为什么 fgetc() 返回 int 而不是 char？

Question

I would like to copy binary file source to file target. Nothing more! The code is inspired from many examples found on the Internet.

#include <stdio.h>

int main(int argc, char **argv) {

    FILE *fp1, *fp2;
    char ch;

    fp1 = fopen("source.pdf", "r");
    fp2 = fopen("target.pdf", "w");

    while((ch = fgetc(fp1)) != EOF)
        fputc(ch, fp2);

    fclose(fp1);
    fclose(fp2);

    return 0;

}

The result differs in file size.

root@vm:/home/coder/test# ls -l
-rwxr-x--- 1 root root 14593 Feb 28 10:24 source.pdf
-rw-r--r-- 1 root root   159 Mar  1 20:19 target.pdf

Ok, so what's the problem?

I know that char is unsigned and get signed when above 80. See here .

This is confirmed when I use printf("%x\\n", ch); which returns approximately 50% of the time something like sometimes FFFFFFE1 .

The solution to the my issue would be to use int iso char .

Examples found with char : example 1 , example 2 example 3 , example 4 , ...

Examples found with int : example a , ...

I don't use fancy compiler options.

Why are virtually all code examples found returning fgetc() to an char iso an int , which would be more correct?

What am I missing?

Answer 1

ISO C mandates that fgetc() returns an int since it must be able to return every possible character in addition to an end-of-file indicator.

So code that places the return value into a char , and uses it to detect EOF, is generally plain wrong and should not be used.

---

Having said that, two of the examples you gave don't actually do that.

One of them uses fseek and ftell to get the number of bytes in the file and then uses that to control the read/write loop. That's could be problematic since the file can actually change in size after the size is retrieved but that's a different problem to trying to force an int into a char .

The other uses feof immediately after the character is read to check if the end of file has been reached.

---

But you're correct in that the easiest way to do it is to simply use the return value correctly, something like:

int charInt;
while ((charInt = fgetc(inputHandle)) != EOF)
    doSomethingWith(charInt);

Answer 2

Well the thing is most of code you saw then is wrong. There are 3 types of char - signed , unsigned and plain char. Now if plain char is by default signed then a character with decimal value 255 will be considered equal to -1 (EOF). This is not what you want. (Yes decimal value 255 won't be representable in signed char but it's implementation defined behavior and on most ones it will store the bit pattern 0xFF in the char ).

Secondly if char is unsigned then it EOF will be considered as 0xFF that is also wrong now and comparison would fail. (Knowing that EOF is -1 it will be converted to CHAR_MAX which is 255 or 0xFF ).

That's why int is considered so that it can hold the value of EOF correctly and that is how you should use it.