使用 GCC 链接到 static 库时遇到问题

Question

I'm working on a big project with glfw but I have been stymied trying to link to a static library with gcc, hence this toy example. I'm not sure why I'm having so much trouble with such a simple thing.

This is the extent of my source code:

#include <stdio.h>
#include <ft2build.h>
#include FT_FREETYPE_H 

int main(){
    FT_Library ft;
    if (FT_Init_FreeType(&ft))
    {
        printf("ERROR::FREETYPE: Could not init FreeType Library\n");
    }

    FT_Face face;
    if (FT_New_Face(ft, "fonts/arial.ttf", 0, &face))
    {
        printf("ERROR::FREETYPE: Failed to load font\n"); 
    }
    return 0;
}

I'm running Linux Mint. I downloaded the FreeType library and built it using CMake and my version of GCC. The libfretype.a file is in a subdirectory called junk . The headers are in a subdirectory called include .

We compile it with the following:

gcc -Wall -Wextra -g -v -Iinclude -Ljunk vex.c -lfreetype -o vex

and I get a ton of errors like sfnt.c:(.text+0x218): undefined reference to 'png_get_error_ptr' .

Thanks in advance for telling me the silly mistake I made.

Answer 1

It basically means that the implementation of the function png_get_error_ptr is missing. So, the compiler could not generate an executable because some code is missing.

The function png_get_error_ptr is implemented in a library named libpng . Sometimes, some libraries have some dependencies on another project, in the general case, you need to include all the dependencies to your build to resolve the linker errors .

You need to include these libraries in the linker:

gcc -Wall -Wextra -g -v -Iinclude -Ljunk vex.c -lfreetype -lpng -lz -o vex 
                                                           ^     ^

-lz is to link against zlib because libpng rely on zlib if I remember correctly.

http://libpng.org/pub/png/libpng-manual.txt