无法将两个 char* 合并为一个

Question

I'm having trouble with creating a function that receives 2 char* and need to return a char* that concatenate the input strings.

char* deviceId = "A1B2C3D4";
char* channelUrl = "/arduino/subscribers/";


char* getSubscriberUrl(char* channelUrl, char* deviceId) {
  char* buffer[sizeof(channelUrl) + sizeof(deviceId)+2];
  strcat(buffer, channelUrl);
  strcat(buffer, deviceId);

  return buffer;
}

I'm getting this error:

initializing argument 1 of 'char* strcat(char*, const char*)'
 char  *strcat (char *__restrict, const char *__restrict);
                ^~~~~~
sketch_sep13a:87:10: error: cannot convert 'char**' to 'char*' in return
   return buffer;

What did I do wrong?

Answer 1

1. channelUrl and deviceId are pointers, so sizeof just returns the size of a pointer, not the lengths of the strings. Use strlen() .
2. You only need to add 1 -- there's only 1 null terminator needed for the combined string.
3. buffer is uninitialized, so you can't concatenate to it.
4. Instead of using multiple calls to strcat() , you can use sprintf() .
5. You can't return a local array. You need to allocate the result with malloc() .

char* getSubscriberUrl(char* channelUrl, char* deviceId) {
  char *buffer = malloc(strlen(channelUrl) + strlen(deviceId)+1);
  sprintf(buffer, "%s%s", channelUrl, deviceId);

  return buffer;
}