用C读取任意长度的文件

Question

What's the most idiomatic/efficient way to read a file of arbitrary length in C?

1. Get the filesize of the file in bytes and issue a single fread()
2. Keep fread() ing a constant size buffer until getting EOF
3. Anything else?

Answer 1

Avoid using any technique which requires knowing the size of the file in advance. That leaves exactly one technique: read the file a bit at a time, in blocks of a convenient size.

Here's why you don't want to try to find the filesize in advance:

1. If it is not a regular file, there may not be any way to tell. For example, you might be reading directly from a console, or taking piped input from a previous data generator. If your program requires the filesize to be knowable, these useful input mechanisms will not be available to your users, who will complain or choose a different tool.

2. Even if you can figure out the filesize, you have no way of preventing it from changing while you are reading the file. If you are not careful about how you read the file, you might open a vulnerability which could be exploited by adversarial programs.

   For example, if you allocate a buffer of the "correct" size and then read until you get an end-of-file condition, you may end up overwriting random memory. (Multiple reads may be necessary if you use an interface like read() which might read less data than requested.) Or you might find that the file has been truncated; if you don't check the amount of data read, you might end up processing uninitialised memory leading to information leakage.

Answer 2

In practice, you usually don't need to keep the entire file content in memory. You'll often parse the file (notably if it is textual), or at least read the file in smaller pieces, and for that you don't need it entirely in memory. For a textual file, reading it line-by-line (perhaps with some state inside your parser) is often enough (using fgets or getline ).

Files exist (notably on disks or SSD s) because usually they can be much "bigger" than your computer RAM. Actually, files have been invented (more than 50 years ago) to be able to deal with data larger than memory. Distributed file systems also can be very big (and accessed remotely even from a laptop, eg by NFS , CIFS , etc...)

Some file systems are capable of storing petabytes of data (on supercomputers), with individual files of many terabytes (much larger than available RAM).

You'll also likely to use some database s. These routinely have terabytes of data. See also this answer (about realistic size of sqlite databases).

If you really want to read a file entirely in memory using stdio (but you should avoid doing that, because you generally want your program to be able to handle a lot of data on files; so reading the entire file in memory is generally a design error), you indeed could loop on fread (or fscanf , or even fgetc ) till end-of-file. Notice that feof is useful only after some input operation.

On current laptop or desktop computers, you could prefer (for efficiency) to use buffers of a few megabytes, and you certainly can deal with big files of several hundreds of gigabytes (much larger than your RAM).

On POSIX file systems, you might do memory mapped IO with eg mmap(2) - but that might not be faster than read(2) with large buffers (of a few megabytes). You could use readahead(2) (Linux specific) and posix_fadvise(2) (or madvise(2) if using mmap ) to tune performance by hinting your OS kernel .

If you have to code for Microsoft Windows, you could study its WinAPI and find some way to do memory mapped IO.

In practice, file data (notably if it was accessed recently) often stays in the page cache , which is of paramount importance for performance. When that is not the case, your hardware (disk, controller, ...) becomes the bottleneck and your program becomes I/O bound (in that case, no software trick could improve significantly the performance).