Double Dollar sign in C Program

Question

I found in some Arm reference software, in the file, "arch/arm/armv7-m/src/arch_exceptions.c," the following lines:

#ifdef __ARMCC_VERSION 
extern char Image$$ARM_LIB_STACKHEAP$$ZI$$Limit;
           
#    define arch_exception_stack (&Image$$ARM_LIB_STACKHEAP$$ZI$$Limit)
#else
extern char __stackheap_end__;

#    define arch_exception_stack (&__stackheap_end__)
#endif

I know ## is used for string concatenation but I've never seen this double $$ sign. Not knowing what that is, I can't search for the right term.

How is this $$ used in C?

The compiler probably allows dollar signs in identifiers as an extension to play nicer with the platform's conventions. — Shawn, Aug 11 '22 at 08:27
Most (if not all) modern compilers allow the `$` sign in identifiers. — Jabberwocky, Aug 11 '22 at 08:36
Yeah, it looks so. I found in some other files in an inline assemble `LDR SP, =Image$$FIQ_STACK$$ZI$$Limit`. So it looks like just a variable. — Chan Kim, Aug 11 '22 at 08:36
Does this answer your question? [dollar sign in variable name?](https://stackoverflow.com/questions/7926394/dollar-sign-in-variable-name) — phuclv, Aug 11 '22 at 13:05
@DavidRanieri this has nothing to do with Unicode identifier. Most compilers have supported dollar symbol in identifiers as an extension for decades — phuclv, Aug 11 '22 at 13:05
@phuclv I did not say otherwise and question is tagged `gcc` — David Ranieri, Aug 11 '22 at 13:47

score 0 · Accepted Answer · answered Aug 11 '22 at 09:18

Whether or not those $$ 'field-separators' have any specific meaning to the target assembler and/or linker, I can't say, but the GNU C compiler does allow dollar signs in identifier names.

So, presumably, Image$$ARM_LIB_STACKHEAP$$ZI$$Limit is the name of an extern char variable defined in one of the support libraries for the Arm platform.

Double Dollar sign in C Program

1 Answers1