CMake GCC can't find curl struct in included header file

Question

I'm trying to create a library with CMake that uses libcurl but GCC is consistently failing to find certain structs and defines even though they were included. I have curl, libssl-dev, and libcurl4-openssl-dev installed. I was able to compile a standalone example program generated by curl http://example.com --libcurl libcurl-out.c with gcc libcurl-out.c -lcurl.

Here is my project structure:

OpenDropC/
├── include/
│   ├── browser.h
│   ├── client.h
│   ├── config.h
│   └── server.h
├── src/
│   ├── browser.c
│   ├── client.c
│   ├── config_private.h
│   ├── config.c
│   ├── server.c
│   ├── utils.c
│   └── utils.h
└── CMakeLists.txt

config.c includes config_private.h which includes <curl/curl.h>. Putting the curl include directly in config.c doesn't fix the problem.

Here is my CMakeLists.txt:

cmake_minimum_required(VERSION 3.18)

project(OpenDropC VERSION 0.0.1 DESCRIPTION "Implementation of OpenDrop in C for Linux")

include(CTest)
enable_testing()

add_library(OpenDropC SHARED
    src/browser.c
    src/client.c
    src/server.c
    src/config.c
    src/utils.c
)

# set_target_properties(OpenDropC PROPERTIES PUBLIC_HEADER include/browser.h)
target_link_libraries(OpenDropC PRIVATE avahi-common avahi-client curl ssl)

Here is the compiler error that I get:

[main] Building folder: OpenDropC 
[build] Starting build
[proc] Executing command: /usr/local/bin/cmake --build /home/parallels/Code/OpenDropC/build --config Debug --target all -j 4 --
[build] [4/8  12% :: 0.043] Building C object CMakeFiles/OpenDropC.dir/src/server.c.o
[build] [5/8  25% :: 0.063] Building C object CMakeFiles/OpenDropC.dir/src/browser.c.o
[build] [6/8  37% :: 0.072] Building C object CMakeFiles/OpenDropC.dir/src/client.c.o
[build] [6/8  50% :: 0.080] Building C object CMakeFiles/OpenDropC.dir/src/utils.c.o
[build] [6/8  62% :: 0.088] Building C object CMakeFiles/OpenDropC.dir/src/config.c.o
[build] FAILED: CMakeFiles/OpenDropC.dir/src/config.c.o 
[build] /bin/gcc-9 -DOpenDropC_EXPORTS  -g -fPIC -MD -MT CMakeFiles/OpenDropC.dir/src/config.c.o -MF CMakeFiles/OpenDropC.dir/src/config.c.o.d -o CMakeFiles/OpenDropC.dir/src/config.c.o -c /home/parallels/Code/OpenDropC/src/config.c
[build] /home/parallels/Code/OpenDropC/src/config.c: In function ‘opendrop_config_new’:
[build] /home/parallels/Code/OpenDropC/src/config.c:42:70: error: invalid application of ‘sizeof’ to incomplete type ‘struct curl_blob’
[build]    42 |     if (!(config_unwrap->root_ca = (struct curl_blob*) malloc(sizeof(struct curl_blob)))) {
[build]       |                                                                      ^~~~~~
[build] /home/parallels/Code/OpenDropC/src/config.c:48:33: error: dereferencing pointer to incomplete type ‘struct curl_blob’
[build]    48 |     if (!(config_unwrap->root_ca->data = malloc(root_ca_len))) {
[build]       |                                 ^~
[build] /home/parallels/Code/OpenDropC/src/config.c:56:37: error: ‘CURL_BLOB_NOCOPY’ undeclared (first use in this function)
[build]    56 |     config_unwrap->root_ca->flags = CURL_BLOB_NOCOPY;
[build]       |                                     ^~~~~~~~~~~~~~~~
[build] /home/parallels/Code/OpenDropC/src/config.c:56:37: note: each undeclared identifier is reported only once for each function it appears in
[build] /home/parallels/Code/OpenDropC/src/config.c: In function ‘opendrop_config_set_record_data’:
[build] /home/parallels/Code/OpenDropC/src/config.c:280:71: error: invalid application of ‘sizeof’ to incomplete type ‘struct curl_blob’
[build]   280 |         if (!(config->record_data = (struct curl_blob*) malloc(sizeof(struct curl_blob)))) {
[build]       |                                                                       ^~~~~~
[build] /home/parallels/Code/OpenDropC/src/config.c:284:38: error: ‘CURL_BLOB_NOCOPY’ undeclared (first use in this function)
[build]   284 |         config->record_data->flags = CURL_BLOB_NOCOPY;
[build]       |                                      ^~~~~~~~~~~~~~~~
[build] [6/8  75% :: 0.097] Building C object CMakeFiles/OpenDropCTest.dir/test/main.c.o
[build] ninja: build stopped: subcommand failed.
[build] Build finished with exit code 1

Running curl-config --cflags returns a blank string, but I tried explicitly including the directory with the curl files and nothing changed. I also tried this answer with no change. Other libraries compile perfectly fine, so this seems to be a libcurl issue. Attempting to use VCPKG for curl specifically did work, but using that would make my project too bloated.

Answer 1

I fixed the problem by installing the latest version of libcurl from source. Ubuntu 20.04 seems to have a broken version of libcurl.