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W1ll
Asked: 2020-05-16 18:20:13 +0800 CST 2020-05-16 18:20:13 +0800 CST

How to implement a function to pass char array to the esp8266

  • 772

I am trying to implement a sketch using a esp8266 como servidor, I need to send a string to the client and for that I am using this piece of code:

  char sbuf[] = "Hello world!\n\r";
  size_t len = strlen(sbuf);
  for (i = 0; i < MAXCLIENTS; i++) {
  if (serverClients[i] && serverClients[i].connected()) {
    serverClients[i].write((char*) &sbuf, len);
    delay(1);
  }
}

Everything funciona OKand the client can receive the characters, now I want to do una funcionthis so I can call it when I need it, so I have tried to implement my function:

void sendDataToClient( char *sbuf[]) {
size_t len = strlen(sbuf) + 1;
for (i = 0; i < MAXCLIENTS; i++) {
 if (serverClients[i] && serverClients[i].connected()) {
   serverClients[i].write((char*) &sbuf, len);
   delay(1);
  }
 }
}

and so I am llamando a la funcion:

char bufferMSGtoClient[] = "Hello world!\n\r\0";

sendDataToClient(bufferMSGtoClient);

But this no funcion, the client does not receive anything, could someone tell me what myerror?

arduino

1 Answers

  1. Best Answer

    When you declare a function with an array as a parameter, the function actually receives a pointer to the first element of the array. In this case:

    void sendDataToClient(char sbuf[]);

    It is equivalent to:

    void sendDataToClient(char *sbuf);

    Then getting the size of sbufcon gives sizeofyou 4 (the size of a char *on your system) and not the size of the array.

    You can fix this by explicitly passing the size when calling the function, for example:

    void sendDataToClient(char sbuf[], size_t len) {
    // ...
}

int main(void) {
    char bufferMSGtoClient[] = "Hello World!\n\r";

    sendDataToClient(bufferMSGtoClient, sizeof bufferMSGtoClient);
}

    If you don't want to pass the size as the second argument, you have two options:

    • Define a macro that uses sizeof on its argument, and be careful not to pass expressions to it that have side effects:
    #define SEND_DATA_TO_CLIENT(sbuf) sendDataToClient(sbuf, sizeof sbuf)
    • If you are only going to send strings (terminated in '\0') and you don't mind the loss of efficiency, you can use strlento calculate the size dynamically:
    void sendDataToClient(char sbuf[]) {
    size_t len = strlen(sbuf) + 1;
    // ...
}

    There is another error in this part:

    serverClients[i].write((char*) &sbuf, len);

    sbufinside the function is a pointer to char ( char *) so you can pass it directly without casting it, so it looks like this:

    serverClients[i].write(sbuf, len);

    the way you use doesn't work because you're taking the address in memory from with the &sbuf operator , but it's already an address in memory (a pointer) so the result you get from is a (pointer to pointer to char).sbuf&sbufchar **

    • 2