_ctrl_topic[100] = "Hello world !";
const char *t = "Have a good day !";
const char *d = "R2D2 & C3P0";

void HandleIncomingMQTTData(const char* topic, const int topic_len, \
                            const char* data, const int data_len)
{
    printf("topic = '%s', topic_len = %d\n", topic, topic_len);
    printf("topic = %ld\n", (unsigned long int)topic);
    int x = strlen(mqtt_meta_ctrl_topic);
    printf("x = %d\n",x);
    // Determine subtopic
    const char *subtopic = topic + x;
    printf("subtopic = %ld, topic + x = %ld\n", (unsigned long int)(subtopic),\
                                                 (unsigned long int)(topic+x));
    printf("topic_len - x = %d\n", topic_len - x);
    printf("subtopic = '%.*s'", topic_len - x, subtopic);
}

int main(){
    HandleIncomingMQTTData(t,(const int)strlen(t),d,(const int)strlen(d));
    return(0);
}

输出如下，没有编译器警告或错误：

topic = 'Have a good day !', topic_len = 17
topic = 4196152
x = 13
subtopic = 4196165, topic + x = 4196165
topic_len - x = 4
subtopic = 'ay !'

error: invalid conversion from 'const char' to 'char' when trying to use pointer arithmetics

error: invalid conversion from 'const char' to 'char' when trying to use pointer arithmetics

c

string

gcc

pointer-arithmetic

error: invalid conversion from 'const char*' to 'char*' when trying to use pointer arithmetics

error: invalid conversion from 'const char*' to 'char*' when trying to use pointer arithmetics

c

string

gcc

pointer-arithmetic

error: invalid conversion from 'const char' to 'char' when trying to use pointer arithmetics

error: invalid conversion from 'const char' to 'char' when trying to use pointer arithmetics