Compiler does not give line number of error undefined reference

Question

Why does the compiler sometimes not give line number of the error? Where is the use case of that "undefined reference". I've already included everything as header files I myself wrote so it needs to give a specific line number. It is not closed source. Have I changed some setting of the compiler by accident or is it another thing whatever that another thing is?

D:\Projects\DanceOfPixels\GLEW>gcc main.c glad.c -IC:\mingw_dev_lib\include\SDL2 -LC:\mingw_dev_lib\lib -lmingw32 -lopengl32 -lSDL2main -lSDL2 -lSDL2_image -o main.exe -ansi -std=c89 -pedantic -w C:\Users\user\AppData\Local\Temp\ccMooHZm.o:main.c:(.text+0x126ce): undefined reference to `drawImagePartScaledHW' collect2.exe: error: ld returned 1 exit status

Edit: I have solved the problem. I have included two different versions of the draw.h, one coming from software renderer, other from OpenGL renderer. Since they use same

#ifndef DRAW_H
#define DRAW_H

...

#endif

structure for both files; the compiler didn't include the second. Once I've changed the DRAW_H to DRAW_HW I managed to compile and run the application.

You should consider compiling all your code with gcc -Wall -g — Basile Starynkevitch
– Basile Starynkevitch, Commented Jun 20, 2018 at 15:18

Steve Summit · Accepted Answer · 2018-06-20 15:51:27Z

4

That error comes from the "linker" (ld), not the compiler proper.

Typically, the compiler compiles each source file into its own, individual object file, containing just the code and data from that source file. Then, the linker combines one or more object files together, and also links in any needed library functions.

Crucially, there's no problem if a single source file (a single object file) calls an undefined function -- that's normal, if the definition of the function is in another source file, or a library. So that's why it's the linker (not the compiler) that finally discovers that there's not a definition for a function anywhere, that it's truly undefined.

But since the linker is working with object files, typically it doesn't know which source file line numbers the functions were originally called on.

(Some C compilers work more closely with their linkers, so that these "undefined external" error messages can, more usefully, contain actual source file line numbers, but that's a relatively recent innovation. For this to work it may be important to compile with debugging enabled, e.g. by using the -g flag, so that the compiler includes source line number information in its object files.)

edited Jun 20, 2018 at 15:51

answered Jun 20, 2018 at 15:17

Steve Summit

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2 Comments

Basile Starynkevitch Over a year ago

On Linux, some linkers are able to use DWARF debug information to report line numbers

Steve Summit Over a year ago

@BasileStarynkevitch Yup, was just adding that.

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