Errors can occur when incorrect assumptions are made about the type of data being read. These assumptions may be violated, for example, when binary data has been read from a file instead of text from a user's terminal or the output of a process is piped to stdin. (See FIO14-C. Understand the difference between text mode and binary mode with file streams.) On some systems, it may also be possible to input a null byte (as well as other binary codes) from the keyboard.
Subclause 7.2123.7.2 of the of the C Standard paragraph 3 [ISO/IEC 9899:20112024] says,
The fgets function returns s if successful. If end-of-file is encountered and no characters have been read into the array, the contents of the array remain unchanged and a null pointer is returned. If a read error occurs during the operation, the members of the array have unspecified values and a null pointer is returned.
The wide-character function fgetws() has the same behavior. Therefore, if fgets() or fgetws() returns a non-null pointer, it is safe to assume that the array contains data. However, it is erroneous to assume that the array contains a nonempty string because the data may contain null characters.
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Bibliography
| [ISO/IEC 9899:20112024] | Subclause 7.2123.7.2, "The |
| [Lai 2006] | |
| [Seacord 2013] | Chapter 2, "Strings" |
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