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0 pointsSomeone6d ago0 comments

> The standard says that fread calls fgetc multiple times for each object:

>> For each object, size calls are made to the fgetc function and the results stored, in the order read, in an array of unsigned char exactly overlaying the object

Aha! That phrase led me to https://man7.org/linux/man-pages/man3/fread.3p.html. I consulted https://man7.org/linux/man-pages/man3/fread.3.html and https://man.openbsd.org/fread.3. Neither mentions that.

Now, I checked https://cplusplus.com/reference/cstdio/fread/. It doesn’t mention it, either.

⇒ this appears to be POSIX-specific.

Finally, if somebody implements fread as “For each object, size calls are made to the fgetc function”, it doesn’t matter whether you ask for 1 object of size 65,536 or 65,536 objects of size 1; both would call fgetc 65,536 times.

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Joker_vD6d ago· 1 in thread

Shouldn't you be using cppreference.com instead of cplusplus.com? Because the former [0] actually has this language:

    Reads up to "count" objects into the array "buffer" from the given input stream "stream"
    as if by calling fgetc "size" times for each object, and storing the results, in the order
    obtained, into the successive positions of buffer, which is reinterpreted as an array of
    "unsigned char".

This whole fread/fwrite's interface is hailing from the time where some OSes used record-based filesystems and were literally unable to read/write less than a record at a time.

[0] https://en.cppreference.com/c/io/fread

SomeoneOP6d ago

Maybe, but that text says “as if” and even “If an error occurs, the resulting value of the file position indicator for the stream is indeterminate. If a partial element is read, its value is indeterminate.” so it doesn’t in any way require implementations to implement it by actually reading one byte at a time.

That description also fits what I saw in the implementations I inspected, both of which simply try to read size × count bytes.

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