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- Integer operands of any pointer arithmetic, including array indexing;
- The assignment expression for the declaration of a variable length array;
- The postfix expression preceding square brackets
[]or the expression in square brackets[]of a subscripted designation of an element of an array object; and - Function arguments of type
size_torrsize_t(for example, an argument to a memory allocation function).
This rule also applies to arguments passed to the following library functions that are converted to unsigned char:
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The only integer type conversions that are guaranteed to be safe for all data values and all possible conforming implementations are conversions of an integral value to a wider type of the same signedness. The C Standard, subclause 6.3.1.3 [ISO/IEC 9899:2011], says,
When a value with integer type is converted to another integer type other than
_Bool, if the value can be represented by the new type, it is unchanged.Otherwise, if the new type is unsigned, the value is converted by repeatedly adding or subtracting one more than the maximum value that can be represented in the new type until the value is in the range of the new type.
Otherwise, the new type is signed and the value cannot be represented in it; either the result is implementation-defined or an implementation-defined signal is raised.
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The time() function returns the value (time_t)(-1) to indicate that the calendar time is not available. The C Standard requires only that the time_t type is only a real type capable of representing time. (The integer and real floating types are collectively called real types.) It is left to the implementor to decide the best real type to use to represent time. If time_t is implemented as an unsigned integer type with less precision than a signed int, the return value of time() will never compare equal to the integer literal -1.
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INT31-C-EX1: The C Standard defines minimum ranges for standard integer types. For example, the minimum range for an object of type unsigned short int is 0 to 65,535, whereas the minimum range for int is −32,767 to +32,767. Consequently, it is not always possible to represent all possible values of an unsigned short int as an int. However, on the IA-32 architecture, for example, the actual integer range is from −2,147,483,648 to +2,147,483,647, meaning that it is quite possible to represent all the values of an unsigned short int as an int for this architecture. As a result, it is not necessary to provide a test for this conversion on IA-32. It is not possible to make assumptions about conversions without knowing the precision of the underlying types. If these tests are not provided, assumptions concerning precision must be clearly documented, as the resulting code cannot be safely ported to a system where these assumptions are invalid. A good way to document these assumptions is to use static assertions. (see See DCL03-C. Use a static assertion to test the value of a constant expression.).
INT31-C-EX2: Conversion from any integer type with a value between SCHAR_MIN and UCHAR_MAX to a character type is permitted provided the value represents a character and not an integer.
Conversions to unsigned character types are well - defined by C to have modular behavior. A character's value is not misinterpreted by the loss of sign or conversion to a negative number. For example, the Euro symbol € is sometimes represented by bit pattern 0x80 which can have the numerical value 128 or −127 depending on the signedness of the type.
Conversions to signed character types are more problematic. The C Standard, subclause 6.3.1.3, paragraph 3 [ISO/IEC 9899:2011], says, regarding conversions:
Otherwise, the new type is signed and the value cannot be represented in it; either the result is implementation-defined or an implementation-defined signal is raised.
Furthermore, subclause 6.2.6.2, paragraph 2, says, regarding integer modifications:
If the sign bit is one, the value shall be modified in one of the following ways:
— the corresponding value with sign bit 0 is negated (sign and magnitude)
— the sign bit has the value −(2M ) (two’s complement);
— the sign bit has the value −(2M − 1) (ones’ complement).
Which of these applies is implementation-defined, as is whether the value with sign bit 1 and all value bits zero (for the first two), or with sign bit and all value bits 1 (for ones’ complement), is a trap representation or a normal value. [See note.]
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However, platforms where this code traps or produces an unexpected value are rare. According to The New C Standard: An Economic and Cultural Commentary by Derek Jones [Jones 2008]:,
Implementations with such trap representations are thought to have existed in the past. Your author was unable to locate any documents describing such processors.
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