It's a pretty simple question. I have had no luck browsing here, or through Google, in finding anything more comprehensive than forum posts on the subject. This is for the builtin SPIFFS functionality:
#include "FS.h"
#include "SPIFFS.h"
Thanks!
Google "esp32 spiffs"
2nd link from the top:
https://docs.espressif.com/projects/esp-idf/en/latest/esp32/api-reference/storage/spiffs.html
Note SPIFFS is deprecated in favour of LittleFS so you may be better off using that instead
The only information on SPIFFS for ESp32 you would find on the Arduino site(s) in likely to be in forum posts.
The ESP32 support for the Arduino IDE is produced and supported by Espressif.
As usual, I could have been more specific. Sorry!
I had found the esp-idf page, but it seemed to refer only to higher-level functions that I am unlikely to have any use for:
esp_err_t esp_spiffs_check(const char* partition_label);
I really don't know or care what that function might do.
What I'm looking for are the arguments for functions like "read" and "write", and a list of all such methods that are available. My "test case" has been "readbytesuntil". I know from the forum postings that this function exists, but I have not been able to find it documented anywhere.
Most particularly, the only example I've found for "read" is in the usage as "read();" meaning, read all currently available bytes, presumably. I'm looking to read a fairly large file, through a limited buffer, and reading all is not an option.
But, rather than simply asking that question, and undoubtedly others to follow, I was hoping to find the list of all available methods. Does such a document exists?
I assume that you have looked at the SPIFFS examples in the IDE
You also have the option to examine the SPIFFS library files to see the functions available, the parameters they take and what they return, if anything
Which ESP board are you using ?
Thanks to all! I can't say that I'm thrilled with the answer, but after all the input, scanning the source code seems to be the best solution.
I find that File has the functions that I'm looking for:
int File::read()
size_t File::read(uint8_t* buf, size_t size)
int File::peek()
etc.
though not the elusive readBytesUntil, which only appears in Stream:
size_t Stream::readBytesUntil(char terminator, char *buffer, size_t length)
If file supports readBytesUntil, it's a mystery to me how. Does it inherit from Stream?
In any case this is the kind of documentation that I was looking for, and I appreciate the help in finding it.
With such an awesomely active community, I'm sure that the subject of better Arduino documentation must come up regularly. Is it a project that is actively worked on today? If so, where do I sign up to help?
Thanks again!
Only when Ops cannot find what they want (to find.)
Authors are often not forum members.
Forum users more concerned with code usage examples.
Glad you got what you were looking for. And yes, looking inside a library is enlightening. 
Maybe ask over in the Espressif forums ........
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