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 has to be added 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",
       
    ]
)

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