According to MISRA 2008, concatenation of wide and narrow string literals leads to undefined behavior. This was once considered implicitly undefined behavior until C90 [ISO/IEC 9899:1990]. However, C99 defined this behavior [ISO/IEC 9899:1999], and C11 further explains in section Section 6.4.5, paragraph 5 [ISO/IEC 9899:2011]:
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If the concatenated string needs to be a wide string literal, each element in the concatenation must be a wide string literal, as in this compliant solution.:
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wchar_t *msg = L"This message is very long, so I want to divide it "
L"into two parts.";
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If wide string literals are unnecessary, it is better to use narrow string literals, as in this compliant solution.:
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char *msg = "This message is very long, so I want to divide it "
"into two parts.";
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