Re: Buffer contents well-defined after fgets() reaches EOF ?

On Sat 2/8/2025 9:59 PM, Janis Papanagnou wrote:

To get the last line of a text file I'm using
      char buf[BUFSIZ];
     while (fgets (buf, BUFSIZ, fd) != NULL)
         ;   // read to last line
 If the end of the file is reached my test shows that the previous
contents of 'buf' are still existing (not cleared or overwritten).
 But the man page does not say anything whether this is guaranteed;
it says: "Reading stops after an EOF or a newline.", but it says
nothing about [not] writing to or [not] resetting the buffer.
 Is that simple construct safe to get the last line of a text file?
 

What situation exactly are you talking about? When end-of-file is encountered _immediately_, before reading the very first character? Of when end-of-file is encountered after reading something (i.e. when the last line in the file does not end with new-line character)?
The former situation is covered by the spec: "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".
The second situation does not need additional clarifications. Per general spec as many characters as available before the end-of-file will be read and then terminated with '\0'. In such case there will be no new-line character in the buffer.
So, in both cases we are perfectly safe when reading the last line of a text file, if you don't forget to check the return value of `fgets`.
(This is all under assumption that size limit does not kick in. I believe your question is not about that.)
Note also that `fgets` is not permitted to assume that the limit value (the second parameter) correctly describes the accessible size of the buffer. E.g. for this reason it is not permitted to zero-out the buffer before reading. For example, this code is valid and has defined behavior
   char buffer[10];
   fgets(buffer, 1000, f);
provided the current line of the file fits into `char[10]`. I.e. even though we "lied" to `fgets` about the limit, it is still required to work correctly if the actual data fits into the actual buffer.
So, why do you care that "the previous contents of 'buf' are still existing"?
--
Best regards,
Andrey