Compiling on Ubuntu 20

[noeldum]

I do have libpng-dev and pkg-config installed but I have a linking error when compiling.

make
cc -std=c11 -Wall -Wextra -g -Iinclude -lpng -D_DEFAULT_SOURCE -D_POSIX_C_SOURCE=200809 -I/usr/include/libpng16 -c src/main.c -o build/main.o
cc -std=c11 -Wall -Wextra -g -Iinclude -lpng -D_DEFAULT_SOURCE -D_POSIX_C_SOURCE=200809 -I/usr/include/libpng16 -c src/ql.c -o build/ql.o
cc -std=c11 -Wall -Wextra -g -Iinclude -lpng -D_DEFAULT_SOURCE -D_POSIX_C_SOURCE=200809 -I/usr/include/libpng16 -c src/loadpng.c -o build/loadpng.o
cc -lpng16 -lz build/main.o build/ql.o build/loadpng.o -o build/qlprint
/usr/bin/ld: build/loadpng.o: in function loadpng': /tmp/qlprint/src/loadpng.c:30: undefined reference to png_sig_cmp'
/usr/bin/ld: /tmp/qlprint/src/loadpng.c:35: undefined reference to png_create_read_struct' /usr/bin/ld: /tmp/qlprint/src/loadpng.c:39: undefined reference to png_create_info_struct'
/usr/bin/ld: /tmp/qlprint/src/loadpng.c:43: undefined reference to png_create_info_struct' /usr/bin/ld: /tmp/qlprint/src/loadpng.c:47: undefined reference to png_set_longjmp_fn'
/usr/bin/ld: /tmp/qlprint/src/loadpng.c:50: undefined reference to png_init_io' /usr/bin/ld: /tmp/qlprint/src/loadpng.c:51: undefined reference to png_set_sig_bytes'
/usr/bin/ld: /tmp/qlprint/src/loadpng.c:53: undefined reference to png_read_info' /usr/bin/ld: /tmp/qlprint/src/loadpng.c:57: undefined reference to png_get_IHDR'
/usr/bin/ld: /tmp/qlprint/src/loadpng.c:61: undefined reference to png_set_strip_alpha' /usr/bin/ld: /tmp/qlprint/src/loadpng.c:63: undefined reference to png_set_rgb_to_gray_fixed'
/usr/bin/ld: /tmp/qlprint/src/loadpng.c:65: undefined reference to png_set_expand_gray_1_2_4_to_8' /usr/bin/ld: /tmp/qlprint/src/loadpng.c:67: undefined reference to png_read_update_info'
/usr/bin/ld: /tmp/qlprint/src/loadpng.c:79: undefined reference to png_set_longjmp_fn' /usr/bin/ld: /tmp/qlprint/src/loadpng.c:85: undefined reference to png_read_image'
/usr/bin/ld: /tmp/qlprint/src/loadpng.c:86: undefined reference to png_read_end' /usr/bin/ld: /tmp/qlprint/src/loadpng.c:98: undefined reference to png_destroy_read_struct'
collect2: error: ld returned 1 exit status
make: *** [Makefile:28: build/qlprint] Error 1

Any help appreciated.

[brtka]

It's an error with the Makefile. Change the following:

build/qlprint: $(OBJS)
	$(CC) $(LDFLAGS) $^ -o $@

to

build/qlprint: $(OBJS)
	$(CC) $^ $(LDFLAGS) -o $@

In GCC, the order in which libraries are linked matters. The -lpng16 (or -lpng, depending on your pkg-config output) should come after the object files in the linking command. This is because the GNU linker scans libraries from left to right and only includes objects from the library that resolve undefined references at that point.