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The ## preprocessing operator is used to merge two tokens into one while expanding macros. This is called token pasting or token concatenation. When a macro is expanded, the two tokens on either side of each ## operator are combined into a single token, which replaces the ## and the two original tokens in the macro expansion [FSF 2005].
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__LINE__ is a predefined macro names which expands name that expands to an integer constant representing the presumed line number of the current source line within the current source file [ISO/IEC 9899:2011]. If the intention is to expand the __LINE__ macro, which is likely the case here, the following definition for JOIN() is noncompliant :
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#define JOIN(x, y) x ## y
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Because because the __LINE__ is not expanded, and the character array is subsequently named assertion_failed_at_line___LINE__.:
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#define JOIN(x, y) x ## y
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Compliant Compliant Solution
To get the macro to expand, a second level of indirection is required, as shown by this compliant solution:
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JOIN(x, y) calls JOIN_AGAIN(x, y) so that , if x or y is a macro, it is expanded before the ## operator pastes them together.
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The macro invocation xstr(foo) expands to 4. This is because s is stringified when it is used in str(), so it is not macro expanded first. However, s is an ordinary argument to xstr(), so it is completely macro expanded before xstr() is expanded. Consequently, by the time str() gets to its argument, it has already been macro expanded.
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[FSF 2005] Section 3.4, "Stringification," and Section 3.5, "Concatenation"
[Saks 2008]
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