What mistakes are in the following paragraph?
Read the string at once instead of reading character by character using the pre-existing functions in C.
That’s a sentence, not a paragraph.
Without further context it looks as if it might need to be:
Read the string all at once rather than reading it character by character using…
‘using the pre-existing functions in C’ relates to the ‘character by character’ reading as it is written. I’m not sure whether that’s what you intended.