Python needs an actual default function
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Can someone explain to me how to compile a C library with "main" and a program with main? How does executing a program actually work? It has an executable flag, but what actually happens in the OS when it encounters a file with an executable file? How does it know to execute "main"? Is it possible to have a library that can be called and also executed like a program?
I haven't done much low level stuff, but I think the 'main' function is something the compiler uses to establish an entry point for the compiled binary. The name 'main' would not exist in the compiled binary at all, but the function itself would still exist. Executable formats aren't all the same, so they'll have different ways of determining where this entry point function is expected to be. You can 'run' a binary library file by invoking a function contained therein, which is how DLL files work.
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Can someone explain to me how to compile a C library with "main" and a program with main? How does executing a program actually work? It has an executable flag, but what actually happens in the OS when it encounters a file with an executable file? How does it know to execute "main"? Is it possible to have a library that can be called and also executed like a program?
wrote on last edited by [email protected]You don't. In C everything gets referenced by a symbol during the link stage of compilation. Libraries ultimately get treated like your source code during compilation and all items land in a symbol table. Two items with the same name result in a link failure and compilation aborts. So a library and a program with main is no bueno.
When Linux loads an executable they basically look at the program's symbol table and search for "main" then start executing at that point
Windows behaves mostly the same way, as does MacOS. Most RTOS's have their own special way of doing things, bare metal you're at the mercy of your CPU vendor. The C standard specifies that "main" is the special symbol we all just happen to use
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Can someone explain to me how to compile a C library with "main" and a program with main? How does executing a program actually work? It has an executable flag, but what actually happens in the OS when it encounters a file with an executable file? How does it know to execute "main"? Is it possible to have a library that can be called and also executed like a program?
wrote on last edited by [email protected]How does executing a program actually work?
Way too long an answer for a lemmy post
It has an executable flag, but what actually happens in the OS when it encounters a file with an executable file?
Depends on OS. Linux will look at the first bytes of the file, either see (ASCII)
#!(called a shebang) or ELF magic, then call the appropriate interpreter with the executable as an argument. When executing e.g. python, it's going to call/usr/bin/envwith parameterspythonand the file name because the shebang was#!/usr/bin/env python.How does it know to execute “main”?
Compiled C programs are ELF so it will go through the ELF header, figure out which
ld.soto use, then start that so that it will find all the libraries, resolve all dynamic symbols, then do some bookkeeping, and jump to_start. That is, it doesn't:mainis a C thing.Is it possible to have a library that can be called and also executed like a program?
Absolutely.
ld.sois an example of that.. Actually, wait, I'm not so sure any more, I'm getting things mixed up withlibdl.so. In any caseld.sois an executable with a file extension that makes it look like a library.EDIT: It does work. My (GNU) libc spits out version info when executed as an executable.
If you want to start looking at the innards like that I would suggest starting here: Hello world in assembly. Note the absence of a
mainfunction, the symbol the kernel actually invokes is_start, the setup necessary to call a Cmainis done bylibc.so. Don't try to understand GNU's libc it's full of hystarical raisins I would suggest musl. -
Shouldn't the third panel just be empty?
It’s fine like this
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Also, do y'all call main() in the if block or do you just put the code you want to run in the if block?
wrote on last edited by [email protected]Still better than having to create a new class just to implement
public static void main(String[] args) {}Relevant Fireship video: https://youtu.be/m4-HM_sCvtQ
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Can someone explain to me how to compile a C library with "main" and a program with main? How does executing a program actually work? It has an executable flag, but what actually happens in the OS when it encounters a file with an executable file? How does it know to execute "main"? Is it possible to have a library that can be called and also executed like a program?
wrote on last edited by [email protected]If you want to have a library that can also be a standalone executable, just put the main function in an extra file and don't compile that file when using the library as a library.
You could also use the preprocessor to do it similar to python but please don't.Just use any build tool, and have two targets, one library and one executable:
LIB_SOURCES = tools.c, stuff.c, more.c EXE_SOURCES = main.c, $LIB_SOURCESEdit: added example
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Now think about this, you have logic that doesn't make sense when run directly, but you need it to be a library.
You have multiple name=main statements in some of your functions
I'm not sure I'm following the implication. Name=main is for scripts primary, is it not?
I've never thought to add more than one of these conditionals anyway...
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Also, do y'all call main() in the if block or do you just put the code you want to run in the if block?
One thing I really dislike about Python is the double underscore thing, just really looks ugly to me and feels excessive. Just give me my flow control characters that aren't whitespace
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What is the point of this?
Not having tons of code in one if statement, but in a function.
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One thing I really dislike about Python is the double underscore thing, just really looks ugly to me and feels excessive. Just give me my flow control characters that aren't whitespace
Php says hello
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Still better than having to create a new class just to implement
public static void main(String[] args) {}Relevant Fireship video: https://youtu.be/m4-HM_sCvtQ
Since Java 21, this has been shortened significantly. https://www.baeldung.com/java-21-unnamed-class-instance-main
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Also, do y'all call main() in the if block or do you just put the code you want to run in the if block?
wrote on last edited by [email protected]It really doesn't. It's a scripting language, functions are there but at it's core it runs a script. The issue is that it was so easy to start with that people started doing everything in it, even though it sucks for anything past complex scripts
It is the excel of databases.
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Since Java 21, this has been shortened significantly. https://www.baeldung.com/java-21-unnamed-class-instance-main
Free standing functions in Java?! This can't be true.
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Free standing functions in Java?! This can't be true.
I know right? It even has var with implicit typing now. While I prefer Kotlin any day, there's been quite a few qol improvements to Java over the last few years.
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Diabolical
isn't that just normal usage? ..or, did I just whoosh and you were sarcastically saying that?
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Tbh reserving "main" is just a hacky if not more so than checking
__name__if you actually understand language design.What is not hacky then in a language design?
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Tbh reserving "main" is just a hacky if not more so than checking
__name__if you actually understand language design.Yeah, this is it.
What's hacky about an introspective language providing environment to all of the executing code, so that the coder can make the decision about what to do?
It would by hacky if Python decided "We'll arbitrarily take functions named "main" and execute them for you, even though we already started execution at the top of the file."
For C, this is less so. The body of the file isn't being executed, it's being read and compiled. Without a function to act as a starting point, it doesn't get executed.
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Just cross your fingers nobody attempts to import it...
Due to the oneness of all things, I refuse to distinguish between library code and executable code. One and Zero are arbitrary limitations.
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Could someone explain this please? I'm still a noob.
wrote on last edited by [email protected]All code needs to have an entry point.
For Python and some other languages, this is the start of the file.
For other languages, this is a special function name reserved for this purpose - generally, "main".
In the first kind of language, the thought process is basically: I have the flow of execution, starting at the top of the file. If I want to make a library, I should build the things I want to build, then get out of the way.
In the other kind of language, the thought process is basically: I am building a library. If I want to make an executable, I should create an entry point they the execution starts at.
The debate is honestly pretty dumb.
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Basically, when you compile a program written in Rust or C/C++ (the first and second panels respectively), the compiler needs to know what's supposed to be executed first when the program is run directly (i.e. when you click on the executable), which in these languages, is denoted by a special function called
main(). Executable files can also contain functions and data structures that can be called by other programs, and when they are, you wouldn't want to run an entire complex and resource intensive program if another program only needs to call a single function from it. In that case, the other program will call the function it wants but not main, so only that function executes and not the entire program.However, Python is a scripting language that's interpreted. So every Python source file is executable provided you have the Python runtime. Python also doesn't have native support for main functions in the same way Rust and C/C++ does, and it will execute every line of code as it reads the source file. This is why a single line Python file that just calls print is valid, it doesn't need to be wrapped in a main function to execute. However, what if your Python file is both meant to be executed directly and provides functions that other Python files can call? If you just put the main routine in the root of the file, it would be executed every time another program tries to import the file in order to call functions from it, since the import causes the file to be interpreted and executed in its entirety. You can still just have a main function in your file, but since Python doesn't natively support it, your main function won't do anything if you run the file directly because as far as Python is concerned, there is no executable code at the root of the file and you haven't called any functions.
The workaround is to have a single if statement at the root of the file that looks like this:
if __name__ == '__main__': main()It checks a special variable called
__name__. If the Python file is directly executed,__name__will have the value of the string'__main__', which satisfies the if statement so main() is called. If another Python file imports it, the value of__name__will be the name of that file, so main() isn't called. It's clunky and not that efficient, but, 1, it works, and 2, if you cared about efficiency, you wouldn't be writing it in Python.Really helpful explanation, thanks.
