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    <p>As Chris Lutz specified, you can define the amount of bits that a variable uses by adding a colon and the size of it: <code>unsigned char myOneBitVariable:1;</code> and for your case 'unsigned char MyFourBitVariable:4'. I'd like to point out how <b>extremely difficult</b> this is and <b>why you should avoid it</b>.</p> <p>Most modern compilers will align the space for your variables in your struct. The most general case today is 4 bytes or even 8 bytes but that varies from platform to platform and compiler to compiler. Some compilers allow you to specify the alignment of the data and its members. On GCC the keyword is <code>__attribute__((aligned(x)))</code> and on MSVC it's <code>__declspec(align(x))</code>. In most cases you will also need to specify how much the compiler should pack the structures. MSVC has the <code>#pragma pack(x)</code>directive: <a href="http://msdn.microsoft.com/en-us/library/2e70t5y1(VS.80).aspx" rel="nofollow noreferrer">http://msdn.microsoft.com/en-us/library/2e70t5y1(VS.80).aspx</a>. You can also read about MSVC alignment here: <a href="http://msdn.microsoft.com/en-us/library/83ythb65(VS.80).aspx" rel="nofollow noreferrer">http://msdn.microsoft.com/en-us/library/83ythb65(VS.80).aspx</a>. GCC has its own implementation called <code>__attribute__ ((__packed__)</code>, which you might have to search around for. An example that doesn't give you what you want, using Microsoft's compiler:</p> <pre><code> #ifndef _MSC_VER #error This alignment solution / packing solution is only valid on MSC #endif /* ifndef _MSC_VER */ #define M_ALIGN(x) __declspec(align(x)) struct S64Bits { unsigned char MyOneBitVariable:1; int My32BitInt; }; // MSVC specific implementation of data-packing in a type. #pragma pack(1) struct S32Bits { D_ALIGN(1) int My16BitVariable:16; D_ALIGN(1) unsigned char Padding8Bits; D_ALIGN(1) unsigned char MyOneBitVariable1:1; D_ALIGN(1) unsigned char MyOneBitVariable2:1; D_ALIGN(1) unsigned char MyOneBitVariable3:1; D_ALIGN(1) unsigned char MyOneBitVariable4:1; D_ALIGN(1) unsigned char MyFourBitVariable:4; }; #pragma pack(pop) </code></pre> <p>'sizeof(S64Bits)' should be 8, which it is. 'sizeof(S32Bits)' should be 4, <b>it isn't</b>. On msvc the latter is 6 bytes. The behavior is also compiler-specific and often have compiler-unique directives. This kind of behavior almost never gives you what you want. I often use a macro to make sure that structures that I require to be of a certain size really are:</p> <pre><code> #define TEST_TYPE_SIZE(Type, Size) assert(sizeof(Type) == Size); </code></pre> <p>That I will use below all my data-types where I try to specify the exact size of them. However, relying on a structure being any size other than sizeof(mystructure) is coding that will likely lead to difficult to debug errors. Alignment-compiler directives are best suited to be used to align data to cache-line size and similar efficiency issues. </p> <p>Karl Bielefeldt provides a good natural solution of storing 4-bit values into an uint8 using bit shift operations, use those instead.</p>
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    1. POHow do i represent bit info in C?
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    1. CO-1 for nonportable hacks, and another -1 for use of bitfields if I could. Simply do the bit arithmetic yourself. It's easier, portable, results in well-defined behavior, and not error-prone.
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    2. CONonportable hacks? I clearly state about compiler-specifics. When it comes to packing data in manners it is very important to know how each compiler packs and aligns data. The question is how to use 4 bits only since space is a constraint and I try to provide information regarding the complexity of getting exactly the size of memory you want. Could you please elaborate your critisism a bit more?
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    3. CO@R..: Perhaps you didn't read my comments regarding the difficulty on achieving correct sizes using these methods? Or maybe you didn't read my text regarding relying on structure-sizes? I thought that they pretty well explained why you shouldn't use these kind of solutions. My argumentation wasn't PRO using these solutions, they are against it.
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