Arduino library: multiple definitions of a global var

Question

I know this has been asked multiple times but could not find a post relating to this specific problem:

# config.h
String g_ssid = "";

# webserver.cpp
#include "config.h"

bool saveConfig(String *ssid, String *pass)
{
  // Open config file for writing.
  File configFile = SPIFFS.open("/cl_conf.txt", "w");
  if (!configFile)
  {
    Serial.println("Failed to open cl_conf.txt for writing");
    return false;
  }

  // Save SSID and PSK.
  configFile.println(*ssid);
  configFile.println(*pass);
  configFile.close();  // <-- error in this line???

  return true;
} // saveConfig

Compile error:

webserver.cpp.o: In function `saveConfig(String*, String*)':
C:\Users\AppData\Local\Temp\build9148105163386366718.tmp/webserver.cpp:114: multiple definition of `g_pass'

Declaring as inline is not possible as the compiler complains:

'g_ssid' declared as an 'inline' variable

Obviously externing the variable from config.h doesn't make much sense as far as the purpose of config.h is concerned. How can this be solved?

The compile error looks like it has nothing to do with the code you posted ? What's inside webserver.cpp thats defining g_pass multiple times? — Unimportant, Nov 13 '15 at 16:01
Updated. I'm wondering if the definition in the header is possible at all as all answers indicate moving definition to c file or inlining. Both are not applicable here. — andig, Nov 13 '15 at 16:06

score 1 · Accepted Answer · edited May 23 '17 at 12:03

1

You would declare the variable in the header file:

String g_ssid;

and define it in a cpp file:

String g_ssid = "";

That way, the header can be included multiple times without re-defining the variable.

See this SO question for more details about the difference between declaring and defining.

edited May 23 '17 at 12:03

Community

1
1

answered Nov 13 '15 at 16:06

Kenney

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That means the best practice for global variables is defining them in a .cpp file, not in .h? – andig Nov 13 '15 at 16:07

Arduino library: multiple definitions of a global var

1 Answers1