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Null-terminated byte strings are implemented as arrays of characters and are susceptible to the same problems as arrays. As a result, rules and recommendations for arrays should also be applied to null-terminated byte strings.

The C standard uses the general philosophy outlined below for choosing character types, though it is not explicitly stated in one place.

signed char and unsigned char:

• Can be used for small integer values.

"plain" char:

• The type of a character constant.
• The type of each element of a string literal.
• Used for character data (where signedness has little meaning) as opposed to integer data.

int:

• Used for data that could be either EOF (a negative value) or character data interpreted as unsigned char and then converted to int.
• Therefore, returned by fgetc(), getc(), getchar(), and ungetc().
• Therefore, accepted by the character handling functions (from <ctype.h>), because they might be passed the result of fgetc() et al.

unsigned char:

• Used internally for string comparison functions, even though these operate on character data.  Therefore, the result of a string comparison does not depend on whether plain char is signed or not.
• Used for situations where the object being manipulated might be of any type, and it is necessary to access all bits of that object, as with fwrite().

Recommendations

STR00-A. Use TR 24731 for remediation of existing string manipulation code

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