While it has been common practice to use integers and pointers interchangeably in C, pointer-to-integer and integer-to- pointer conversions are implementation - defined.
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The only value that can be considered interchangeable between pointers and integers is the constant 0. Except in this case, conversions between integers and pointers may have undesired consequences depending on the [implementation|BB. Definitions#implementation]. According to C99 \[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\]: |
An integer may be converted to any pointer type. Except as previously specified, the result is implementation - defined, might not be correctly aligned, might not point to an entity of the referenced type, and might be a trap representation.
Any pointer type may be converted to an integer type. Except as previously specified, the result is implementation - defined. If the result cannot be represented in the integer type, the behavior is undefined. The result need not be in the range of values of any integer type.
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These conversions are machine dependent and should only be coded only when absolutely necessary.
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In this noncompliant code example, the pointer ptr is converted to an integer value. Both a pointer and an int are assumed to be 32 bits. The high-order nine 9 bits of the number are used to hold a flag value, and the result is converted back into a pointer.
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\[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.3.2.3, "Pointers"
\[[ISO/IEC PDTR 24772|AA. C References#ISO/IEC PDTR 24772]\] "HFC Pointer casting and pointer type changes"
\[[MITRE 07|AA. C References#MITRE 07]\] [CWE ID 466|http://cwe.mitre.org/data/definitions/466.html], "Return of Pointer Value Outside of Expected Range," and [CWE ID 587|http://cwe.mitre.org/data/definitions/587.html], "Assignment of a Fixed Address to a Pointer" |
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