从文件读取数据到 C 中的结构

Question

Lets say i used the fread function to read data from a file to a struct. How exactly is data read to the struct? Lets say my struct has the following:

Int x;
Char c;

Will the first 4 bytes read go into x and the next byte go into c? And if i read in more bytes than the elements in my struct can hold what's gonna happen?

Answer 1

Will the first 4 bytes read go into x and the next byte go into c?

Yes, unless your compiler has extremely strange padding rules (eg every member must be 8 byte aligned). And assuming Int is 4 bytes and Char is 1 byte.

And if i read in more bytes than the elements in my struct can hold what's gonna happen?

That's undefined behavior, unless perhaps the over-long write is not more than sizeof(YourStruct) in which case you'll only be writing to the padding bytes (which on a lot of platforms will be 3 bytes after the char).

Answer 2

fread reads data byte-for-byte from a file (stream) into memory. Therefore, if what you're trying to read is a struct, the byte layout of the struct in the file must exactly match the layout your compiler has chosen for the struct in memory.

So the question of "How does fread read from a file?" really boils down to, "How does the compiler lay out structs in memory?"

And the answer to that question is, it's partly determined by the rules of the C language, and it's partly up to the compiler.

So if you want to read structures from a file, you have three choices:

1. Learn everything you can about the C rules for laying out structures in memory, and the choices compilers can make in interpreting these rules. Keep all these rules in mind as you design your structures and your data file formats. (This is not an impossible task. Many programmers take this approach to file i/o all the time.)

2. Don't worry about the layout too much. Define your structures, and write them out to files using fwrite . Then the files are automatically readable using fread -- at least, as long as the program doing the reading is running on the same kind of machine, and was compiled by the same compiler using the same settings. (This, too, is a popular strategy, and works much of the time.)

3. Don't use fread to read structures form a file. (And although it sounds defeatist, this is my own preferred argument.)

There's much, much more that could be said abut this question. If you choose approach 1, as I've already said, you're going to have to learn everything you can about the C rules for laying out structures in memory, and the choices compilers can make in interpreting these rules. If you choose approach 3, you have to learn some decent techniques for doing so without using fwrite and fread . But I'm not going to launch into long explanations of either of those topics here. I'm sure someone else will post some links, or you could start with Chapter 17 of these C programming notes .