I had read the GCC documentation and I did not know about the forward parameter declaration. I did know about the other stuff that is mentioned there (and in the first part).
Declarations in for loops is something that I had only ever used in macros (I had not found it useful in other circumstances), such as:
The first two are obvious, but the third is also legal. It works because array indexing is just sugar for pointer arithmetic, so array[2]=4 is identical in meaning to *(array+2)=4. Therefore 3[array]=27 is identical to *(3+array)=27 and so is legal. But just because you can doesn't mean you should.
Well it could (and I agree with WalterBright that it should) have been disallowed. a[b] being implemented as an early stage rewrite rule expanding to *(a+b) is an uninteresting implementation detail. And I doubt it is even implemented that way in modern compilers anyway. It certainly can't be in C++ as a[b] and b[a] mean different things when [] is overloaded.
Agreed - I've only been programming C for 38 years but I've also never found a legitimate use. However I have used it to illustrate a point when teaching C to beginners - it looks so odd they tend to remember it.
I'm going to speculate a bit on why these silly things are in C.
C was developed on a PDP-11 that had 64Kb of memory. That's not much of any at all. Therefore, the compiler must be extremely tightly coded.
The fundamental rules of the C language are pretty simple. But articles like these expose consequences of such simple rules. Fixing them requires adding more code. Adding more code means less room for the code being compiled.
Therefore, if the intended use of the language works, the pragmatic approach would be to simply not worry about the quirky consequences.
A more interesting question would be "why do these characteristics persist in modern C compilers?"
The stock answer is "backwards compatibility", "Obfuscated C Code contests" and "gotcha job interview questions". My argument would be that there is no reason for the persistence of such "junk DNA" and it should be deprecated and removed.
I've done my part. D doesn't support that stuff, even though the basic use of the language is easily confused with C.
For example:
#include <stdio.h>
void main()
{
int i;
for (i = 0; i < 10; ++i);
printf("%d\n", i);
}
I've died on that hill. I know others who lost an entire day staring at it wondering what's wrong with it. I saw it on X recently as "99% of C programmers will not be able to find the bug."
The equivalent D code:
import core.stdc.stdio;
void main()
{
int i;
for (i = 0; i < 10; ++i);
printf("%d\n", i);
}
gets you:
test.d(5): Error: use `{ }` for an empty statement, not `;`
> I know others who lost an entire day staring at it wondering what's wrong with it. I saw it on X recently as "99% of C programmers will not be able to find the bug."
Both gcc and clang give a warning[1] for that code with just "-Wall", so I's hard to imagine it being a real problem these days.
I know modern compilers do it, too. But still the language needs to be fixed. The proof is C programmers still get victimized by this useless feature.
Compiler warnings are a good source of material for things that need to be fixed in the language. Unfortunately, every compiler has their own set of warnings, and sometimes warnings from different compilers contradict each other. That encourages programmers to not use the warning feature. That's another reason why the language should be fixed.
I added a warning for it in my C compiler back in the mid 1980s.
Fun story. A friend of mine (Eric Engstrom!) bought himself a backhoe. I'd never driven one before and he offered to let me drive it. Sure!
The clutch pedal works backwards from that in a car. Press on the clutch to engage it, release the pedal to disengage it. After some struggling with my reflexes being all wrong, I came within a couple feet of taking out the side of his barn - by switching off the key.
There was nothing wrong with that user interface, other than being insane.
Without information about how identifiers are declared, you do not know how to parse this:
(A)(B);
It could be a cast of B to type A, or function A being called with argument B.
Or this (like the puts(puts) in the article):
A(B):
Could be a declaration of B as an identifier of type A, or a call to a function A with argument B.
Back in 1999 I made a small C module called "sfx" (side effects) which parses and identifies C expressions that could plausibly contain side effects. This is one of the bits provided in a small collection called Kazlib.
This can be used to make macros safer; it lets you write a #define macro that inserts an argument multiple times into the expansion. Such a macro could be unsafe if the argument has side effects. With this module, you can write the macro in such a way that it will catch the situation (albeit at run time!). It's like a valgrind for side effects in macros, so to speak.
In the sfx.c module, there is a rudimentary C expression parser which has to work in the absence of declaration info. In other words it has to make sense of an input like (A)(B).
I made it so that when the parser encounters an ambiguity, it will try parsing it both ways, using backtracking via exception handling (provided by except.c). When it hits a syntax error, it can backtrack to an earlier point and parse alternatively.
Consider (A)(A+B). When we are looking at the left part (A), that could plausibly be a cast or declaration. In recursive descent mode, we are going left to right and looking at left derivations. If we parse it as a declaration, we will hit a syntax error on the +, because there is no such operator in the declarator grammar. So we backtrack and parse it as a cast expression, and then we are good.
Hard to believe that was 26 years ago now. I think I was just on the verge of getting into Lisp.
I see the sfx.c code assumes it would never deal with negative character values, so it cheerfully uses the <ctype.h> functions without a cast to unsigned char. It's a reasonable assumption there since the inputs under the intended use case would be expressions in the user's program, stringified by the preprocessor. Funny bytes would only occur in a multi-byte string literal (e.g. UTF-8). When I review code today, this kind of potential issue immediately stands out.
The same exception module is (still?) used in the Ethereal/Wireshark packet capture and analysis tool. It's used to abort "dissecting" packets that are corrupt or truncated.
C is a small language. People confuse simple with small quite often. As languages get smaller, using them gets more difficult once below a certain size. The "Turing tarpit" languages like Brainfuck are extremely difficult to write complex programs in, mostly because they're so small.
C is clearly too small to be simple. C++ is too large to be simple. Somewhere in between, there may exist a simple language waiting to be invented.
Note that this is GNU C, not standard C. GNU has extended the normal C language with features such as forward parameter declarations and numeric ranges in switch cases. Lots of people don't know about these things.
Basically dead. The main motivation would be to make it easier to use variably modified types in function parameters, where the (length) identifier is declared after the variably modified type, as in
> void foo(int a[m][m], int m)
Currently you can only do:
> void foo(int m, int a[m][m])
The holy grail is being able to update the prototypes of functions like snprintf to something like:
> int snprintf(char buf[bufsiz], size_t bufsiz, const char *, ...);
However, array pointer decay means that foo above is actually:
> void foo(int (*a)[m], int m)
Likewise, the snprintf example above would be little different than the current definition.
There's related syntax, like
> foo (int m, int a[static m])
But a is still just a pointer, and while it can help some static analyzers to detect mismatched buffer size arguments at the call site, the extent of the analysis is very limited as decay semantics effectively prevent tracing the propagation of buffer sizes across call chains, even statically.
There's no active proposal at the moment to make it possible to pass VM arrays (or rather, array references) directly to functions--you can only pass pointers to VM array types. That actually works (sizeof *a == sizeof (int) * m when declaring int (*a)[m] in the prototype), but the code in the function body becomes very stilted with all the syntactical dereferencing--and it's just syntactical as the same code is generated for a function parameter of `int (*a)[m]` as for `int *a` (underneath it's the same pointer value rather than an extra level of memory indirection). There are older proposals but they all lost steam because there aren't any existing implementation examples in any major production C compilers. Without that ability, the value of forward declarations is greatly diminished. Because passing VM array types to functions already requires significant refactoring, most of the WG14 felt it wasn't worth the risk of adopting GCC's syntax when everybody could (and should?) just start declaring size parameters before their respective buffer parameters in new code.
I hope it is not "basically" dead. I just resubmitted it at the request of several people.
And yes, for new APIs you could just change the order, but it does help also with legacy APIs. It does even when not using pointers to arrays: https://godbolt.org/z/TM5Mn95qK (I agree that new APIs should pass a pointer to a VLA).
(edited because I am agreeing with most of what you said)
> everybody could (and should?) just start declaring size parameters before their respective buffer parameters in new code
I know that was a common opinion pre-C23, but it feels like the committee trying to reshape the world to their desires (and their designs). It's a longstanding convention that C APIs accept (address, length) pairs in that order. So changing that will already get you a score of -4 on the Hard to Misuse List[1], for "Follow common convention and you'll get it wrong". (The sole old exception in the standard is the signature of main(), but that's somewhat vindicated by the fact that nobody really needs to call main(); there is a new exception in the standard in the form of Meneide's conversion APIs[2], which I seriously dislike for that reason.)
The reason I was asking is that 'uecker said it was requested at the committee draft stage for C23 by some of the national standards orgs. That's already ancient history of course, but I hoped the idea itself was still alive, specifically because I don't want to end up in the world where half of C APIs are (address, length) and half are (length, address), when the former is one of the few C conventions most everyone agrees on currently.
No implicit cast except for literals and void* (explicit compile time/runtime casts), one loop statement (loop{}), no switch/enum/generic/_thread/typeof/etc, no integer promotion, only sized primitive types (u64 s32 f32 etc...), no anonymous code block, real compiler hard/compile time constant declaration, many operators have to go (--,++, a?b:c, etc)... and everything I am forgetting right now (the dangerous struct pack attribute...). But we need inline keywords for memory barriers, atomics for modern hardware architecture programming.
There is C0, a stripped-down version of C popular in academia [1]. Great for teaching because it's conceptually simple and easy to write a compiler for. But with a couple of additions (like sized primitive types) it might match what you are imagining
No. Assembly is not portable, not typed and not structured (as in structured programming).
Another usecase for microC: most decompilers decompile to a custom C-like language. It pretends to be C, but in reality this is just a representation of a recovered AST that is often-but not always-a valid C code. MicroC would be a better target, because it would supposedly have less weird edge cases.
I'd argue that unsafe Rust is a better target here (although I don't know if &raw has made it into stable Rust yet, which you need for doing an address-of that doesn't have a risk of UB like & does). Rust's typesystem is less silly (there's only one multiple-types-for-same-value, namely usize), there's no implicit conversions, the core CFG primitives are a little bit richer (labeled break/continue, although those are now added in C2y), and there's a decent usable intrinsic class for common operators-not-in-C like rotations or popcount.
If your goal is a simple language to compile, well, Rust won't ever fit the bill; but as a target for a decompiler, I think the unsafe subset is worth exploring.
For the specific question of decompiling, as noted in the GP comment, you're already forced to decompile to a not-quite-C language because C's semantics just aren't conducive to accurately representing the hardware semantics. Not that Rust doesn't have its own foibles, but I suspect its core semantics are easier to map to than C's semantics without having to resort as much to a not-quite-Rust target.
It's definitely something I would like to see at least explored!
Yep, assembly... but a royalty free, brutally simple ISA...
Wait... we have it... RISC-V.
But we need performant µArchitectures of all major use cases (server/desktop/mobile/etc) and that on the best silicon process.
If RISC-V is a success, no need for a µC, just go RISC-V assembly BUT... do not abuse that macro preprocessor, because if it is to move complexity from the C syntax to some macro preprocessor, that would be pointless.
Forth is kind of weak dealing with value types of unknown size. For example, suppose you're writing a cross-compiler, and an address on the target machine might take one or two cells on the host machine depending on the host bitness. Now suppose you need to accept a (host-machine) cell and a target-machine address (e.g. an opcode and an immediate) and manipulate them a bit. Trivial in C, definitely possible in Forth, but supremely annoying and the elegance kind of falls apart.
I doubt that, zig is allocators land. Even stdlib datastructures required an allocators to be instanciated. Have a look at the selection of allocators: https://zig.guide/standard-library/allocators .
It creates a compound literal [1] of type array of int, and initializes the specified array positions using designated initializers [2] with the results of calls to puts().
Using designated initializers without the = symbol is an obsolete extension.
That's not valid standard C; gcc and clang give a warning with '-pedantic'. It's valid C++ though.
And IMO it's quite a nice feature, useful sometimes for reducing boilerplate in early returns. It's the obvious consequence if you don't treat void as some extremely-special syntax but rather as just another type, perhaps alike an empty struct (though that's not valid C either ¯\_(ツ)_/¯) that's just implicitly returned at the end of a void-returning function, and a "return;" statement implicitly "creates" a value of void.
In fact in Rust (and probably a bunch of other languages that I'm too lazy to remember) void-returning functions are done via returning a 0-tuple.
In C function prototypes with no arguments are strange.
void g();
Means that g is a function which takes a not-specified number of arguments (but not a variable number of arguments).
Almost always what you want to do is
void g(void)
Which says that g takes 0 arguments.
Having said that, declaring it as g() should work fine as long as you always provided that you always invoke it with the correct arguments. If you try invoking it with the wrong arguments, then the compiler will let you, and your program may just break in fun and exciting ways.
Edit: looking closer, it looks like the intent might have been to alias f and g. But, as discussed above, it does so in a way that will break horribly if g expects any arguments.
Well, at the risk of stating the obvious: `return expr;` is meant to return whatever expr evaluates to.
Here, g()'s return type is void, so there's no value to return and at the same time, f()'s return type is void, so return should not have an expr to begin with.
This statement is effectively equivalent to: "g(); return;".
> so return should not have an expr to begin with.
Thats one way to think about it. Another is that void is a type - which is obviously true given you can have void* pointers and functions can return void. In this example, f() returns a void expression, so that’s a perfectly fine thing to return from g.
void* is leaps-and-bounds different from void. It is an expression, it has storage, you can actually assign something to it. You can't have a variable of type void; "void var;" is meaningless. A function doesn't "return" void. void simply denotes that a function doesn't return anything.
You can't have a variable of type void, but you can have an expression of type void - calling a function that returns void does just that (in C++, "throw" is also a void-typed expression).
Usually I'm more in the camp of "let's preserve everything we can as cultural heritage, yes even those awful Nazi propaganda material" and I'm confident that some distant archeologist (or current close neighbor) will be glad we did.
But as time pass, I'm more and more convinced that wiping-out every peace of C that was ever produced would be one of the greatest possible gesture for the future of humanity.
I also have a theory that universe exits so we can have some opportunities to taste chocolate. Surely in that perspective, even C can be an unfortunate but acceptable byproduct.
Too many people still don't understand C's greatest failure: the undefined behavior. Most people assume that if you write past an array then the result may be a program crash; but actually undefined behavior includes other wonderful options such as stealing all your money, encrypting all your files and extorting you for some bitcoin, or even partially destroying a nuclear isotopes processing facility. Undefined really means undefined, theoretically some demons may start flying out of your nose and it would be completely up to spec. If you think that this is justified by "performance gains" or some other nonsense then I really don't know what to tell you!
No, it doesn't. It means the C standard imposes no requirements. It does not mean the compiler becomes unconstrained by statutory law or physics, and if your compiler is actually malware then it is not politely waiting for undefined behaviour before injecting a payload.
The notorious nasal demons may not be in conflict with the C standard, but they are not going to actually happen, because they only exist in the imagination. The example is given to illustrate by absurdity that the scope of consequential defects is greater than "your program may crash", that's all. If you do wish to produce a similar effect then I suggest consuming a bowl of Buldak instant noodles whilst inducing a sneeze during compilation. Warning: your sinuses will not thank you. And cover your keyboard.
The biggest hazard with undefined behaviour is that the compiler is not required to issue warnings or errors when encountered.
Remember that C's contemporary languages were either inefficient (e.g. ALGOL 68, PL/1, Lisp), functionally obsolete (e.g. FORTRAN didn't have recursion or heap allocation), or even lower level (Assembly, B). C eliminated the need for need for assembly in programs that were low level (like OS kernels) or high performance (math, graphics, signal processing), and that was surely a huge improvement in type safety and expressiveness.
I don't get that C hate. That terse syntax can be misused to produce
unreadable code in C, does not change that I usually find it more
readable than more verbose syntax.
Declarations in for loops is something that I had only ever used in macros (I had not found it useful in other circumstances), such as:
(The compiler will optimize out the loop and the declared variable in the use of the lpt_document macro; I had tested this.)https://www.goodreads.com/book/show/198207.Expert_C_Programm...
D doesn't have that bug!
In 44 years of C programming, I've never encountered a legitimate use for the 3rd. (Other than Obfuscated C, that is.))
But I call it a bug because it has no use and just pointlessly confuses people.
C was developed on a PDP-11 that had 64Kb of memory. That's not much of any at all. Therefore, the compiler must be extremely tightly coded.
The fundamental rules of the C language are pretty simple. But articles like these expose consequences of such simple rules. Fixing them requires adding more code. Adding more code means less room for the code being compiled.
Therefore, if the intended use of the language works, the pragmatic approach would be to simply not worry about the quirky consequences.
A more interesting question would be "why do these characteristics persist in modern C compilers?"
The stock answer is "backwards compatibility", "Obfuscated C Code contests" and "gotcha job interview questions". My argument would be that there is no reason for the persistence of such "junk DNA" and it should be deprecated and removed.
I've done my part. D doesn't support that stuff, even though the basic use of the language is easily confused with C.
For example:
I've died on that hill. I know others who lost an entire day staring at it wondering what's wrong with it. I saw it on X recently as "99% of C programmers will not be able to find the bug."The equivalent D code:
gets you: C'mon, Standard C! Fix that!Both gcc and clang give a warning[1] for that code with just "-Wall", so I's hard to imagine it being a real problem these days.
[1] https://godbolt.org/z/vfPzhc596
Compiler warnings are a good source of material for things that need to be fixed in the language. Unfortunately, every compiler has their own set of warnings, and sometimes warnings from different compilers contradict each other. That encourages programmers to not use the warning feature. That's another reason why the language should be fixed.
I always use brackets too. That's simply a less error-prone style.
Fun story. A friend of mine (Eric Engstrom!) bought himself a backhoe. I'd never driven one before and he offered to let me drive it. Sure!
The clutch pedal works backwards from that in a car. Press on the clutch to engage it, release the pedal to disengage it. After some struggling with my reflexes being all wrong, I came within a couple feet of taking out the side of his barn - by switching off the key.
There was nothing wrong with that user interface, other than being insane.
But I'm feeling much better.
https://news.ycombinator.com/item?id=40835274 (113 comments)
Or this (like the puts(puts) in the article):
Could be a declaration of B as an identifier of type A, or a call to a function A with argument B.Back in 1999 I made a small C module called "sfx" (side effects) which parses and identifies C expressions that could plausibly contain side effects. This is one of the bits provided in a small collection called Kazlib.
This can be used to make macros safer; it lets you write a #define macro that inserts an argument multiple times into the expansion. Such a macro could be unsafe if the argument has side effects. With this module, you can write the macro in such a way that it will catch the situation (albeit at run time!). It's like a valgrind for side effects in macros, so to speak.
https://git.savannah.gnu.org/cgit/kazlib.git/tree/sfx.c
In the sfx.c module, there is a rudimentary C expression parser which has to work in the absence of declaration info. In other words it has to make sense of an input like (A)(B).
I made it so that when the parser encounters an ambiguity, it will try parsing it both ways, using backtracking via exception handling (provided by except.c). When it hits a syntax error, it can backtrack to an earlier point and parse alternatively.
Consider (A)(A+B). When we are looking at the left part (A), that could plausibly be a cast or declaration. In recursive descent mode, we are going left to right and looking at left derivations. If we parse it as a declaration, we will hit a syntax error on the +, because there is no such operator in the declarator grammar. So we backtrack and parse it as a cast expression, and then we are good.
Hard to believe that was 26 years ago now. I think I was just on the verge of getting into Lisp.
I see the sfx.c code assumes it would never deal with negative character values, so it cheerfully uses the <ctype.h> functions without a cast to unsigned char. It's a reasonable assumption there since the inputs under the intended use case would be expressions in the user's program, stringified by the preprocessor. Funny bytes would only occur in a multi-byte string literal (e.g. UTF-8). When I review code today, this kind of potential issue immediately stands out.
The same exception module is (still?) used in the Ethereal/Wireshark packet capture and analysis tool. It's used to abort "dissecting" packets that are corrupt or truncated.
How to Get Fired Using Switch Statements & Statement Expressions:
https://blog.robertelder.org/switch-statements-statement-exp...
C is clearly too small to be simple. C++ is too large to be simple. Somewhere in between, there may exist a simple language waiting to be invented.
There's related syntax, like
But a is still just a pointer, and while it can help some static analyzers to detect mismatched buffer size arguments at the call site, the extent of the analysis is very limited as decay semantics effectively prevent tracing the propagation of buffer sizes across call chains, even statically.There's no active proposal at the moment to make it possible to pass VM arrays (or rather, array references) directly to functions--you can only pass pointers to VM array types. That actually works (sizeof *a == sizeof (int) * m when declaring int (*a)[m] in the prototype), but the code in the function body becomes very stilted with all the syntactical dereferencing--and it's just syntactical as the same code is generated for a function parameter of `int (*a)[m]` as for `int *a` (underneath it's the same pointer value rather than an extra level of memory indirection). There are older proposals but they all lost steam because there aren't any existing implementation examples in any major production C compilers. Without that ability, the value of forward declarations is greatly diminished. Because passing VM array types to functions already requires significant refactoring, most of the WG14 felt it wasn't worth the risk of adopting GCC's syntax when everybody could (and should?) just start declaring size parameters before their respective buffer parameters in new code.
And yes, for new APIs you could just change the order, but it does help also with legacy APIs. It does even when not using pointers to arrays: https://godbolt.org/z/TM5Mn95qK (I agree that new APIs should pass a pointer to a VLA).
(edited because I am agreeing with most of what you said)
I know that was a common opinion pre-C23, but it feels like the committee trying to reshape the world to their desires (and their designs). It's a longstanding convention that C APIs accept (address, length) pairs in that order. So changing that will already get you a score of -4 on the Hard to Misuse List[1], for "Follow common convention and you'll get it wrong". (The sole old exception in the standard is the signature of main(), but that's somewhat vindicated by the fact that nobody really needs to call main(); there is a new exception in the standard in the form of Meneide's conversion APIs[2], which I seriously dislike for that reason.)
The reason I was asking is that 'uecker said it was requested at the committee draft stage for C23 by some of the national standards orgs. That's already ancient history of course, but I hoped the idea itself was still alive, specifically because I don't want to end up in the world where half of C APIs are (address, length) and half are (length, address), when the former is one of the few C conventions most everyone agrees on currently.
[1] https://ozlabs.org/~rusty/index.cgi/tech/2008-04-01.html
[2] https://thephd.dev/_vendor/future_cxx/papers/C%20-%20Restart...
I can't wait to slip this into some production code to confuse the hell out of some intern in a few years
We need a C- ore µC:
No implicit cast except for literals and void* (explicit compile time/runtime casts), one loop statement (loop{}), no switch/enum/generic/_thread/typeof/etc, no integer promotion, only sized primitive types (u64 s32 f32 etc...), no anonymous code block, real compiler hard/compile time constant declaration, many operators have to go (--,++, a?b:c, etc)... and everything I am forgetting right now (the dangerous struct pack attribute...). But we need inline keywords for memory barriers, atomics for modern hardware architecture programming.
1: https://c0.cs.cmu.edu/docs/c0-reference.pdf
Another usecase for microC: most decompilers decompile to a custom C-like language. It pretends to be C, but in reality this is just a representation of a recovered AST that is often-but not always-a valid C code. MicroC would be a better target, because it would supposedly have less weird edge cases.
That depends on the assembly language. Some have structure constructs, some are typed. Portability is out.
But if you accept a slightly higher abstraction, WebAssembly is portable, typed, and structured.
Does WebAssembly support AOT compilation to native binaries? I thought it was just a VM.
Or use an existing implement like wasmedge, I guess.
If your goal is a simple language to compile, well, Rust won't ever fit the bill; but as a target for a decompiler, I think the unsafe subset is worth exploring.
And even before that release, there were macros the poly fill it, so you could have used those to target an earlier version.
It's definitely something I would like to see at least explored!
Wait... we have it... RISC-V.
But we need performant µArchitectures of all major use cases (server/desktop/mobile/etc) and that on the best silicon process.
If RISC-V is a success, no need for a µC, just go RISC-V assembly BUT... do not abuse that macro preprocessor, because if it is to move complexity from the C syntax to some macro preprocessor, that would be pointless.
Read up on Forth languages. It's pretty much exactly what you're after.
In short, you can initialize an array like this, by specifying each element in order:
However, you can also initialize specific array elements: "BASIC compatibility" mode uses the above syntax.Using designated initializers without the = symbol is an obsolete extension.
[1] https://gcc.gnu.org/onlinedocs/gcc/Compound-Literals.html [2] https://gcc.gnu.org/onlinedocs/gcc/Designated-Inits.html
void g(); void f() { return g(); }
And IMO it's quite a nice feature, useful sometimes for reducing boilerplate in early returns. It's the obvious consequence if you don't treat void as some extremely-special syntax but rather as just another type, perhaps alike an empty struct (though that's not valid C either ¯\_(ツ)_/¯) that's just implicitly returned at the end of a void-returning function, and a "return;" statement implicitly "creates" a value of void.
In fact in Rust (and probably a bunch of other languages that I'm too lazy to remember) void-returning functions are done via returning a 0-tuple.
Sanity restored.
Almost always what you want to do is
Which says that g takes 0 arguments.Having said that, declaring it as g() should work fine as long as you always provided that you always invoke it with the correct arguments. If you try invoking it with the wrong arguments, then the compiler will let you, and your program may just break in fun and exciting ways.
Edit: looking closer, it looks like the intent might have been to alias f and g. But, as discussed above, it does so in a way that will break horribly if g expects any arguments.
Here, g()'s return type is void, so there's no value to return and at the same time, f()'s return type is void, so return should not have an expr to begin with.
This statement is effectively equivalent to: "g(); return;".
Thats one way to think about it. Another is that void is a type - which is obviously true given you can have void* pointers and functions can return void. In this example, f() returns a void expression, so that’s a perfectly fine thing to return from g.
But as time pass, I'm more and more convinced that wiping-out every peace of C that was ever produced would be one of the greatest possible gesture for the future of humanity.
I also have a theory that universe exits so we can have some opportunities to taste chocolate. Surely in that perspective, even C can be an unfortunate but acceptable byproduct.
The notorious nasal demons may not be in conflict with the C standard, but they are not going to actually happen, because they only exist in the imagination. The example is given to illustrate by absurdity that the scope of consequential defects is greater than "your program may crash", that's all. If you do wish to produce a similar effect then I suggest consuming a bowl of Buldak instant noodles whilst inducing a sneeze during compilation. Warning: your sinuses will not thank you. And cover your keyboard.
The biggest hazard with undefined behaviour is that the compiler is not required to issue warnings or errors when encountered.
Should this misfortune befall you, please don't get on an airplane (with me).