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I'm trying to gain some insight on how Apples OS signpost implementation works. I'm working with the C API (there is also a Swift API). My ultimate goal is trying to build a RAII style C++ wrapper class for them, which is harder as it might seem.
Expanding the os_signpost_emit_with_type macro reveals that it creates static strings from the string literals passed to that macro that look like this:
__attribute__((section("__TEXT,__oslogstring,cstring_literals"), internal_linkage)) static const char string_name[] __asm (OS_STRINGIFY(OS_CONCAT(LOS_##_ns, __COUNTER__))) = "string literal";
These strings will later appear as names for the signposts in the instruments profiler. What I get from reading that code, is that the string is placed in a specific section of the binary so that the profiler can find it. What's confusing me is the __asm statement before the assignment. Obviously via the __COUNTER__ macro, it expands to something like __asm ("LOS_##_ns0"), __asm ("LOS_##_ns1") with the number being unique for every string. I have very little in depth knowledge when it comes to assembly, I tried to research the meaning of that statement a bit but got no useful results.
My try-and-error testing revealed that the uniqueness of that numerical appendix generated by the __COUNTER__ macro matters, if two duplicated values occur the string with that duplicated value will shadow the other one in the profiler output.
Can anyone with assembly know how explain what's going on here to a C++ developer like me?
Bonus question: Would there be any way to generate that instruction from within C++ code where the unique numerical value here generated by __COUNTER__would be taken from some variable?
A general note: for information on clang extensions, you generally have to refer to the gcc documentation instead. clang aims to be compatible with gcc and so they didn't bother to write independent docs.
So in your example, a few different extensions are being used. Note that none of them are part of standard C or C++.
__attribute__((section ("foo")) places the variable in the section named foo, by having the compiler emit a .section directive into the assembly before placing the label for the variable. See https://gcc.gnu.org/onlinedocs/gcc-12.2.0/gcc/Common-Variable-Attributes.html#Common-Variable-Attributes. It sounds like you already know about this.
asm in a declaration isn't really inline assembly per se; it simply tells the compiler what symbol name to use for this variable when it emits the assembly code. The __asm is just a variant spelling of asm. See https://gcc.gnu.org/onlinedocs/gcc-12.2.0/gcc/Asm-Labels.html#Asm-Labels. So int foo asm("bar") = 7; defines a variable which will be referred to as foo in C source, but whose label in assembly will be named bar.
__COUNTER__ is a special macro defined by the gcc/clang preprocessor that simply increments every time it is expanded. See https://gcc.gnu.org/onlinedocs/cpp/Common-Predefined-Macros.html#Common-Predefined-Macros
NOTE: This is NOT a duplicate of the question linked by Paul T, because I am asking if it is possible to determine if a type is of a certain broader incomplete type/kind at compile time, not if a symbol has been registered at compile time. This seems like a fundamental misunderstanding of the question.
I am writing a library in C that deals with pseudo-generic functions which take a type as an argument through a macro wrapper.
To spare the details (because they are rather complicated) there are two possible features that could help, I think:
Being able to detect if a type is a pointer at compile time. (No, the "use _Generic to test if you get ptrdiff_t from subtraction" trick won't work, because structures are a possibility, and you can't subtract structures.)
Being able to detect if a type is a struct at compile time. (If this was possible, then the aforementioned _Generic trick could be used if the type was detected as not being a struct.)
I've tried everything I could think of on Godbolt (even trying to compare types to incomplete anonymous structs and toying with __builtin_types_compatible_p) and wasn't able to find any solutions.
If anyone has any solutions I'd love to see them, otherwise I may just end up having to complicate the design a bit-- so not the end of the world if it's impossible, but it would be ideal if it can be done.
To give a basic idea of what one of these macros might look like or their expected output:
int *a;
assert(!IS_STRUCT(a));
assert(IS_POINTER(a));
struct {} b;
assert(IS_STRUCT(b));
assert(!IS_POINTER(b));
shouldn't throw any errors.
Complete Answer (if EDG Front End used):
If your IDE / compiler is using the EDG C++ Front End (which a lot are), and you are using C, not C++ (which your tag suggests), and you say you ARE using typeof, then you can detect a structs as follows (see the latest manual, page 75):
/* Test if EDG Front End is used*/
#if defined(__EDG__) && defined(__EDG_VERSION__)
#define IS_STRUCT(expression_or_type_name) __is_class(typeof (expression_or_type_name)))
#endif
since in C __is_class() will only be true for a struct (http://www.cplusplus.com/reference/type_traits/is_class/).
Further, pointers can similarly be detected as follows:
/* Test if EDG Front End is used*/
#if defined(__EDG__) && defined(__EDG_VERSION__)
#define IS_POINTER(expression_or_type_name) (__is_convertible_to(typeof (expression_or_type_name), void*) || __is_convertible_to(typeof (expression_or_type_name), void const*) || __is_convertible_to(typeof (expression_or_type_name), void volatile*) || __is_convertible_to(typeof (expression_or_type_name), void const volatile*))
#endif
(http://www.cplusplus.com/reference/type_traits/is_convertible/)
It’s not possible in standard C. Whatever solutions there may be must be implementation-defined.
It seems that C is not a suitable language for your problem domain. Short of esoteric platforms that come without C++ support, there seems to be little reason to have to solve this in C. On most any platform where C is available, so is C++. Since you claim that you use gcc, then C++ is surely available and you can process some of the input using a C++ compiler. You could do the input processing as a generator step: run the code through a C++ compiler to generate source code that defines constants that capture type properties sought.
I have worked on several projects in college on C, but never used it in professional capacity.
Recently I started reading through cpython's source code and the following syntax confused me: github
What does PyAPI_FUNC(int) PyToken_OneChar(int); the part before the function name mean? Is it a wrapper function that dynamically constructs the return type?
I am not even sure what to Google search for, in this case!
PyAPI_FUNC() is a macro defined in pyport.h. The particular definition depends on the platform you're building on, but here's an example:
#define PyAPI_FUNC(RTYPE) __declspec(dllimport) RTYPE
So the line in your question, PyAPI_FUNC(int) PyToken_OneChar(int); expands to:
__declspec(dllimport) int PyToken_OneChar(int);
Basically, it just declares the name PyToken_OneChar as a function that takes an int as its parameter and returns an int, but it does it in a way that lets the compiler embed storage information with those types. See What is __declspec and when do I need to use it? for more information about the __declspec directive if you're interested. Another of the definitions for PyAPI_FUNC is:
#define PyAPI_FUNC(RTYPE) RTYPE
which skips all that and just expands the line above to:
int PyToken_OneChar(int);
So the main thing to take away from this is that source code that's meant to compile on multiple platforms often uses macros that make it easier to write code once and use it on each of those platforms. In this case, it lets the programmers write declarations for PyToken_OneChar() and many other functions once instead of having to write (and maintain!) different versions for each platform. This is fairly advanced stuff -- not something you should worry about if you're getting started.
It's a C Macro they wrote which allows them to do different things on different OS platforms, for instance, on windows, this will export the function as part of the public interface for a DLL.
I have a lot of preprocessor macro definitions, like this:
#define FOO 1
#define BAR 2
#define BAZ 3
In the real application, each definition corresponds to an instruction in an interpreter virtual machine. The macros are also not sequential in numbering to leave space for future instructions; there may be a #define FOO 41, then the next one is #define BAR 64.
I'm now working on a debugger for this virtual machine, and need to effectively 'reverse' these preprecessor macros. In other words, I need a function which takes the number and returns the macro name, e.g. an input of 2 returns "BAR".
Of course, I could create a function using a switch myself:
const char* instruction_by_id(int id) {
switch (id) {
case FOO:
return "FOO";
case BAR:
return "BAR";
case BAZ:
return "BAZ";
default:
return "???";
}
}
However, this will a nightmare to maintain, since renaming, removing or adding instructions will require this function to be modified too.
Is there another macro which I can use to create a function like this for me, or is there some other approach? If not, is it possible to create a macro to perform this task?
I'm using gcc 6.3 on Windows 10.
You have the wrong approach. Read SICP if you have not read it.
I have a lot of preprocessor macro definitions, like this:
#define FOO 1
#define BAR 2
#define BAZ 3
Remember that C or C++ code can be generated, and it is quite easy to instruct your build automation tool to generate some particular C file (with GNU make or ninja you just add some rule or recipe).
For example, you could use some different preprocessor (liek GPP or m4), or some script -e.g. in awk or Python or Guile, etc..., or write your own program (in C, C++, Ocaml, etc...), to generate the header file containing these #define-s. And another script or program (or the same one, invoked differently) could generate the C code of instruction_by_id
Such basic metaprogramming techniques (of generating some or several C files from something higher level but specific) have been used since at least the 1980s (e.g. with yacc or RPCGEN). The C preprocessor facilitates that with its #include directive (since you can even include lines inside some function body, etc...). Actually, the idea that code is data (and proof) and data is code is even older (Church-Turing thesis, Curry-Howard correspondence, Halting problem). The Gödel, Escher, Bach book is very entertaining....
For example, you could decide to have a textual file opcodes.txt (or even some sqlite database containing stuff....) like
# ignore lines starting with an hashsign
FOO 1
BAR 2
and have two small awk or Python scripts (or two tiny C specialized programs), one generating the #define-s (into opcode-defines.h) and another generating the body of instruction_by_id (into opcode-instr.inc). Then you need to adapt your Makefile to generate these, and put #include "opcode-defines.h" inside some global header, and have
const char* instruction_by_id(int id) {
switch (id) {
#include "opcode-instr.inc"
default: return "???";
}
}
this will a nightmare to maintain,
Not so with such a metaprogramming approach. You'll just maintain opcodes.txt and the scripts using it, but you express a given "knowledge element" (the relation of FOO to 1) only once (in a single line of opcode.txt). Of course you need to document that (at the very least, with comments in your Makefile).
Metaprogramming from some higher-level, declarative formalization, is a very powerful paradigm. In France, J.Pitrat pioneered it (and he is writing an interesting blog today, while being retired) since the 1960s. In the US, J.MacCarthy and the Lisp community also.
For an entertaining talk, see Liam Proven FOSDEM 2018 talk on The circuit less traveled
Large software are using that metaprogramming approach quite often. For example, the GCC compiler have about a dozen of C++ code generators (in total, they are emitting more than a million of C++ lines).
Another way of looking at such an approach is the idea of domain-specific languages that could be compiled to C. If you use an operating system providing dynamic loading, you can even write a program emitting C code, forking a process to compile it into some plugin, then loading that plugin (on POSIX or Linux, with dlopen). Interestingly, computers are now fast enough to enable such an approach in an interactive application (in some sort of REPL): you can emit a C file of a few thousand lines, compile it into some .so shared object file, and dlopen that, in a fraction of second. You could also use JIT-compiling libraries like GCCJIT or LLVM to generate code at runtime. You could embed an interpreter (like Lua or Guile) into your program.
BTW, metaprogramming approaches is one of the reasons why basic compilation techniques should be known by most developers (and not only just people in the compiler business); another reason is that parsing problems are very common. So read the Dragon Book.
Be aware of Greenspun's tenth rule. It is much more than a joke, actually a profound truth about large software.
In a similar case I've resorted to defining a text file format that defines the instructions, and writing a program to read this file and write out the C source of the actual instruction definitions and the C source of functions like your instruction_by_id(). This way you only need to maintain the text file.
As awesome as general code generation is, I’m surprised that nobody mentioned that (if you relax your problem definition just a bit) the C preprocessor is perfectly capable of generating the necessary code, using a technique called X macros. In fact every simple bytecode VM in C that I’ve seen uses this approach.
The technique works as follows. First, there is a file (call it insns.h) containing the authoritative list of instructions,
INSN(FOO, 1)
INSN(BAR, 2)
INSN(BAZ, 3)
or alternatively a macro in some other header containing the same,
#define INSNS \
INSN(FOO, 1) \
INSN(BAR, 2) \
INSN(BAZ, 3)
whichever is more conveinent for you. (I’ll use the first option in the following.) Note that INSN is not defined anywhere. (Traditionally it would be called X, thus the name of the technique.) Wherever you want to loop over your instructions, define INSN to generate the code you want, include insns.h, then undefine INSN again.
In your disassembler, write
const char *instruction_by_id(int id) {
switch (id) {
#define INSN(NAME, VALUE) \
case NAME: return #NAME;
#include "insns.h" /* or just INSNS if you use a macro */
#undef INSN
default: return "???";
}
}
using the prefix stringification operator # to turn names-as-identifiers into names-as-string-literals.
You obviously can’t define the constants this way, because macros cannot define other macros in the C preprocessor. However, if you don’t insist that the instruction constants be preprocessor constants, there’s a different perfectly serviceable constant facility in the C language: enumerations. Whether or not you use an enumerated type, the enumerators defined inside it are regular integer constants from the point of view of the compiler (though not the preprocessor—you cannot use #ifdef with them, for example). So, using an anonymous enumeration type, define your constants like this:
enum {
#define INSN(NAME, VALUE) \
NAME = VALUE,
#include "insns.h" /* or just INSNS if you use a macro */
#undef INSN
NINSNS /* C89 doesn’t allow trailing commas in enumerations (but C99+ does), and you may find this constant useful in any case */
};
If you want to statically initialize an array indexed by your bytecodes, you’ll have to use C99 designated initializers {[FOO] = foovalue, [BAR] = barvalue, /* ... */} whether or not you use X macros. However, if you don’t insist on assigning custom codes to your instructions, you can eliminate VALUE from the above and have the enumeration assign consecutive codes automatically, and then the array can be simply initialized in order, {foovalue, barvalue, /* ... */}. As a bonus, NINSNS above then becomes equal to the number of the instructions and the size of any such array, which is why I called it that.
There are more tricks you can use here. For example, if some instructions have variants for several data types, the instruction list X macro can call the type list X macro to generate the variants automatically. (The somewhat ugly second option of storing the X macro list in a large macro and not an include file may be more handy here.) The INSN macro may take additional arguments such as the mode name, which would ignored in the code list but used to call the appropriate decoding routine in the disassembler. You can use token pasting operator ## to add prefixes to the names of the constants, as in INSN_ ## NAME to generate INSN_FOO, INSN_BAR, etc. And so on.
I am a first year computer science student and my professor said #define is banned in the industry standards along with #if, #ifdef, #else, and a few other preprocessor directives. He used the word "banned" because of unexpected behaviour.
Is this accurate? If so why?
Are there, in fact, any standards which prohibit the use of these directives?
First I've heard of it.
No; #define and so on are widely used. Sometimes too widely used, but definitely used. There are places where the C standard mandates the use of macros — you can't avoid those easily. For example, §7.5 Errors <errno.h> says:
The macros are
EDOM
EILSEQ
ERANGE
which expand to integer constant expressions with type int, distinct positive values, and which are suitable for use in #if preprocessing directives; …
Given this, it is clear that not all industry standards prohibit the use of the C preprocessor macro directives. However, there are 'best practices' or 'coding guidelines' standards from various organizations that prescribe limits on the use of the C preprocessor, though none ban its use completely — it is an innate part of C and cannot be wholly avoided. Often, these standards are for people working in safety-critical areas.
One standard you could check the MISRA C (2012) standard; that tends to proscribe things, but even that recognizes that #define et al are sometimes needed (section 8.20, rules 20.1 through 20.14 cover the C preprocessor).
The NASA GSFC (Goddard Space Flight Center) C Coding Standards simply say:
Macros should be used only when necessary. Overuse of macros can make code harder to read and maintain because the code no longer reads or behaves like standard C.
The discussion after that introductory statement illustrates the acceptable use of function macros.
The CERT C Coding Standard has a number of guidelines about the use of the preprocessor, and implies that you should minimize the use of the preprocessor, but does not ban its use.
Stroustrup would like to make the preprocessor irrelevant in C++, but that hasn't happened yet. As Peter notes, some C++ standards, such as the JSF AV C++ Coding Standards (Joint Strike Fighter, Air Vehicle) from circa 2005, dictate minimal use of the C preprocessor. Essentially, the JSF AV C++ rules restrict it to #include and the #ifndef XYZ_H / #define XYZ_H / … / #endif dance that prevents multiple inclusions of a single header. C++ has some options that are not available in C — notably, better support for typed constants that can then be used in places where C does not allow them to be used. See also static const vs #define vs enum for a discussion of the issues there.
It is a good idea to minimize the use of the preprocessor — it is often abused at least as much as it is used (see the Boost preprocessor 'library' for illustrations of how far you can go with the C preprocessor).
Summary
The preprocessor is an integral part of C and #define and #if etc cannot be wholly avoided. The statement by the professor in the question is not generally valid: #define is banned in the industry standards along with #if, #ifdef, #else, and a few other macros is an over-statement at best, but might be supportable with explicit reference to specific industry standards (but the standards in question do not include ISO/IEC 9899:2011 — the C standard).
Note that David Hammen has provided information about one specific C coding standard — the JPL C Coding Standard — that prohibits a lot of things that many people use in C, including limiting the use of of the C preprocessor (and limiting the use of dynamic memory allocation, and prohibiting recursion — read it to see why, and decide whether those reasons are relevant to you).
No, use of macros is not banned.
In fact, use of #include guards in header files is one common technique that is often mandatory and encouraged by accepted coding guidelines. Some folks claim that #pragma once is an alternative to that, but the problem is that #pragma once - by definition, since pragmas are a hook provided by the standard for compiler-specific extensions - is non-standard, even if it is supported by a number of compilers.
That said, there are a number of industry guidelines and encouraged practices that actively discourage all usage of macros other than #include guards because of the problems macros introduce (not respecting scope, etc). In C++ development, use of macros is frowned upon even more strongly than in C development.
Discouraging use of something is not the same as banning it, since it is still possible to legitimately use it - for example, by documenting a justification.
Some coding standards may discourage or even forbid the use of #define to create function-like macros that take arguments, like
#define SQR(x) ((x)*(x))
because a) such macros are not type-safe, and b) somebody will inevitably write SQR(x++), which is bad juju.
Some standards may discourage or ban the use of #ifdefs for conditional compilation. For example, the following code uses conditional compilation to properly print out a size_t value. For C99 and later, you use the %zu conversion specifier; for C89 and earlier, you use %lu and cast the value to unsigned long:
#if __STDC_VERSION__ >= 199901L
# define SIZE_T_CAST
# define SIZE_T_FMT "%zu"
#else
# define SIZE_T_CAST (unsigned long)
# define SIZE_T_FMT "%lu"
#endif
...
printf( "sizeof foo = " SIZE_T_FMT "\n", SIZE_T_CAST sizeof foo );
Some standards may mandate that instead of doing this, you implement the module twice, once for C89 and earlier, once for C99 and later:
/* C89 version */
printf( "sizeof foo = %lu\n", (unsigned long) sizeof foo );
/* C99 version */
printf( "sizeof foo = %zu\n", sizeof foo );
and then let Make (or Ant, or whatever build tool you're using) deal with compiling and linking the correct version. For this example that would be ridiculous overkill, but I've seen code that was an untraceable rat's nest of #ifdefs that should have had that conditional code factored out into separate files.
However, I am not aware of any company or industry group that has banned the use of preprocessor statements outright.
Macros can not be "banned". The statement is nonsense. Literally.
For example, section 7.5 Errors <errno.h> of the C Standard requires the use of macros:
1 The header <errno.h> defines several macros, all relating to the reporting of error conditions.
2 The macros are
EDOM
EILSEQ
ERANGE
which expand to integer constant expressions with type int, distinct
positive values, and which are suitable for use in #if preprocessing
directives; and
errno
which expands to a modifiable lvalue that has type int and thread
local storage duration, the value of which is set to a positive error
number by several library functions. If a macro definition is
suppressed in order to access an actual object, or a program defines
an identifier with the name errno, the behavior is undefined.
So, not only are macros a required part of C, in some cases not using them results in undefined behavior.
No, #define is not banned. Misuse of #define, however, may be frowned upon.
For instance, you may use
#define DEBUG
in your code so that later on, you can designate parts of your code for conditional compilation using #ifdef DEBUG, for debug purposes only. I don't think anyone in his right mind would want to ban something like this. Macros defined using #define are also used extensively in portable programs, to enable/disable compilation of platform-specific code.
However, if you are using something like
#define PI 3.141592653589793
your teacher may rightfully point out that it is much better to declare PI as a constant with the appropriate type, e.g.,
const double PI = 3.141592653589793;
as it allows the compiler to do type checking when PI is used.
Similarly (as mentioned by John Bode above), the use of function-like macros may be disapproved of, especially in C++ where templates can be used. So instead of
#define SQ(X) ((X)*(X))
consider using
double SQ(double X) { return X * X; }
or, in C++, better yet,
template <typename T>T SQ(T X) { return X * X; }
Once again, the idea is that by using the facilities of the language instead of the preprocessor, you allow the compiler to type check and also (possibly) generate better code.
Once you have enough coding experience, you'll know exactly when it is appropriate to use #define. Until then, I think it is a good idea for your teacher to impose certain rules and coding standards, but preferably they themselves should know, and be able to explain, the reasons. A blanket ban on #define is nonsensical.
That's completely false, macros are heavily used in C. Beginners often use them badly but that's not a reason to ban them from industry. A classic bad usage is #define succesor(n) n + 1. If you expect 2 * successor(9) to give 20, then you're wrong because that expression will be translated as 2 * 9 + 1 i.e. 19 not 20. Use parenthesis to get the expected result.
No. It is not banned. And truth to be told, it is impossible to do non-trivial multi-platform code without it.
No your professor is wrong or you misheard something.
#define is a preprocessor macro, and preprocessor macros are needed for conditional compilation and some conventions, which aren't simply built in the C language. For example, in a recent C standard, namely C99, support for booleans had been added. But it's not supported "native" by the language, but by preprocessor #defines. See this reference to stdbool.h
Macros are used pretty heavily in GNU land C, and without conditional preprocessor commands there's be no way to properly handle multiple inclusions of the same source files, so that makes them seem like essential language features to me.
Maybe your class is actually on C++, which despite many people's failure to do so, should be distinguished from C as it is a different language, and I can't speak for macros there. Or maybe the professor meant he's banning them in his class. Anyhow I'm sure the SO community would be interested in hearing which standard he's talking about, since I'm pretty sure all C standards support the use of macros.
Contrary to all of the answers to date, the use of preprocessor directives is oftentimes banned in high-reliability computing. There are two exceptions to this, the use of which are mandated in such organizations. These are the #include directive, and the use of an include guard in a header file. These kinds of bans are more likely in C++ rather than in C.
Here's but one example: 16.1.1 Use the preprocessor only for implementing include guards, and including header files with include guards.
Another example, this time for C rather than C++: JPL Institutional Coding Standard for the C Programming Language . This C coding standard doesn't go quite so far as banning the use of the preprocessor completely, but it comes close. Specifically, it says
Rule 20 (preprocessor use)
Use of the C preprocessor shall be limited to file inclusion and simple macros. [Power of Ten Rule 8].
I'm neither condoning nor decrying those standards. But to say they don't exist is ludicrous.
If you want your C code to interoperate with C++ code, you will want to declare your externally visible symbols, such as function declarations, in the extern "C" namespace. This is often done using conditional compilation:
#ifdef __cplusplus
extern "C" {
#endif
/* C header file body */
#ifdef __cplusplus
}
#endif
Look at any header file and you will see something like this:
#ifndef _FILE_NAME_H
#define _FILE_NAME_H
//Exported functions, strucs, define, ect. go here
#endif /*_FILE_NAME_H */
These define are not only allowed, but critical in nature as each time the header file is referenced in files it will be included separately. This means without the define you are redefining everything in between the guard multiple times which best case fails to compile and worse case leaves you scratching your head later why your code doesn't work the way you want it to.
The compiler will also use define as seen here with gcc that let you test for things like the version of the compiler which is very useful. I'm currently working on a project that needs to compile with avr-gcc, but we have a testing environment that we also run our code though. To prevent the avr specific files and registers from keeping our test code from running we do something like this:
#ifdef __AVR__
//avr specific code here
#endif
Using this in the production code, the complementary test code can compile without using the avr-gcc and the code above is only compiled using avr-gcc.
If you had just mentioned #define, I would have thought maybe he was alluding to its use for enumerations, which are better off using enum to avoid stupid errors such as assigning the same numerical value twice.
Note that even for this situation, it is sometimes better to use #defines than enums, for instance if you rely on numerical values exchanged with other systems and the actual values must stay the same even if you add/delete constants (for compatibility).
However, adding that #if, #ifdef, etc. should not be used either is just weird. Of course, they should probably not be abused, but in real life there are dozens of reasons to use them.
What he may have meant could be that (where appropriate), you should not hardcode behaviour in the source (which would require re-compilation to get a different behaviour), but rather use some form of run-time configuration instead.
That's the only interpretation I could think of that would make sense.