为什么fseek不起作用？

Question

I wrote a function that list my file that it's binary and write it with fwrite func from my struct:

void ReadFile::printList(){
clearerr(bookFilePtr);
fseek(bookFilePtr,0L,SEEK_SET); // set to begin of file
int counter = 1;
cout << "***************************************************" << endl;
struct book tmp ;
while (!feof(bookFilePtr)){
            fread(bookPtrObj,sizeof(struct book),1,bookFilePtr);   
    cout << bookPtrObj->name << "s1"<< endl;
    cout << bookPtrObj->publisher << "s2"<< endl;
    cout << bookPtrObj->author << "s3" <<endl;
    cout << bookPtrObj->stock << endl;
    cout << bookPtrObj->translation << endl;
    cout << bookPtrObj->trasnlator << "s4" <<endl;
    cout << bookPtrObj->delayDays << endl;
    cout << bookPtrObj->delayPay << endl;
    cout << "***************************************************" << endl;
    fseek(bookFilePtr,counter * sizeof(struct book) ,SEEK_SET); // seek to next data
    counter ++;
}

It prints for once all of my file , but didn't quit from my loop. and my func continue to print last data in file.How i do to quit my func and find out to end of file? does fseek work?

Answer 1

while(!feof(bookFilePtr)) is a bad way to do a reading loop. !feof(...) does not guarantee that fread will succeed. You should loop while fread succeeds instead.

while(fread(bookPtrObj, sizeof(struct book), 1, bookFilePtr) == 1) {
    //  blah blah do the things
}

The call to fseek is redundant too: fread already advances the file cursor itself, so you don't need to seek.

Answer 2

while (!feof(f)) does not mean what you think it means. It does not mean 'current position is at end of file', it means 'last read attempted to read past end of file': it's a flag that's set by fread and others, and cleared by fseek and others. fread has a return value. That return value indicates whether the read was successful. Use that as your loop condition.

You're ignoring fseek 's return value as well, which is bad for the same reasons, but that isn't the cause of your problem, at least not directly.

BTW, I'm assuming this is just code to experiment with fseek , but your fseek 's in the loop set the position to where the position should already be, so it adds no value.