Error Compiling for board Arduino Nano while using Adafruit GFX

Question

Whys is this happening to me?? I have to meet a deadline please help!! Here is my code.

#include <SPI.h>
#include <Wire.h>
#include <Adafruit_GFX.h>
#include <Adafruit_SSD1306.h>
#define SCREEN_WIDTH 128 // OLED display width, in pixels
#define SCREEN_HEIGHT 64 // OLED display height, in pixels
// Declaration for SSD1306 display connected using software SPI (default case):
#define OLED_MOSI 11
#define OLED_CLK 12
#define OLED_DC 9
#define OLED_CS 8
#define OLED_RESET 10
Adafruit_SSD1306 display(SCREEN_WIDTH, SCREEN_HEIGHT,
 OLED_MOSI, OLED_CLK, OLED_DC, OLED_RESET, OLED_CS);
void drawLine(uint16_t x0, uint16_t y0, uint16_t x1, uint16_t y1, uint16_t color);
void setup() 
{
 Serial.begin(9600);
 // SSD1306_SWITCHCAPVCC = generate display voltage from 3.3V internally
 if(!display.begin(SSD1306_SWITCHCAPVCC)) {
 Serial.println(F("SSD1306 allocation failed"));
 for(;;); // Don't proceed, loop forever
}
 
 display.display();
 delay(2000); 
 
 display.clearDisplay();
 
 display.drawPixel(10, 10, SSD1306_WHITE);
 
 display.display();
 delay(2000);
 display.invertDisplay(true);
 delay(1000);
 display.invertDisplay(false);
 delay(1000);
 
 
 }
void loop()
{
drawLine(20,60,5,50,256);
 
}

The Error

C:\Users\hp\AppData\Local\Temp\ccyB6dO1.ltrans0.ltrans.o: In function loop': C:\Users\hp\Desktop\Sunfox\Sunfox\BP_Plus_ECG\February_11\OLED_Testing_w_Adafruit\Test_2\Test_2/Test_2.ino:51: undefined reference to drawLine(unsigned int, unsigned int, unsigned int, unsigned int, unsigned int)' collect2.exe: error: ld returned 1 exit status exit status 1 Error compiling for board Arduino Nano.

• Shrie Sharma: Kind Sir @MichelKeijzers what does that even mean?
• Michel Keijzers: I will make an answer.
• Shrie Sharma: @MichelKeijzers But I am using Adafruits built in library. I thought they would have done that for me.
• Michel Keijzers: I thought initially that you want to create your own function, but I made an answer to use the library instead of your own function.

Answer 1

Currently, you defined a forward declaration for drawLine, however I assume you want to call the method (function) inside display.

So change

drawLine(20,60,5,50,256);

into

display.drawLine(20,60,5,50,256);

And you can remove the forward declaration, because this method is already defined in the library.

void drawLine(uint16_t x0, uint16_t y0, uint16_t x1, uint16_t y1, uint16_t color);

Note, if you call drawLine without an instance (which is display), then the linker tries to find the drawLine function which it cannot find, so you get a linker error (see ld returned in your error, together with undefined reference).