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I would like to generate a random number or string using the C Preprocessor ... um ... I don't even know if this is possible, but I am trying to create variables on the fly (strings would be helpful here) and assign them values (integers). So there are a few things I am trying to do but the basic question remains - can I create a random string or number using the preprocessor.

Brian Tompsett - 汤莱恩
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Xofo
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2 Answers2

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Based on 1999-01-15 Jeff Stout (thanks to @rlb.usa)

#define UL unsigned long
#define znew  ((z=36969*(z&65535)+(z>>16))<<16)
#define wnew  ((w=18000*(w&65535)+(w>>16))&65535)
#define MWC   (znew+wnew)
#define SHR3  (jsr=(jsr=(jsr=jsr^(jsr<<17))^(jsr>>13))^(jsr<<5))
#define CONG  (jcong=69069*jcong+1234567)
#define KISS  ((MWC^CONG)+SHR3)
/*  Global static variables: 
    (the seed changes on every minute) */
static UL z=362436069*(int)__TIMESTAMP__, w=521288629*(int)__TIMESTAMP__, \
   jsr=123456789*(int)__TIMESTAMP__, jcong=380116160*(int)__TIMESTAMP__;


int main(int argc, _TCHAR* argv[]){
    cout<<KISS<<endl;
    cout<<KISS<<endl;
    cout<<KISS<<endl;
}

Output:

247524236
3009541994
1129205949
nergeia
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    Very good implementation for an embedded system. Both answers are good. – Xofo May 27 '15 at 17:45
  • What does it mean: "the seed changes on every minute"? What event changes it? –  Oct 18 '18 at 15:14
  • @user10133158 `__TIMESTAMP__` is a compiler variable that has a 1-minute precision. So, every different minute you compile, the seed changes – nergeia Jan 29 '19 at 09:05
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    This is misleading because it does not use the preprocessor to compute a random number, as OP requested. Instead it inserts a block of C code that will be compiled into code that computes a number. – ScottJ Aug 07 '21 at 20:10
  • @ScottJ No, the code is at preprocessor stage – nergeia Aug 12 '21 at 13:36
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    @nergeia Nonsense. Run it with `gcc -E` (preproccesor only) to see the result: cout<<(((((z=36969*(z&65535)+(z>>16))<<16)+((w=18000*(w&65535)+(w>>16))&65535))^(jcong=69069*jcong+1234567))+(jsr=(jsr=(jsr=jsr^(jsr<<17))^(jsr>>13))^(jsr<<5)))< – ScottJ Aug 13 '21 at 21:47
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I take your question that you want to have a way of creating unique identifier tokens through the preprocessor.

gcc has an extension that is called __COUNTER__ and does what you expect from its name. You can combine this with macro concatenation ## to obtain unique identifiers.

If you have a C99 compiler you can use P99. It has macros called P99_LINEID and P99_FILEID. They can be used as 

#include "p99_id.h"

P99_LINEID(some, other, tokens, to, make, it, unique, on, the, line)

and similarily for P99_FILEID.

The first mangles a name from your tokens and the line number and a hash that depends on the number of times the file "p99_id.h" had been included. The second macro just uses that hash and not the line number such that a name is reproducible at several places inside the same compilation unit.

These two macros also have counterparts P99_LINENO and P99_FILENO that just produce large numbers instead of identifier tokens.

Jens Gustedt
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    Isn't that extension `__COUNTER__`? https://gcc.gnu.org/onlinedocs/cpp/Common-Predefined-Macros.html – Alyssa Haroldsen Aug 06 '15 at 04:28
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    Wanted to add that `__COUNTER__` is also available for other compilers like [MSVC](https://msdn.microsoft.com/en-us/library/b0084kay.aspx). Just be aware that `__COUTER__` is only unique for the file the pre-processor is currently parsing (it starts with `0` for each source file parsed). – Florian Feb 28 '17 at 13:50