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CERT C++ Secure Coding Standard: STR34-CPP. Cast characters to unsigned types before converting to larger integer sizes
ISO/IEC 9899:2011 Section Section 6.2.5, "Types"
ISO/IEC TR 17961 (Draft) Conversion of signed characters to wider integer types before a check for EOF [signconv]
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MITRE CWE: CWE-704, "Incorrect type conversion or cast"
Bibliography
[xorl 2009] "CVE-2009-0887: Linux-PAM Singedness Issue"
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