奇怪的C程序fgets

Question

I was going through an open source codebase and I see the following:-

char *buf;
char *line;
#define BUFSIZE 5000

buf = malloc(BUFSIZE)
line = buf;

while(fgets(line, (unsigned)(buf + BUFSIZE -line), in) != NULL) {
    // do stuff
    // ....
}

Why is the second argument to fgets given as buf + BUFSIZE - line ?

Answer 1

That gives the number of characters from line to end of buf . Your //do stuff likely increments line

buf + BUFSIZE is a char * pointing to the first char after the memory allocated for buf

buf + BUFSIZE - line is an integral, the number of chars from line to buf + BUFSIZE - and therefore the number of characters you can safely write to line without overflowing buf

Answer 2

buf + BUFSIZE - line gives the free space in the buffer.

This way line can be a scrolling pointer pointing to the first free byte, where the next read operation can put the data.

Answer 3

Line will probably get incremented during the loop. Thus this expression shrinks the value of BUFSIZE by the size of text already read.

It's a guess, sicne you didn't post the loop.