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offsetof
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<p>C's offsetof() macro is an <a href="/facts/ANSI_C/rPKeEuPm">ANSI C</a> library feature found in <a href="/facts/Stddef.h/49S2hMa1">stddef.h</a>. It evaluates to the <a href="/facts/Offset_(computer_science)/Qv0N3nND">offset</a> (in bytes) of a given member within a <a href="/facts/Struct/1fvSgenB">struct</a> or <a href="/facts/Union_(computer_science)/GSV1DsyV">union</a> type, an expression of type <a href="/facts/Size_t/49S2hMa1">size_t</a>. The offsetof() macro takes two <a href="/facts/Parameters/zFvVFMv9">parameters</a>, the first being a structure or union name, and the second being the name of a subobject of the structure/union that is not a <a href="/facts/Bit_field/fWgTjqvX">bit field</a>. It cannot be described as a C prototype. </p>