signed integers used as bit masks #142
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A work around for C++ client to not get clang-tidy warning is and use the alias in the remainder of your code. |
bvanassche
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Jun 29, 2020
The C language requires that the operands of a bitwise or expression are promoted before the bitwise OR happens. From the C standard: "The following may be used in an expression wherever an int or unsigned int may be used: - An object or expression with an integer type (other than int or unsigned int) whose integer conversion rank is less than or equal to the rank of int and unsigned int. - A bit-field of type _Bool , int, signed int, or unsigned int. If an int can represent all values of the original type (as restricted by the width, for a bit-field), the value is converted to an int; otherwise, it is converted to an unsigned int. These are called the integer promotions. All other types are unchanged by the integer promotions." In other words, the result of a bitwise OR of two unsigned char operands has type 'int' instead of unsigned int. Hence change the type of constants in header files from unsigned char into unsigned int. See also net-snmp#142.
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Does commit 31a2c80 fix the hicpp-signed-bitwise warnings? |
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Found with clang-tidy hicpp-signed-bitwise with use of SNMP_MSG_GET
Explored with
https://godbolt.org/z/Ank87j
#define u_char unsigned char
#define ASN_CONTEXT ((u_char)0x80)
#define ASN_CONSTRUCTOR ((u_char)0x20)
The expression
ASN_CONTEXT | ASN_CONSTRUCTOR
becomes a signed integer so even if you change
(ASN_CONTEXT | ASN_CONSTRUCTOR | 0x3)
to
(ASN_CONTEXT | ASN_CONSTRUCTOR | 0x03U)
you sill still get the same warning.
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