I'm trying to run the "Hello World" example from the official GTK documentation:
GTK Hello World Example

I followed the instructions and ran the following command:

gcc $(pkg-config --cflags gtk4) -o hello-world-gtk hello-world-gtk.c $(pkg-config --libs gtk4)

However, I got this error:

hello-world-gtk.c:1:10: fatal error: gtk/gtk.h: No such file or directory
    1 | #include <gtk/gtk.h>
      |          ^~~~~~~~~~~
compilation terminated.

I have GTK4 installed, and the directory /usr/include/gtk-4.0/gtk exists, so the file gtk.h is present. It seems like GCC can't find the library despite using pkg-config.

Additional Information:

• System: Arch Linux, kernel version 6.11.9-arch1-1

• Installed GTK version: gtk4 (confirmed via pacman -Q gtk4)

• pkg-config seems to work when I test it:

  pkg-config --cflags gtk4
  

  Outputs something like:

  -I/usr/include/gtk-4.0
  

  Similarly, pkg-config --libs gtk4 outputs the correct linker flags.

Despite all this, GCC still can't find the header.

1. Verified the gtk.h file exists in /usr/include/gtk-4.0/gtk.
2. Reinstalled gtk4 and pkg-config via pacman.
3. Checked that pkg-config outputs the correct flags (as shown above).

What might I be missing, and how can I fix this issue?

(btw. clangd has the same issue and prints tons of errors because it cant find the library)

Here some additional log based on the comments:

1. gtk.h exists and is accessible:
   The file /usr/include/gtk-4.0/gtk/gtk.h is present and readable:

   .rw-r--r-- 11k root  1 Nov 20:21 /usr/include/gtk-4.0/gtk/gtk.h
   

2. Expanded compilation command:
   The full command generated by pkg-config is:

   gcc -I/usr/include/gtk-4.0 -I/usr/include/pango-1.0 -I/usr/include/harfbuzz -I/usr/include/gdk-pixbuf-2.0 -I/usr/include/cairo -I/usr/include/glib-2.0 -I/usr/lib/glib-2.0/include -I/usr/include/freetype2 -I/usr/include/graphene-1.0 -I/usr/lib/graphene-1.0/include -mfpmath=sse -msse -msse2 -I/usr/include/libmount -I/usr/include/blkid -I/usr/include/fribidi -I/usr/include/sysprof-6 -pthread -I/usr/include/libpng16 -I/usr/include/pixman-1 -o hello-world-gtk hello-world-gtk.c -lgtk-4 -lharfbuzz -lpangocairo-1.0 -lpango-1.0 -lgdk_pixbuf-2.0 -lcairo-gobject -lcairo -lvulkan -lgraphene-1.0 -lgio-2.0 -lglib-2.0 -lgobject-2.0
   

3. Direct -I flag:
   Attempting to compile with -I/usr/include/gtk-4.0 results in:

   fatal error: glib.h: No such file or directory
   

4. Output of pkg-config:
   pkg-config --cflags gtk4 correctly includes:

   -I/usr/include/glib-2.0 -I/usr/lib/glib-2.0/include
   

5. Environment variable check:
   PKG_CONFIG_PATH is empty, so there should be no interference:

   echo $PKG_CONFIG_PATH
   (no output)
   

6. Output of gcc -v

Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/14.2.1/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /build/gcc/src/gcc/configure --enable-languages=ada,c,c++,d,fortran,go,lto,m2,objc,obj-c++,rust --enable-bootstrap --prefix=/usr --libdir=/usr/lib --libexecdir=/usr/lib --mandir=/usr/share/man --infodir=/usr/share/info --with-bugurl= --with-build-config=bootstrap-lto --with-linker-hash-style=gnu --with-system-zlib --enable-__cxa_atexit --enable-cet=auto --enable-checking=release --enable-clocale=gnu --enable-default-pie --enable-default-ssp --enable-gnu-indirect-function --enable-gnu-unique-object --enable-libstdcxx-backtrace --enable-link-serialization=1 --enable-linker-build-id --enable-lto --enable-multilib --enable-plugin --enable-shared --enable-threads=posix --disable-libssp --disable-libstdcxx-pch --disable-werror
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 14.2.1 20240910 (GCC)

7. Output of pacman -Qi gcc

Name            : gcc
Version         : 14.2.1+r134+gab884fffe3fc-1
Description     : The GNU Compiler Collection - C and C++ frontends
Architecture    : x86_64
URL             : 
Licenses        : GPL-3.0-with-GCC-exception  GFDL-1.3-or-later
Groups          : None
Provides        : gcc-multilib
Depends On      : gcc-libs=14.2.1+r134+gab884fffe3fc-1  binutils>=2.28  libmpc
                  zstd  libisl.so=23-64
Optional Deps   : lib32-gcc-libs: for generating code for 32-bit ABI
Required By     : base-devel  clang  dmd  rust  vala
Optional For    : dmd  node-gyp  x-xrdb
Conflicts With  : None
Replaces        : gcc-multilib
Installed Size  : 202,47 MiB
Packager        : Frederik Schwan <[email protected]>
Build Date      : 2024-09-10T15:07:33 CEST
Install Date    : 2024-11-09T22:09:28 CET
Install Reason  : Installed as a dependency for another package
Install Script  : No
Validated By    : Signature