Question

I am using fscanf in C++ as follows:

Here is my text file:

abcd
efgh

There is no space or new line after “efgh”.
Now here is my loop:

FILE* fp;
char eof = 0;
do
{
    char str[20];
    fscanf(fp, "%s", str);
    std::cout<<str<<std::endl;
}
while((eof = fgetc(fp)) != eof)

The output i expect is:

abcd
efgh

But the actual output i get is:

abcd
efgh
efgh

I debugged a found that after reading “efgh” the value read into eof is ‘\n’ and not EOF.
The environment is linux mint.I want to know why always the last string is read 2 times.Please advice

Answer 1

The last string is not being read twice. The problem is the loop’s test for continuing:

(eof = fgetc(fp)) != eof

This assigns fgetc()‘s return value to eof and checks that it is not equal to eof. Something which is hard to do logically. However, when fgetc() is called when the file is at EOF, it returns -1. That is cast to a char for the assignment, but the sub-expression in parenthesis retains the value -1 (due to type promotion rules). Comparing -1 to 255 or -127 (depending on whether char is signed or unsigned) finally terminates the loop.

The third time through the loop, fscanf() fails and does not update str: that’s why the same value seems to have been read twice.

To fix it, the most straightforward technique would be:

do {
 ...
} while (!feof (fp));

However, on many operating systems, feof() doesn’t work very well with fscanf() because the end of file indication isn’t reliably set until fscanf() fails. A more reliable, O/S-resistent technique is to use

do {
    int result = fscanf (fp, ...whatever...);
    if (result < 0)   // end of file or i/o error?
         break;
} while (!feof (fp));