Handling OS function name mismatch with macros - c

My current code is:
int my_func(int a)
{
#ifdef _WIN32
return _func(a);
#else
return func(a);
#endif
}
But I think it would be better if it was something like:
#ifdef _WIN32
#define common_func(a) _func(a);
#else
#define common_func(a) func(a);
#endif
int my_func(int a)
{
return common_func(a);
}
Now, I have never done this and I don't know what I am doing. The K&R cat() example is confusing. I basically just want to get the #ifdefs out of the function because it would get too messy because I have to run the function several times.
func() and _func() are the same function, just Windows thought it would be a great idea to prepend it with an underscore. So it's not even a macro function but more like a function alias maybe. Or a wrapper?
Does this impact performance? Does the generated code differ from version 1?
Is there some trick, since the difference is just the underscore?
I want to do it correctly and properly.
Please help.

Windows thought it would be a great idea to prepend it with an underscore. So it's not even a macro function but more like a function alias maybe. Or a wrapper?
Most likely it's simply a differently-named function that has the same specifications. Probably not a wrapper or alias. Functions such as POSIX open(), read(), and write() are not defined by the C language specification, unlike corresponding fopen(), fread(), and fwrite(). Generally speaking, C implementations must provide the latter group, by they have no obligation to provide the former group.
Does this impact performance?
Conditional compilation directives such as #ifdef are evaluated at compile time. They themselves have no runtime impact at all, and they are pretty cheap at compile time.
Does the generated code differ from version 1?
No. The two versions of your code are 100% equivalent.
Is there some trick, since the difference is just the underscore?
If there are several functions you want to use where the Windows versions differ in name from (I suppose) the POSIX version by a leading underscore, then you might think it worth your while to define a common macro that you can reuse for all of them, instead of rolling a separate thing for each individual function. Maybe something like this:
#ifdef _WIN32
#define POSIX_MANGLE(f) _ ## f
#else
#define POSIX_MANGLE(f) f
#endif
You would use that something like so:
void do_something(int a) {
POSIX_MANGLE(func1)(a);
POSIX_MANGLE(func2)(a);
}
On Windows (technically, wherever _WIN32 is defined), that is then equivalent to ...
void do_something(int a) {
_func1(a);
_func2(a);
}
Anywhere else, it is equivalent to ...
void do_something(int a) {
func1(a);
func2(a);
}

Related

Why to replace static function as object-like macro, #define STATIC static?

I just spotted this in the legacy code. I know that using the macro, whenever the name is used, it is replaced by the contents of the macro. They are most commonly used to give symbolic names to numeric constants.What I know is preprocess has no notion of type safety, scope.
What is the real benefit of doing like this?
#define STATIC static
STATIC function1() { /*Do something*/ }
I knew that static functions are visible only in that module or translation unit. Not visible outside of the C file they are defined.
Why not just declare like this, instead of macro replacement?
static function1() { /*Do something*/ }
I thought I will find an answer in SO but I didn't find any proper answer.
There is no rational reason why you would do this. Generally it is bad practice to hide keywords behind #define in this manner, because the code turns cryptic and harder to read.
I would suspect it has to do with coding style, it is common to write various function specifiers in upper case, particularly in Windows programming. Often it is done to specify a certain calling convention. For example CALLBACK in Windows, or this example from the old "Windows bible" (Petzold):
#define EXPORT __declspec (dllexport)
(Which could be modified to also contain extern "C" in case of C++.) You'd then have a function such as EXPORT void CALLBACK func (void). Similarly there's also WINAPI in Windows.
Sometimes we also see things like
#define PRIVATE static
which is kind of horrible, because static doesn't really have the same meaning as private in C++.

How to #ifdef the __builtin_prefetch function

How do I keep __builtin_prefetch() in my code, but make compilers that do not have it compile successfully? (Just doing nothing where it is found).
__builtin_prefetch() is recognised by the compiler (gcc) not the preprocessor, so you won't be able to detect it using the C preprocessor.
Since an identifier with two leading underscores is reserved for use by the implementation (so any code you use which defines such an identifier has undefined behaviour) I'd do it the other way around.
#ifdef __GNUC__
#define do_prefetch(x) __builtin_prefetch(x)
#else
#define do_prefetch(x)
#endif
and then use
do_prefetch(whatever);
where needed.
That way there is no code emitted unless it is actually needed.
Since __builtin_prefetch() accepts a variable number of arguments, you might want to adapt the above to use variadic macros (C99 and later) - if you use it with different numbers of arguments in different places in your code.
It is not exactly the best solution, but it will disable __builtin_prefetch() on all other compilers other than GCC.
#ifndef __GNUC__
# define __builtin_prefetch(x)
#endif

Is there a tool that checks what predefined macros a C file depends on?

To avoid impossible situation one could reduce the problem to two cases.
Case 1
The first (simplest) case is situation where the preprocessor has a chance to detect it, that is there's a preprocessor directive that depends on a macro being predefined (that is defined before the first line of input) or not. For example:
#ifdef FOO
#define BAR 42
#else
#define BAR 43
#endif
depends on FOO being predefined or not. However the file
#undef FOO
#ifdef FOO
#define BAR 42
#endif
does not. A harder case would be to detect if the dependency actually does matter, which it doesn't in the above cases (as neither FOO or BAR affects the output).
Case 2
The second (harder) case is where successful compilation depends on predefined macros:
INLINE int fubar(void) {
return 42;
}
which is perfectly fine as far as the preprocessor is concerned whether or not ENTRY_POINT is predefined, but unless INLINE is carefully defined that code won't compile. Similarily we could in this case it might be possible to exclude cases where the output isn't affected, but I can't find an example of that. The complication here is that in the example:
int fubar(void) {
return 42;
}
the fubar being predefined can alter the successful compilation of this, so one would probably need to restrict it to cases where a symbol need to be predefined in order to compile successfully.
I guess such a tool would be something similar to a preprocessor (and C parser in the second case). The question is if there is such a tool? Or is there a tool that only handles the first case? Or none at all?
In C everything can be (re)defined, so there is no way to know in advance what is intended to be (re)defined. Usually some naming conventions helps us to figure out what is meant to be a macro (like upper-case). Therefore it is not possible to have such tool. Of course if you assume that the compilation errors are caused by missing macro definitions then you can use them to analyze what is missing.

Preventing incorrect macro expansion in gcc

Is there any way to prevent gcc from expanding a macro in this:
#define putc(a) fputc(a)
...
void _putc(char ch) {}
struct foo { void *(putc)(char ch); }
struct foo f = {_putc;}
(&f)->putc('X'); // this is an error because it gets expanded into fputc, which is very inappropriate.
I don't want to use #undef putc because it messes up other things.
Including <stdio.h> may or may not define macro functions. In either case, a real function is provided.
It's probably not the best idea to name a function pointer like a standard library function, but you can do it. To prevent macro expansion, you have basically three options:
#undef it. You said this would mess up other things, though this shouldn't be a problem -- a real function with that name still exists. For some functions, you may miss optimizations or warnings (for functions like printf, for example), however (depending on your compiler).
Don't include the header file and declare the function yourself. I mention this for sake of completeness rather than as a real suggestion. This doesn't work if you need a type definition provided only in the header you don't want to include.
Don't put an opening parenthesis after the macro name, as in
((&f)->putc)('X'); // or (f.putc)('X'); -- looking less confusing.

What are C macros useful for?

I have written a little bit of C, and I can read it well enough to get a general idea of what it is doing, but every time I have encountered a macro it has thrown me completely. I end up having to remember what the macro is and substitute it in my head as I read. The ones that I have encountered that were intuitive and easy to understand were always like little mini functions, so I always wondered why they weren't just functions.
I can understand the need to define different build types for debug or cross platform builds in the preprocessor but the ability to define arbitrary substitutions seems to be useful only to make an already difficult language even more difficult to understand.
Why was such a complex preprocessor introduced for C? And does anyone have an example of using it that will make me understand why it still seems to be used for purposes other than simple if #debug style conditional compilations?
Edit:
Having read a number of answers I still just don't get it. The most common answer is to inline code. If the inline keyword doesn't do it then either it has a good reason to not do it, or the implementation needs fixing. I don't understand why a whole different mechanism is needed that means "really inline this code" (aside form the code being written before inline was around). I also don't understand the idea that was mentioned that "if its too silly to be put in a function". Surely any piece of code that takes an input and produces an output is best put in a function. I think I may not be getting it because I am not used to the micro optimisations of writing C, but the preprocessor just feels like a complex solution to a few simple problems.
I end up having to remember what the macro is and substitute it in my head as I read.
That seems to reflect poorly on the naming of the macros. I would assume you wouldn't have to emulate the preprocessor if it were a log_function_entry() macro.
The ones that I have encountered that were intuitive and easy to understand were always like little mini functions, so I always wondered why they weren't just functions.
Usually they should be, unless they need to operate on generic parameters.
#define max(a,b) ((a)<(b)?(b):(a))
will work on any type with an < operator.
More that just functions, macros let you perform operations using the symbols in the source file. That means you can create a new variable name, or reference the source file and line number the macro is on.
In C99, macros also allow you to call variadic functions such as printf
#define log_message(guard,format,...) \
if (guard) printf("%s:%d: " format "\n", __FILE__, __LINE__,__VA_ARGS_);
log_message( foo == 7, "x %d", x)
In which the format works like printf. If the guard is true, it outputs the message along with the file and line number that printed the message. If it was a function call, it would not know the file and line you called it from, and using a vaprintf would be a bit more work.
This excerpt pretty much sums up my view on the matter, by comparing several ways that C macros are used, and how to implement them in D.
copied from DigitalMars.com
Back when C was invented, compiler
technology was primitive. Installing a
text macro preprocessor onto the front
end was a straightforward and easy way
to add many powerful features. The
increasing size & complexity of
programs have illustrated that these
features come with many inherent
problems. D doesn't have a
preprocessor; but D provides a more
scalable means to solve the same
problems.
Macros
Preprocessor macros add powerful features and flexibility to C. But they have a downside:
Macros have no concept of scope; they are valid from the point of definition to the end of the source. They cut a swath across .h files, nested code, etc. When #include'ing tens of thousands of lines of macro definitions, it becomes problematical to avoid inadvertent macro expansions.
Macros are unknown to the debugger. Trying to debug a program with symbolic data is undermined by the debugger only knowing about macro expansions, not the macros themselves.
Macros make it impossible to tokenize source code, as an earlier macro change can arbitrarily redo tokens.
The purely textual basis of macros leads to arbitrary and inconsistent usage, making code using macros error prone. (Some attempt to resolve this was introduced with templates in C++.)
Macros are still used to make up for deficits in the language's expressive capability, such as for "wrappers" around header files.
Here's an enumeration of the common uses for macros, and the corresponding feature in D:
Defining literal constants:
The C Preprocessor Way
#define VALUE 5
The D Way
const int VALUE = 5;
Creating a list of values or flags:
The C Preprocessor Way
int flags:
#define FLAG_X 0x1
#define FLAG_Y 0x2
#define FLAG_Z 0x4
...
flags |= FLAG_X;
The D Way
enum FLAGS { X = 0x1, Y = 0x2, Z = 0x4 };
FLAGS flags;
...
flags |= FLAGS.X;
Setting function calling conventions:
The C Preprocessor Way
#ifndef _CRTAPI1
#define _CRTAPI1 __cdecl
#endif
#ifndef _CRTAPI2
#define _CRTAPI2 __cdecl
#endif
int _CRTAPI2 func();
The D Way
Calling conventions can be specified in blocks, so there's no need to change it for every function:
extern (Windows)
{
int onefunc();
int anotherfunc();
}
Simple generic programming:
The C Preprocessor Way
Selecting which function to use based on text substitution:
#ifdef UNICODE
int getValueW(wchar_t *p);
#define getValue getValueW
#else
int getValueA(char *p);
#define getValue getValueA
#endif
The D Way
D enables declarations of symbols that are aliases of other symbols:
version (UNICODE)
{
int getValueW(wchar[] p);
alias getValueW getValue;
}
else
{
int getValueA(char[] p);
alias getValueA getValue;
}
There are more examples on the DigitalMars website.
They are a programming language (a simpler one) on top of C, so they are useful for doing metaprogramming in compile time... in other words, you can write macro code that generates C code in less lines and time that it will take writing it directly in C.
They are also very useful to write "function like" expressions that are "polymorphic" or "overloaded"; e.g. a max macro defined as:
#define max(a,b) ((a)>(b)?(a):(b))
is useful for any numeric type; and in C you could not write:
int max(int a, int b) {return a>b?a:b;}
float max(float a, float b) {return a>b?a:b;}
double max(double a, double b) {return a>b?a:b;}
...
even if you wanted, because you cannot overload functions.
And not to mention conditional compiling and file including (that are also part of the macro language)...
Macros allow someone to modify the program behavior during compilation time. Consider this:
C constants allow fixing program behavior at development time
C variables allow modifying program behavior at execution time
C macros allow modifying program behavior at compilation time
At compilation time means that unused code won't even go into the binary and that the build process can modify the values, as long as it's integrated with the macro preprocessor. Example: make ARCH=arm (assumes forwarding macro definition as cc -DARCH=arm)
Simple examples:
(from glibc limits.h, define the largest value of long)
#if __WORDSIZE == 64
#define LONG_MAX 9223372036854775807L
#else
#define LONG_MAX 2147483647L
#endif
Verifies (using the #define __WORDSIZE) at compile time if we're compiling for 32 or 64 bits. With a multilib toolchain, using parameters -m32 and -m64 may automatically change bit size.
(POSIX version request)
#define _POSIX_C_SOURCE 200809L
Requests during compilation time POSIX 2008 support. The standard library may support many (incompatible) standards but with this definition, it will provide the correct function prototypes (example: getline(), no gets(), etc.). If the library doesn't support the standard it may give an #error during compile time, instead of crashing during execution, for example.
(hardcoded path)
#ifndef LIBRARY_PATH
#define LIBRARY_PATH "/usr/lib"
#endif
Defines, during compilation time a hardcode directory. Could be changed with -DLIBRARY_PATH=/home/user/lib, for example. If that were a const char *, how would you configure it during compilation ?
(pthread.h, complex definitions at compile time)
# define PTHREAD_MUTEX_INITIALIZER \
{ { 0, 0, 0, 0, 0, 0, { 0, 0 } } }
Large pieces of text may that otherwise wouldn't be simplified may be declared (always at compile time). It's not possible to do this with functions or constants (at compile time).
To avoid really complicating things and to avoid suggesting poor coding styles, I'm wont give an example of code that compiles in different, incompatible, operating systems. Use your cross build system for that, but it should be clear that the preprocessor allows that without help from the build system, without breaking compilation because of absent interfaces.
Finally, think about the importance of conditional compilation on embedded systems, where processor speed and memory are limited and systems are very heterogeneous.
Now, if you ask, is it possible to replace all macro constant definitions and function calls with proper definitions ? The answer is yes, but it won't simply make the need for changing program behavior during compilation go away. The preprocessor would still be required.
Remember that macros (and the pre-processor) come from the earliest days of C. They used to be the ONLY way to do inline 'functions' (because, of course, inline is a very recent keyword), and they are still the only way to FORCE something to be inlined.
Also, macros are the only way you can do such tricks as inserting the file and line into string constants at compile time.
These days, many of the things that macros used to be the only way to do are better handled through newer mechanisms. But they still have their place, from time to time.
Apart from inlining for efficiency and conditional compilation, macros can be used to raise the abstraction level of low-level C code. C doesn't really insulate you from the nitty-gritty details of memory and resource management and exact layout of data, and supports very limited forms of information hiding and other mechanisms for managing large systems. With macros, you are no longer limited to using only the base constructs in the C language: you can define your own data structures and coding constructs (including classes and templates!) while still nominally writing C!
Preprocessor macros actually offer a Turing-complete language executed at compile time. One of the impressive (and slightly scary) examples of this is over on the C++ side: the Boost Preprocessor library uses the C99/C++98 preprocessor to build (relatively) safe programming constructs which are then expanded to whatever underlying declarations and code you input, whether C or C++.
In practice, I'd recommend regarding preprocessor programming as a last resort, when you don't have the latitude to use high level constructs in safer languages. But sometimes it's good to know what you can do if your back is against the wall and the weasels are closing in...!
From Computer Stupidities:
I've seen this code excerpt in a lot of freeware gaming programs for UNIX:
/*
* Bit values.
*/
#define BIT_0 1
#define BIT_1 2
#define BIT_2 4
#define BIT_3 8
#define BIT_4 16
#define BIT_5 32
#define BIT_6 64
#define BIT_7 128
#define BIT_8 256
#define BIT_9 512
#define BIT_10 1024
#define BIT_11 2048
#define BIT_12 4096
#define BIT_13 8192
#define BIT_14 16384
#define BIT_15 32768
#define BIT_16 65536
#define BIT_17 131072
#define BIT_18 262144
#define BIT_19 524288
#define BIT_20 1048576
#define BIT_21 2097152
#define BIT_22 4194304
#define BIT_23 8388608
#define BIT_24 16777216
#define BIT_25 33554432
#define BIT_26 67108864
#define BIT_27 134217728
#define BIT_28 268435456
#define BIT_29 536870912
#define BIT_30 1073741824
#define BIT_31 2147483648
A much easier way of achieving this is:
#define BIT_0 0x00000001
#define BIT_1 0x00000002
#define BIT_2 0x00000004
#define BIT_3 0x00000008
#define BIT_4 0x00000010
...
#define BIT_28 0x10000000
#define BIT_29 0x20000000
#define BIT_30 0x40000000
#define BIT_31 0x80000000
An easier way still is to let the compiler do the calculations:
#define BIT_0 (1)
#define BIT_1 (1 << 1)
#define BIT_2 (1 << 2)
#define BIT_3 (1 << 3)
#define BIT_4 (1 << 4)
...
#define BIT_28 (1 << 28)
#define BIT_29 (1 << 29)
#define BIT_30 (1 << 30)
#define BIT_31 (1 << 31)
But why go to all the trouble of defining 32 constants? The C language also has parameterized macros. All you really need is:
#define BIT(x) (1 << (x))
Anyway, I wonder if guy who wrote the original code used a calculator or just computed it all out on paper.
That's just one possible use of Macros.
I will add to whats already been said.
Because macros work on text substitutions they allow you do very useful things which wouldn't be possible to do using functions.
Here a few cases where macros can be really useful:
/* Get the number of elements in array 'A'. */
#define ARRAY_LENGTH(A) (sizeof(A) / sizeof(A[0]))
This is a very popular and frequently used macro. This is very handy when you for example need to iterate through an array.
int main(void)
{
int a[] = {1, 2, 3, 4, 5};
int i;
for (i = 0; i < ARRAY_LENGTH(a); ++i) {
printf("a[%d] = %d\n", i, a[i]);
}
return 0;
}
Here it doesn't matter if another programmer adds five more elements to a in the decleration. The for-loop will always iterate through all elements.
The C library's functions to compare memory and strings are quite ugly to use.
You write:
char *str = "Hello, world!";
if (strcmp(str, "Hello, world!") == 0) {
/* ... */
}
or
char *str = "Hello, world!";
if (!strcmp(str, "Hello, world!")) {
/* ... */
}
To check if str points to "Hello, world". I personally think that both these solutions look quite ugly and confusing (especially !strcmp(...)).
Here are two neat macros some people (including I) use when they need to compare strings or memory using strcmp/memcmp:
/* Compare strings */
#define STRCMP(A, o, B) (strcmp((A), (B)) o 0)
/* Compare memory */
#define MEMCMP(A, o, B) (memcmp((A), (B)) o 0)
Now you can now write the code like this:
char *str = "Hello, world!";
if (STRCMP(str, ==, "Hello, world!")) {
/* ... */
}
Here is the intention alot clearer!
These are cases were macros are used for things functions cannot accomplish. Macros should not be used to replace functions but they have other good uses.
One of the case where macros really shine is when doing code-generation with them.
I used to work on an old C++ system that was using a plugin system with his own way to pass parameters to the plugin (Using a custom map-like structure). Some simple macros were used to be able to deal with this quirk and allowed us to use real C++ classes and functions with normal parameters in the plugins without too much problems. All the glue code being generated by macros.
Given the comments in your question, you may not fully appreciate is that calling a function can entail a fair amount of overhead. The parameters and key registers may have to be copied to the stack on the way in, and the stack unwound on the way out. This was particularly true of the older Intel chips. Macros let the programmer keep the abstraction of a function (almost), but avoided the costly overhead of a function call. The inline keyword is advisory, but the compiler may not always get it right. The glory and peril of 'C' is that you can usually bend the compiler to your will.
In your bread and butter, day-to-day application programming this kind of micro-optimization (avoiding function calls) is generally worse then useless, but if you are writing a time-critical function called by the kernel of an operating system, then it can make a huge difference.
Unlike regular functions, you can do control flow (if, while, for,...) in macros. Here's an example:
#include <stdio.h>
#define Loop(i,x) for(i=0; i<x; i++)
int main(int argc, char *argv[])
{
int i;
int x = 5;
Loop(i, x)
{
printf("%d", i); // Output: 01234
}
return 0;
}
It's good for inlining code and avoiding function call overhead. As well as using it if you want to change the behaviour later without editing lots of places. It's not useful for complex things, but for simple lines of code that you want to inline, it's not bad.
By leveraging C preprocessor's text manipulation one can construct the C equivalent of a polymorphic data structure. Using this technique we can construct a reliable toolbox of primitive data structures that can be used in any C program, since they take advantage of C syntax and not the specifics of any particular implementation.
Detailed explanation on how to use macros for managing data structure is given here - http://multi-core-dump.blogspot.com/2010/11/interesting-use-of-c-macros-polymorphic.html
Macros let you get rid of copy-pasted fragments, which you can't eliminate in any other way.
For instance (the real code, syntax of VS 2010 compiler):
for each (auto entry in entries)
{
sciter::value item;
item.set_item("DisplayName", entry.DisplayName);
item.set_item("IsFolder", entry.IsFolder);
item.set_item("IconPath", entry.IconPath);
item.set_item("FilePath", entry.FilePath);
item.set_item("LocalName", entry.LocalName);
items.append(item);
}
This is the place where you pass a field value under the same name into a script engine. Is this copy-pasted? Yes. DisplayName is used as a string for a script and as a field name for the compiler. Is that bad? Yes. If you refactor you code and rename LocalName to RelativeFolderName (as I did) and forget to do the same with the string (as I did), the script will work in a way you don't expect (in fact, in my example it depends on did you forget to rename the field in a separate script file, but if the script is used for serialization, it would be a 100% bug).
If you use a macro for this, there will be no room for the bug:
for each (auto entry in entries)
{
#define STR_VALUE(arg) #arg
#define SET_ITEM(field) item.set_item(STR_VALUE(field), entry.field)
sciter::value item;
SET_ITEM(DisplayName);
SET_ITEM(IsFolder);
SET_ITEM(IconPath);
SET_ITEM(FilePath);
SET_ITEM(LocalName);
#undef SET_ITEM
#undef STR_VALUE
items.append(item);
}
Unfortunately, this opens a door for other types of bugs. You can make a typo writing the macro and will never see a spoiled code, because the compiler doesn't show how it looks after all preprocessing. Someone else could use the same name (that's why I "release" macros ASAP with #undef). So, use it wisely. If you see another way of getting rid of copy-pasted code (such as functions), use that way. If you see that getting rid of copy-pasted code with macros isn't worth the result, keep the copy-pasted code.
One of the obvious reasons is that by using a macro, the code will be expanded at compile time, and you get a pseudo function-call without the call overhead.
Otherwise, you can also use it for symbolic constants, so that you don't have to edit the same value in several places to change one small thing.
Macros .. for when your &#(*$& compiler just refuses to inline something.
That should be a motivational poster, no?
In all seriousness, google preprocessor abuse (you may see a similar SO question as the #1 result). If I'm writing a macro that goes beyond the functionality of assert(), I usually try to see if my compiler would actually inline a similar function.
Others will argue against using #if for conditional compilation .. they would rather you:
if (RUNNING_ON_VALGRIND)
rather than
#if RUNNING_ON_VALGRIND
.. for debugging purposes, since you can see the if() but not #if in a debugger. Then we dive into #ifdef vs #if.
If its under 10 lines of code, try to inline it. If it can't be inlined, try to optimize it. If its too silly to be a function, make a macro.
While I'm not a big fan of macros and don't tend to write much C anymore, based on my current tasking, something like this (which could obviously have some side-effects) is convenient:
#define MIN(X, Y) ((X) < (Y) ? (X) : (Y))
Now I haven't written anything like that in years, but 'functions' like that were all over code that I maintained earlier in my career. I guess the expansion could be considered convenient.
I didn't see anyone mentioning this so, regarding function like macros, eg:
#define MIN(X, Y) ((X) < (Y) ? (X) : (Y))
Generally it's recommended to avoid using macros when not necessary, for many reasons, readability being the main concern. So:
When should you use these over a function?
Almost never, since there's a more readable alternative which is inline, see https://www.greenend.org.uk/rjk/tech/inline.html
or http://www.cplusplus.com/articles/2LywvCM9/ (the second link is a C++ page, but the point is applicable to c compilers as far as I know).
Now, the slight difference is that macros are handled by the pre-processor and inline is handled by the compiler, but there's no practical difference nowadays.
when is it appropriate to use these?
For small functions (two or three liners max). The goal is to gain some advantage during the run time of a program, as function like macros (and inline functions) are code replacements done during the pre-proccessing (or compilation in case of inline) and are not real functions living in memory, so there's no function call overhead (more details in the linked pages).

Resources