Link against pre-installed system libraries

In the previous example we wrote a simple C program that only prints “hello world” and includes a standard header file. Real programs often use external code/libraries installed via system package manager (apt or brew) and link against it. It’s possible to use Bazel to do the same thing - rather than a “hermetic” build. This isn’t typically what you want, but might be useful during a migration, or for certain fixed development environments. In this example we will write a simple program that reads a file and tries to determine what kind of file it is by using libmagic. Before we do that, we need to install libmagic on our computers.

MacOS users can brew install libmagic followed by brew link libmagic
Debian users can apt-get install libmagic-dev
Windows users can follow along by using Windows Subsystem for Linux (WSL).

Here is an example C program that opens a file and tells the mime type using libmagic. Let’s name this src/magic.c.

#include <stdio.h>
#include <stdlib.h>
#include <magic.h>

int main(int argc, char *argv[]) {
    // Check if filename is provided
    if (argc != (int)2) {
        fprintf(stderr, "Usage: %s <filename>\n", argv[0]);
        return 1;
    }

    // Initialize magic handle
    magic_t magic = magic_open(MAGIC_MIME_TYPE);
    if (magic == NULL) {
        fprintf(stderr, "Failed to initialize libmagic\n");
        return 1;
    }

    // Load magic database
    if (magic_load(magic, NULL) != 0) {
        fprintf(stderr, "Cannot load magic database: %s\n", magic_error(magic));
        magic_close(magic);
        return 1;
    }

    // Get MIME type
    const char *mime_type = magic_file(magic, argv[1]);
    if (mime_type == NULL) {
        fprintf(stderr, "Error determining MIME type: %s\n", magic_error(magic));
        magic_close(magic);
        return 1;
    }

    // Print result
    printf("MIME type: %s\n", mime_type);

    // Cleanup
    magic_close(magic);
    return 0;
}

Outside of Bazel, compiling this program can be accomplished by running gcc main.c -lmagic -o main where the compiler is told to link against libmagic via -lmagic. Here’s an equivalent Makefile, though note the portability issues with finding the lib and include folders:

main:
	gcc main.c -lmagic -o main \
	-L $(shell brew --prefix)/lib -I $(shell brew --prefix)/include

❔On macOS -L $(brew --prefix)/lib -I $(brew --prefix)/include flags have to be added to the command line to allow the compiler to find brew managed libraries. On Linux this is not necessary as apt installs everything under system /usr

When compiling on macOS some additional configuration is also needed to allow linking against homebrew managed libraries.

llvm.toolchain(
    # Add /opt/homebrew to allowlist of linking against system libraries.
    cxx_builtin_include_directories = {
        "darwin-aarch64": ["/opt/homebrew"]
    },
)

Finally we can create a cc_binary target for our program and run it via bazel run src:magic and should see the Usage: line from earlier.

cc_binary(
    name = "magic",
    srcs = ["magic.cc"],
    copts = [
        # on macOS search for headers on homebrew managed /include directory.
        "-I/opt/homebrew/include",
    ],
    linkopts = [
        # link the magic library.
        "-lmagic",
        # on macOS allow linking against homebrew installed dylibs.
        "-L/opt/homebrew/lib",
       
    ]
)