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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 this format:
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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;
unsigned char *ptr;
data.m1 = 0;
data.m2 = 0;
data.m3 = 0;
data.m4 = 0;
char* ptr = (unsigned char *) &data;
(*ptr)++; /* could increment data.m1 or data.m4 */
}
|
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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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struct bf {
unsigned m1 : 6;
unsigned m2 : 4;
};
void function() {
unsigned char *ptr;
struct bf data;
data.m1 = 0;
data.m2 = 0;
char* ptr = (unsigned char *) &data;
ptr++;
*ptr += 1; /* what does this increment? */
}
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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.
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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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| Wiki Markup |
|---|
\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.7.2, "Type specifiers"
\[[ISO/IEC PDTR 24772|AA. C References#ISO/IEC PDTR 24772]\] "BQF Unspecified Behaviour" and "STR Bit Representations"
\[[MISRA 04|AA. C References#MISRA 04]\] Rule 3.5
\[[Plum 85|AA. C References#Plum 85]\] Rule 6-5 |
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