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I have a code which uses bit-fields declared as follows
typedef struct my{
const char *name;
uint8_t is_alpha : 1;
uint8_t is_hwaccel : 1;
uint8_t x_chroma_shift;
uint8_t y_chroma_shift;
} mystr;
uint8_t is typedef'ed to unsigned char.
Building the code in MS-VS 2008 using this bit fields gives a warning as below:
imgconvert.c(60) : warning C4214: nonstandard extension used : bit-field types other than int.
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In programming terminology, a bit field is a data structure that allows the programmer to allocate memory to structures and unions in bits in order to utilize computer memory in an efficient manner. Since structures and unions are user-defined data types in C, the user has an idea of how much memory will they occupy.
Both C and C++ allow integer members to be stored into memory spaces smaller than the compiler would ordinarily allow. These space-saving structure members are called bit fields, and their width in bits can be explicitly declared.
Bit fields can be used to reduce memory consumption when a program requires a number of integer variables which always will have low values. For example, in many systems storing an integer value requires two bytes (16-bits) of memory; sometimes the values to be stored actually need only one or two bits.
In C, we can specify size (in bits) of structure and union members. The idea is to use memory efficiently when we know that the value of a field or group of fields will never exceed a limit or is within a small range. For example, consider the following declaration of date without the use of bit fields.
1] Is there any problems/potential issues in using bit fields of type other than int? Why the warning?
Since bit-fields are low-level, there may be issues with portability if you are using non-standard types. Hence the warning -- note it is still a warning and not an error.
2] Are other than int type bit-fileds they allowed by C99 C language specification?
From the draft of C99:
6.7.2.1 Structure and union specifiers
4 A bit-field shall have a type that is a qualified or unqualified version of _Bool, signed int, unsigned int, or some other implementation-defined type.
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