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Consequently, it is impossible to write portable code that makes assumptions about the layout of bit-field structures.
Non-Compliant Code Example (
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Alignment)
Bit-fields can be used to allow flags or other integer values with small ranges to be packed together to save storage space. Bit-fields can improve the storage efficiency of structures. Compilers typically allocate consecutive bit-field structure members into the same int-sized storage, as long as they fit completely into that storage unit. However, the order of allocation within a storage unit is implementation-defined. Some implementations are "right-to-left": the first member occupies the low-order position of the storage unit. Others are "left-to-right": the first member occupies the high-order position of the storage unit. Calculations that depend on the order of bits within a storage unit may produce different results on different implementations.
Consider the following structure made up of four 8-bit bit-field members.
| Code Block |
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struct bf {
unsigned m1 : 8;
unsigned m2 : 8;
unsigned m3 : 8;
unsigned m4 : 8;
}; /* 32 bits total */
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Right-to-left implementations will allocate struct bf as one storage unit with the this format:
| Code Block |
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m4 m3 m2 m1 |
Conversely, left-to-right implementations will allocate struct bf as one storage unit with the this format:
| Code Block |
|---|
m1 m2 m3 m4 |
...
| Code Block | ||
|---|---|---|
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struct bf {
unsigned m1 : 8;
unsigned m2 : 8;
unsigned m3 : 8;
unsigned m4 : 8;
}; /* 32 bits total */
void function() {
struct bf data;
data.m1 = 0;
data.m2 = 0;
data.m3 = 0;
data.m4 = 0;
char* ptr = (char*) &data;
(*ptr)++; /* could increment data.m1 or data.m4 */
}
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Compliant Solution (
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Alignment)
This code is explicit about the fields it modifies.
| Code Block | ||
|---|---|---|
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struct bf {
unsigned m1 : 8;
unsigned m2 : 8;
unsigned m3 : 8;
unsigned m4 : 8;
}; /* 32 bits total */
void function() {
struct bf data;
data.m1 = 0;
data.m2 = 0;
data.m3 = 0;
data.m4 = 0;
data.m1++;
}
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Non-Compliant Code Example (
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Overlap)
In this non-compliant example, assuming eight bits to a byte, if bit-fields of six and four bits are declared, is each bit-field contained within a byte or are they be the bit-fields split across multiple bytes?
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In the above example, if each bit-field lives within its own byte, then m2 (or m1, depending on alignment) is incremented by 1. If the bit-fields are indeed packed across 8-bit bytes, then m2 might be incremented by 4.
Compliant Solution (
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Overlap)
| Code Block | ||
|---|---|---|
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struct bf {
unsigned m1 : 6;
unsigned m2 : 4;
};
void function() {
struct bf data;
data.m1 = 0;
data.m2 = 0;
data.m2 += 1;
}
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