C - why does fread() have two possible behaviours if EOF is reached? - c

Question

I can't understand why function fread() behaves differently in these 2 examples:
1)
I have a structure with a short and a char (size is 4 bytes including padding) and an array of three such structures.If I write each short and char of each structure separately with fwrite() and then read that file with fread() to a variable whose type is that structure, I will read 4 bytes at a time (there will be 9 bytes in the file) so you can see that one byte will be left in 3rd iteration (and one byte will be lost in each iteration).What happens is that there is no 3rd read because I'm left with one byte and fread has to read 4 bytes.
2)
A simpler example, if I write a 1 byte char to a file with fwrite() and then put the content of that file into a 4 byte int with fread(), the integer will get that data.
Why does this happen?Why does the data get read in one case but not in the other if EOF is reached?
Here is the first example:
int main()
{
struct X { short int s; char c; } y, x[]=
{{0x3132,'3'},{0x3435,'6'},{0x3738,'9'}};
FILE *fp=fopen("FILE.DAT","wb+");
if (fp)
{
for(int i=0;i<sizeof(x)/sizeof(x[i]);)
{
fwrite(&x[i].s,sizeof(x[i].s),1,fp);
fwrite(&x[i].c,sizeof(x[i].c),1,fp);
i++;
}
rewind(fp);
for(int i=0;fread(&y,sizeof(y),1,fp);)
printf("%d:%x %c\n",++i, y.s, y.c);
fclose(fp);
}
return 0;
}
Second example:
int main()
{
FILE *fp=fopen("FILE.DAT","wb+");
char c = 'a';
fwrite(&c, sizeof(c), 1, fp);
rewind(fp);
int num;
fread(&num, sizeof(num), 1, fp);
fclose(fp);
return 0;
}

Answer 1

Why does the data get read in one case but not in the other if EOF is reached?
"What happens is that there is no 3rd read because I'm left with one byte and fread has to read 4 bytes." is a questionable premise.
1st Code did read 3 times. There are with no bytes left to read.
In both codes, the last read was a partial read with a fread() return value of 0.#Useless
(The first code did not print the result of the 3rd read.)
With fread(), a return value of 0 does not mean "end-of-file" was immediately encountered - nothing read. Instead, 0 means an complete read did not occur due to :
* "end-of-file" or partial read.
* rare I/O error.
Why does this happen?
In the 2nd code, results may differ due to Indeterminate behavior
fread() ... If a partial element is read, its value is indeterminate1 C11dr §7.21.8.1 2
fread(&num, sizeof(num), 1, fp) result may or may not be as expected.
A more informative example
int main(void) {
FILE *fp = fopen("FILE.DAT", "wb+");
char c = 'a';
printf(" %8X\n", c);
fwrite(&c, sizeof(c), 1, fp);
rewind(fp);
unsigned num = rand();
printf(" %8X\n", num);
size_t len = fread(&num, sizeof(num), 1, fp);
printf("%zu %8X\n", len, num);
len = fread(&num, sizeof(num), 1, fp);
printf("%zu\n", len);
fclose(fp);
return 0;
}
Output
61 as expected
5851F42D as expected - some random value
0 5851F461 Indeterminate! (in this case, looks like the LSByte was replaced.)
0 as expected
Moral of the story: assess the return value of fread() before relying on what was read into the buffer.
1 indeterminate value
either an unspecified value or a trap representation

Answer 2

... when EOF is reached ...
EOF isn't "reached". Many <stdio.h> functions return EOF as a signal that something went wrong, giving no indication what that something is. If you want to know what went wrong after receiving the signal, test with feof() and/or ferror().