Is it possible to define a macro off of the content of a macro?
For example:
#define SET(key,value) #define key value
SET(myKey,"value")
int main(){
char str[] = myKey;
printf("%s",str);
}
would result in
int main(){
char str[] = "value";
printf("%s",str);
}
after being preprocessed.
Why would I do this? Because I'm curious ;)
No, its not possible to define a macro within another macro.
The preprocessor only iterates once before the compiler. What you're suggesting would require an undetermined amount of iterations.
No you can't - # in a replacment list of a macro means QUOTE NEXT TOKEN. It's more of a spelling issue, than any logical puzzle :)
(If you require this kind of solution in your code, than there are ways and tricks of using macro's, but you need to be specific about the use cases you need - as your example can be achieved by defining: #define mykey "value")
Here it is from the ansi C99 standard
6.10.3.2 The # operator
Constraints
1 Each # preprocessing token in the replacement list for a
function-like macro shall be followed by a parameter as the next
preprocessing token in the replacement list. Semantics 2 If, in the
replacement list, a parameter is immediately preceded by a #
preprocessing token, both are replaced by a single character string
literal preprocessing token that contains the spelling of the
preprocessing token sequence for the corresponding argument. Each
occurrence of white space between the argument’s preprocessing tokens
becomes a single space character in the character string literal.
White space before the first preprocessing token and after the last
preprocessing token composing the argument is deleted. Otherwise, the
original spelling of each preprocessing token in the argument is
retained in the character string literal, except for special handling
for producing the spelling of string literals and character constants:
a \ character is inserted before each " and \ character of a character
constant or string literal (including the delimiting " characters),
except that it is implementation-defined whether a \ character is
inserted before the \ character beginning a universal character name.
If the replacement that results is not a valid character string
literal, the behavior is undefined. The character string literal
corresponding to an empty argument is "". The order of evaluation of #
and ## operators is unspecified.
Macros are a simple text substitution. Generating new preprocessor directives from a macro would require the preprocessor to continue preprocessing from the beginning of the substitution. However, the standard defined preprocessing to continue behind the substitution.
This makes sense from a streaming point of view, viewing the unprocessed code as the input stream and the processed (and substituted) code as the output stream. Macro substitutions can have an arbitrary length, which means for the preprocessing from the beginning that an arbitrary number of characters must be inserted at the beginning of the input stream to be processed again.
When the processing continues behind the substitution, then the input simply is handled in one single run without any insertion or buffering, because everything directly goes to the output.
whilst it is not possible to use a macro to define another macro, depending on what you are seeking to achieve, you can use macros to effectively achieve the same thing by having them define constants. for example, i have an extensive library of c macros i use to define objective C constant strings and key values.
here are some snippets of code from some of my headers.
// use defineStringsIn_X_File to define a NSString constant to a literal value.
// usage (direct) : defineStringsIn_X_File(constname,value);
#define defineStringsIn_h_File(constname,value) extern NSString * const constname;
#define defineStringsIn_m_File(constname,value) NSString * const constname = value;
// use defineKeysIn_X_File when the value is the same as the key.
// eg myKeyname has the value #"myKeyname"
// usage (direct) : defineKeysIn_X_File(keyname);
// usage (indirect) : myKeyDefiner(defineKeysIn_X_File);
#define defineKeysIn_h_File(key) defineStringsIn_h_File(key,key)
#define defineKeysIn_m_File(key) defineStringsIn_m_File(key,##key)
// use defineKeyValuesIn_X_File when the value is completely unrelated to the key - ie you supply a quoted value.
// eg myKeyname has the value #"keyvalue"
// usage: defineKeyValuesIn_X_File(keyname,#"keyvalue");
// usage (indirect) : myKeyDefiner(defineKeyValuesIn_X_File);
#define defineKeyValuesIn_h_File(key,value) defineStringsIn_h_File(key,value)
#define defineKeyValuesIn_m_File(key,value) defineStringsIn_m_File(key,value)
// use definePrefixedKeys_in_X_File when the last part of the keyname is the same as the value.
// eg myPrefixed_keyname has the value #"keyname"
// usage (direct) : definePrefixedKeys_in_X_File(prefix_,keyname);
// usage (indirect) : myKeyDefiner(definePrefixedKeys_in_X_File);
#define definePrefixedKeys_in_h_File_2(prefix,key) defineKeyValuesIn_h_File(prefix##key,##key)
#define definePrefixedKeys_in_m_File_2(prefix,key) defineKeyValuesIn_m_File(prefix##key,##key)
#define definePrefixedKeys_in_h_File_3(prefix,key,NSObject) definePrefixedKeys_in_h_File_2(prefix,key)
#define definePrefixedKeys_in_m_File_3(prefix,key,NSObject) definePrefixedKeys_in_m_File_2(prefix,key)
#define definePrefixedKeys_in_h_File(...) VARARG(definePrefixedKeys_in_h_File_, __VA_ARGS__)
#define definePrefixedKeys_in_m_File(...) VARARG(definePrefixedKeys_in_m_File_, __VA_ARGS__)
// use definePrefixedKeyValues_in_X_File when the value has no relation to the keyname, but the keyname has a common prefixe
// eg myPrefixed_keyname has the value #"bollocks"
// usage: definePrefixedKeyValues_in_X_File(prefix_,keyname,#"bollocks");
// usage (indirect) : myKeyDefiner(definePrefixedKeyValues_in_X_File);
#define definePrefixedKeyValues_in_h_File(prefix,key,value) defineKeyValuesIn_h_File(prefix##key,value)
#define definePrefixedKeyValues_in_m_File(prefix,key,value) defineKeyValuesIn_m_File(prefix##key,value)
#define VA_NARGS_IMPL(_1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, N, ...) N
#define VA_NARGS(...) VA_NARGS_IMPL(X,##__VA_ARGS__, 11, 10,9, 8, 7, 6, 5, 4, 3, 2, 1, 0)
#define VARARG_IMPL2(base, count, ...) base##count(__VA_ARGS__)
#define VARARG_IMPL(base, count, ...) VARARG_IMPL2(base, count, __VA_ARGS__)
#define VARARG(base, ...) VARARG_IMPL(base, VA_NARGS(__VA_ARGS__), __VA_ARGS__)
and a usage example that invokes it:
#define sw_Logging_defineKeys(defineKeyValue) \
/** start of key list for sw_Logging_ **/\
/**/defineKeyValue(sw_Logging_,log)\
/**/defineKeyValue(sw_Logging_,time)\
/**/defineKeyValue(sw_Logging_,message)\
/**/defineKeyValue(sw_Logging_,object)\
/**/defineKeyValue(sw_Logging_,findCallStack)\
/**/defineKeyValue(sw_Logging_,debugging)\
/**/defineKeyValue(sw_Logging_,callStackSymbols)\
/**/defineKeyValue(sw_Logging_,callStackReturnAddresses)\
/** end of key list for sw_Logging_ **/
sw_Logging_defineKeys(definePrefixedKeys_in_h_File);
the last part may be a little difficult to get your head around.
the sw_Logging_defineKeys() macro defines a list that takes the name of a macro as it's parameter (defineKeyValue) this is then used to invoke the macro that does the actual definition process. ie, for each item in the list, the macro name passed in is used to define the context ( "header", or "implementation", eg either "h" or "m" file, if you understand the objective c file extensions) whilst this is used for objective c, it is simply plain old c macros, used for a "higher purpose" than possibly Kernighan and Richie ever envisaged. :-)
Related
I have a #define'd value named HEIGHT with a value of 20.
I want to use the ASCI escape code "\033[HA" (where H is the number of lines the cursor is moved up.
However, when my code reads "\033[HEIGHTA", it is reading the 'H' as a different escape code (return cursor home). How can I include a #define'd value within an escape code?
Thanks
There are several alternatives, among them
Use a function instead of a macro to generate the escape code as needed. For example,
const char *cursor_up_seq() {
static char sequence[12];
if (sequence[0] == '\0') {
// one-time initialization
sprintf(sequence, "\033[%dA", HEIGHT);
}
return sequence;
}
As a variation on (1), do not produce the escape sequence as a standalone entity at all. Instead, embed it in whatever else you are printing, where it is natural to use (say) printf() to print the value of the HEIGHT macro.
But if you really want to produce a macro for a string literal containing the whole escape sequence, then you can do so by combining two C features:
the stringification (#) macro operator, and
automatic concatenation of adjacent string literals
Another answer, now deleted, attempted to demonstrate that, but floundered on one of the gotchas in that area. Here is a variation that works:
#define HEIGHT 20
#define STRINGIFY(x) #x
#define STRINGIFY_VALUE(x) STRINGIFY(x)
#define SEQUENCE "\033[" STRINGIFY_VALUE(HEIGHT) "A"
The resulting SEQUENCE macro expands to "\033[" "20" "A", which is 100% equivalent to "\033[20A" because of string literal concatenation. The gotcha here is that you cannot use STRINGIFY() directly for this purpose, because that does not macro-expand its argument before converting it to a string (per the standard behavior of #). Wrapping it in another macro layer (STRINGIFY_VALUE) results in that outer layer expanding the argument before presenting the result for stringification.
ATOMIC_JOIN(prefix, detail_platform) is an macro which will output some string as follows:
base/atomic/gcc_gnu_x64
in another macro ATOMIC_DETAIL_HEADER, which output expected to be:
"base/atomic/gcc_gnu_x64.hpp" // notice: double quotes included in the output
I try to write the ATOMIC_DETAIL_HEADER, such as：
#define ATOMIC_DETAIL_HEADER(prefix) "ATOMIC_JOIN(prefix, ATOMIC_DETAIL_PLATFORM).hpp"
#define ATOMIC_DETAIL_HEADER(prefix) \"ATOMIC_JOIN(prefix, ATOMIC_DETAIL_PLATFORM).hpp\"
#define ATOMIC_DETAIL_HEADER(prefix) "##ATOMIC_JOIN(prefix, ATOMIC_DETAIL_PLATFORM).hpp##"
... failed!
but if i hope output is:
<base/atomic/gcc_gnu_x64.hpp>
The follow macro define can do right thing:
#define ATOMIC_DETAIL_HEADER(prefix) <ATOMIC_JOIN(prefix, ATOMIC_DETAIL_PLATFORM).hpp>
A cpp macro cannot build strings this way. It can join tokens to form new tokens, but at every stage it must be a valid token. Your example with angle-brackets works because the bracket characters are distinct tokens whereas the double-quotes cannot exist floating-off like that, and you cannot apply ## to it.
In most contexts, the compiler will concatenate adjacent string literals, so it may be sufficient to #stringify each piece at let the compiler do that.
While luser droog correctly stated why your use of quotes didn't work, he didn't show exactly how the goal can be accomplished. Indeed the # operator replaces a parameter by a string literal, i. e. puts quotation marks around the argument. This is slightly complicated by the fact that your token sequence has to be expanded first, so an additional level of macro substitution is needed:
#define QUOTED(a) #a
#define QUOTE(a) QUOTED(a)
#define ATOMIC_DETAIL_HEADER(prefix) QUOTE(ATOMIC_JOIN(prefix, ATOMIC_DETAIL_PLATFORM).hpp)
I have a problem with macro expansion deferral. Here is an example:
#include <stdio.h>
#define CONST_ABC 15
#define CONST_5 7
#define ABC 5
#define PRINT(x) printf("CONST=%d\n", CONST_ ## x)
// The problematic macro
#define PRINT2(x) PRINT(x)
int main(int argc, char *argv[])
{
PRINT(ABC); // Prints 15 - OK
PRINT2(ABC); // Prints 7 - Not OK.
}
How to define PRINT2 macro so that it will use PRINT and result would be 15? I'm getting:
CONST=15
CONST=7
And want to get:
CONST=15
CONST=15
It requires you to have at least a C99 compiler, since C99 allows empty macro arguments. However some compilers may allow them as an extension, even in C89 mode. Here is the code:
#include <stdio.h>
#define CONST_ABC 15
#define CONST_5 7
#define ABC 5
#define PRINT(x) printf("CONST=%d\n", CONST_ ## x)
// The problematic macro
#define PRINT2(x, y) PRINT(x ## y)
int main(int argc, char *argv[])
{
PRINT(ABC); // Prints 15 - OK
PRINT2(ABC,); // Prints 7 - Not OK.
}
The second argument (i.e. the y) is empty, making it an empty preprocessing token. The ## operator prevents argument expansion, so the result of the concatenation is the same as x argument.
C11 6.10.3.1/p1 Argument substitution (emphasis mine):
After the arguments for the invocation of a function-like macro have
been identified, argument substitution takes place. A parameter in the
replacement list, unless preceded by a # or ## preprocessing token or
followed by a ## preprocessing token (see below), is replaced by the
corresponding argument after all macros contained therein have been
expanded. Before being substituted, each argument’s preprocessing
tokens are completely macro replaced as if they formed the rest of the
preprocessing file; no other preprocessing tokens are available.
The macro replacement, basically, proceeds as follows:
A token is found, which is a macro name
The arguments of the macro are collected
The arguments are substituted for the formal parameters in the body of the macro definition
Thus substituted parameters are completely macro replaced, not counting the rest the of input; stringification may be performed at this stage as well;
The token paste operators are performed
Thus substituted sequence is re-scanned, together with the rest of the input for further macro replacements
(plus some not well defined rules when a macro is forbidden for replacement)
The only way to prevent a macro argument from being macro-replaced in 4.
is for it to be followed or preceded by a token paste operator (##).
However, in 5. the paste operator has to perform the operation with
the argument under discussion and a special placemarker token. A placemarker token is inserted only for empty argument substitution.
Check this, it might give you an idea for your real code:
#define PRINT2(noreplace,x) PRINT(noreplace ## x)
PS. and yeah, "noreplace" is meant to be empty :)
PRINT2(,ABC)
C seems to be pretty permissive when it comes to whitespace.
We can use or omit whitespace around an operator, between a function name and its parenthesized list of arguments, between an array name and its index, etc. in order to make code more readable. I understand this is a matter of preference.
The only place I can think of where whitespace is NOT allowed is this:
#include < stdio.h > // fatal error: stdio.h : No such file or directory
What are the other contexts in C where whitespace cannot be used for readability?
In most cases, adding whitespace within a single token either makes the program invalid or changes the meaning of the token. An obvious example: "foo" and " foo " are both valid string literals with different values, because a string literal is a single token. Changing 123456 to 123 456 changes it from a single integer constant to two integer constants, resulting in a syntax error.
The exceptions to this involve the preprocessor.
You've already mentioned the #include directive. Note that given:
#include "header.h"
the "header.h" is not syntactically a string literal; it's processed before string literals are meaningful. The syntax is similar, but for example a \t sequence in a header name isn't necessarily replaced by a tab character.
Newlines (which are a form of whitespace) are significant in preprocessor directives; you can't legally write:
#ifdef
FOO
/* ... */
#endif
But whitespace other than newlines is permitted:
# if SPACES_ARE_ALLOWED_HERE
#endif
And there's one case I can think of where whitespace is permitted between preprocessor tokens but it changes the meaning. In the definition of a function-like macro, the ( that introduces the parameter list must immediately follow the macro name. This:
#define TWICE(x) ((x) + (x))
defines TWICE as a function-like macro that takes one argument. But this:
#define NOT_TWICE (x) ((x) + (x))
defines NOT_TWICE as an ordinary macro with no arguments that expands to (x) ((x) + (x)).
This rule applies only to macro definitions; a macro invocation follows the normal rules, so you can write either TWICE(42) or TWICE ( 42 ).
White spaces are not allowed for readability (are significant) within a lexical token. I.e. within an identifier (foo bar is different from foobar), within a number (123 456 is different from 123456), within a string (that's your example basically) or within an operator (+ + is different from ++ and + = is different from +=). Between those you can add as much white space as you want, but when you add white space inside such a token you will break the lexical token into two separate tokens (or change the value in case of string constants), thus changing the meaning of your code .
In most cases the code with the added white space is either equivalent to the original code or results in a syntax error. But there are exceptions. For example:
return a +++ b;
is the same as
return a ++ + b;
but is different from:
return a + ++ b;
As I recall you need to be very careful with function-like macros, as in such dummy example:
#include <stdio.h>
#define sum(x, y) ((x)+(y))
int main(void)
{
printf("%d\n", sum(2, 2));
return 0;
}
the:
#define sum(x, y) ((x)+(y))
is different thing than say:
#define sum (x, y) ((x)+(y))
The latter one is object-like macro, that replaces exactly with (x, y) ((x)+(y)), that is parameters are not being subsituted (as it happens in function-like macro).
I came across one more piece of code that is even more confusing..
#include "stdio.h"
#define f(a,b) a##b
#define g(a) #a
#define h(a) g(a)
int main(void)
{
printf("%s\n",h(f(1,2)));
printf("%s\n",g(1));
printf("%s\n",g(f(1,2)));
return 0;
}
output is
12
1
f(1,2)
My assumption was
1) first f(1,2) is replaced by 12 , because of macro f(a,b)
concantenates its arguments
2) then g(a) macro replaces 1 by a string literal "1"
3) the output should be 1
But why is g(f(1,2)) not getting substituted to 12.
I'm sure i'm missing something here.
Can someone explain me this program ?
Macro replacement occurs from the outside in. (Strictly speaking, the preprocessor is required to behave as though it replaces macros one at a time, starting from the beginning of the file and restarting after each replacement.)
The standard (C99 §6.10.3.2/2) says
If, in the replacement list, a parameter is immediately preceded by a # preprocessing
token, both are replaced by a single character string literal preprocessing token that
contains the spelling of the preprocessing token sequence for the corresponding
argument.
Since # is present in the replacement list for the macro g, the argument f(1,2) is converted to a string immediately, and the result is "f(1,2)".
On the other hand, in h(f(1,2)), since the replacement list doesn't contain #, §6.10.3.1/1 applies,
After the arguments for the invocation of a function-like macro have been identified,
argument substitution takes place. A parameter in the replacement list, unless preceded
by a # or ## preprocessing token or followed by a ## preprocessing token (see below), is
replaced by the corresponding argument after all macros contained therein have been
expanded.
and the argument f(1, 2) is macro expanded to give 12, so the result is g(12) which then becomes "12" when the file is "re-scanned".
Macros can't expand into preprocessing directives. From C99 6.10.3.4/3 "Rescanning and further replacement":
The resulting completely macro-replaced preprocessing token sequence
is not processed as a preprocessing directive even if it resembles
one,
Source: https://stackoverflow.com/a/2429368/2591612
But you can call f(a,b) from g like you did with h. f(a,b) is interpreted as a string literal as #Red Alert states.