need of fseek() in c

问题

Part of Code 1:-

while(1)
    {
        ch=fgetc(pt);
        if(c==EOF)
        {
            break;
        }
        if(c==' ')
        {
            fputc('z',pt);
        }
    }

Part of Code 2:-

while(1)
{
    ch=fgetc(pt);
    if(c==EOF)
    {
        break;
    }
    if(c==' ')
    {
        fseek(pt,0,SEEK_CUR);
        fputc('z',pt);
        fseek(pt,0,SEEK_CUR);
    }
}

I want to replace next character after every space in a file. That file is pointed by the pointer pt.

Both the code shows no error and runs fine, but when I externally opens the .txt file, first code did nothing whereas the second code replaces the next character after space successfully.

Clearly fseek(pt,0,SEEK_CUR); is making the difference.

So I am unable to understand that what it is doing in the second code?

回答1:

The use of fseek() here - The C standard requires a positioning operation between a read and a write operation on an update stream, or between a write and a read. This is a positioning operation between a write and a read. It is not a no-op; it places the stream into a mode which allows the next fgetc() to work correctly, reliably, across platforms, as required by the C standard.

EDIT:

2 fseek() calls are required because the first one acts as the "no-op" call between an fgetc() and a subsequent fputc() call. After the fputc(), the second one acts as the "no-op" between the fputc() and the subsequent fgetc() call. (since a loop is running)

来源：https://stackoverflow.com/questions/21965142/need-of-fseek-in-c