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/GCC 7

6.40 Dollar Signs in Identifier Names

In GNU C, you may normally use dollar signs in identifier names. This is because many traditional C implementations allow such identifiers. However, dollar signs in identifiers are not supported on a few target machines, typically because the target assembler does not allow them.

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https://gcc.gnu.org/onlinedocs/gcc-7.1.0/gcc/Dollar-Signs.html