CPP-更改库中的#define

Question

I am modifying CPP Arduino library. purpose is to add hostname in user code.

Here is Arduino ethernet library: Arduino Ethernet Library

I am using ethernet.h in my source code.

in ethernet.h there is include statement.

#include "dhcp.h"

and in dhcp.h macro is there.

#define HOST_NAME "MYBOARD"

in dhcp.cpp this macro used

#include "dhcp.h" 
buffer[17] = strlen(HOST_NAME) ; // length of hostname + last 3 bytes of mac address
strcpy((char*)&(buffer[18]), HOST_NAME);

can this macro replaced with variable, such that I can assign value for HOST_NAME is my source code?

example. in my source code

 variable_name ="MYBOARD1"

should pass value to HOST_NAME in dhcp.cpp

I dont know much about c++ , need help with example code.

Thank You

Sudhir

Answer 1

#define is a preprocessor directive ("macro"), not a variable. There's no way you can modify it at runtime.

One possibility is to replace #define HOST_NAME "MYBOARD" with anything you want. If you can't do this for some reason, search where the HOST_NAME is used and replace it with your custom variable. I can't help you much more without other details.

Answer 2

you can not change HOST_NAME because it is already defined in your dhcp.h file,

if you want yo change you have to declare it as a global variable instead of #define

More info about #define:

Why use #define instead of a variable

Answer 3

I was able to achive the result

in dhcp.h I replaced

#define HOST_NAME "my-host-name"

with

extern const char  HOST_NAME[10] ;

then in my project source code I used

const char HOST_NAME[] = "MYHOSTONE";

after compiling I ran the code, observed in DHCP server logs, its showing correct host name. I think its dirty trick, I request expers to help me clean up this line.

thanks