Is there a getline function that uses fread (I / O block) instead of fgetc (character input / output)?
There is a performance penalty when reading a file symbol by symbol through fgetc . We believe that to improve performance, we can use block reading via fread in the inner getline loop. However, this leads to a potentially undesirable effect of reading beyond the end of the line. At the very least, implementing a getline to track the "unread" part of a file will require an abstraction other than the ANSI C FILE semantics. This is not what we want to realize ourselves!
We have profiled our application, and slow performance is isolated by the fact that we consume large files with a symbol through fgetc . The rest of the overhead actually has trivial costs compared. We always sequentially read every line of the file, from beginning to end, and we can lock the entire file for the duration of reading. This probably simplifies the implementation of fread getline .
So, is there a getline function that uses fread (block I / O) instead of fgetc (character input / output)? We are sure that this is so, but if not, how can we implement it?
Update Find a helpful article, Handling User Input in C Paul Ssieh. This is an approach based on fgetc , but it has an interesting discussion of alternatives (starting with how bad gets , and then discussing fgets ):
On the other hand, a common retort from C programmers (even those considered experienced) is that fgets () should be used as an alternative. Of course, fgets () itself does not really handle user input as such. Besides the presence of a strange condition for line termination (when \ n or EOF, but not \ 0), the mechanism chosen to complete when the buffer has reached power should just abruptly stop the fgets () operation and \ 0 stop it. Therefore, if user input exceeds the length of the previously allocated buffer, fgets () returns a partial result. To solve this problem, programmers have several options; 1) just handle truncated user input (there is no way to return to the user that the input has been truncated while they provide input). 2) Simulate a growing array of characters and fill it with consecutive calls to fgets () . The first solution is almost always a very poor solution for user input of variable length, because the buffer will inevitably be too large in most cases, because it tries to capture too many ordinary cases and is too small for unusual cases. The second solution is fine, except that it can be difficult to implement correctly. Not related to fgets' odd with respect to '\ 0'.
Reader exercise: to determine how many bytes were actually read by calling fgets () , you can try scanning like 'it for' \ n 'and skip any' \ 0 'without exceeding the size passed to fgets () . Explain why this is not enough for the very last line of the stream. What weakness of ftell () does not completely solve this problem?
Reader Exercise: Solve a problem that determines the length of the data consumed by fgets () by overwriting the entire buffer with a nonzero value between each call to fgets () .
Thus, with fgets (), it remains for us to choose a lot of writing code and live with the condition of line termination, which is incompatible with the rest of the C library or has an arbitrary -off cut. If this is not enough, then what are we left with? scanf () mixes parsing with reading in a way that cannot be split, and fread () will read past the end of the line. In short, the C library leaves us with nothing. We are forced to collapse our own based on fgetc () . So let's take a picture.
So, is there a getline function based on fgets (and not clipping input)?