I've read through the "C bindings" in the tutorial but I'm a novice at C stuff.
Could someone please let me know if a Crystal program can be built as a static library to link to, and if so could you please provide a simple example?
In the C programming language, a static library is a compiled object file containing all symbols required by the main program to operate (functions, variables etc.) as opposed to having to pull in separate entities. Static libraries aren't loaded by the compiler at run-time; only the executable file need be loaded.
Shared libraries (also called dynamic libraries) are linked into the program in two stages. First, during compile time, the linker verifies that all the symbols (again, functions, variables and the like) required by the program, are either linked into the program, or in one of its shared libraries.
Static libraries are created by copying all necessary library modules used in a program into the final executable image. The linker links static libraries as a last step in the compilation process. An executable is created by resolving external references, combining the library routines with program code.
What are the differences between static and dynamic libraries? Static libraries, while reusable in multiple programs, are locked into a program at compile time. Dynamic, or shared libraries, on the other hand, exist as separate files outside of the executable file.
Yes, but it is not recommended to do so. Crystal depends on a GC which makes it less desirable to produce shared (or static) libraries. Thus there are also no syntax level constructs to aid in the creation of such nor a simple compiler invocation to do so. The C bindings section in the documentation is about making libraries written in C available to Crystal programs.
Here's a simple example anyhow:
logger.cr
fun init = crystal_init : Void
# We need to initialize the GC
GC.init
# We need to invoke Crystal's "main" function, the one that initializes
# all constants and runs the top-level code (none in this case, but without
# constants like STDOUT and others the last line will crash).
# We pass 0 and null to argc and argv.
LibCrystalMain.__crystal_main(0, Pointer(Pointer(UInt8)).null)
end
fun log = crystal_log(text: UInt8*): Void
puts String.new(text)
end
logger.h
#ifndef _CRYSTAL_LOGGER_H
#define _CRYSTAL_LOGGER_H
void crystal_init(void);
void crystal_log(char* text);
#endif
main.c
#include "logger.h"
int main(void) {
crystal_init();
crystal_log("Hello world!");
}
We can create a shared library with
crystal build --single-module --link-flags="-shared" -o liblogger.so
Or a static library with
crystal build logger.cr --single-module --emit obj
rm logger # we're not interested in the executable
strip -N main logger.o # Drop duplicated main from the object file
ar rcs liblogger.a logger.o
Let's confirm our functions got included
nm liblogger.so | grep crystal_
nm liblogger.a | grep crystal_
Alright, time to compile our C program
# Folder where we can store either liblogger.so or liblogger.a but
# not both at the same time, so we can sure to use the right one
rm -rf lib
mkdir lib
cp liblogger.so lib
gcc main.c -o dynamic_main -Llib -llogger
LD_LIBRARY_PATH="lib" ./dynamic_main
Or the static version
# Folder where we can store either liblogger.so or liblogger.a but
# not both at the same time, so we can sure to use the right one
rm -rf lib
mkdir lib
cp liblogger.a lib
gcc main.c -o static_main -Llib -levent -ldl -lpcl -lpcre -lgc -llogger
./static_main
With much inspiration from https://gist.github.com/3bd3aadd71db206e828f
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With