Everyone knows that the gets function in the C standard library is dangerous to use because it offers no protection against buffer overflows.
What should people use instead?
The typical answer is to use fgets. Unfortunately, although safe, fgets in non-trivial cases is much harder to use properly:
-
How do you determine what a good maximum buffer size is? The reason why using gets is dangerous in the first place is because you don’t know how big the input can be.
-
Unlike
gets,fgetsincludes the trailing newline character, but only if the entire line fits within the buffer. This can be remedied easily by replacing the newline character with NUL, but it’s a nuisance. -
If the input line exceeded the buffer size,
fgetsleaves the excess in the input stream. Now your input stream is a bad state, and you either need to discard the excess (and possibly throw away the incomplete line you just read), or you need to grow your buffer and try again.Discarding the excess usually involves calling
fscanf, and I don’t know anyone who usesfscanfwithout disdain, becausefscanfis hard to use properly too. Furthermore, discarding the line by itself won’t necessarily make you any better off, because you’ve accepted incomplete input and still have to walk the road to recovery.Growing the buffer is also a hassle. You either need to grow the buffer exponentially as you fill the buffer to capacity, or you need to read ahead, find out how many more bytes you need, grow the buffer, backtrack, and read the excess bytes again. (The latter isn’t even possible with
stdin.) And, of course, this means you also need to handle allocation failure.
In all, this means to use fgets properly, you need to make several more library calls than you want. For example, here’s an implementation that discards line excess:
/** Returns the number of characters read or (size_t) -1 if the
* line exceeded the buffer size.
*/
size_t fgetline(char* line, size_t max, FILE* fp)
{
if (fgets(line, max, fp) == NULL)
{
return 0;
}
else
{
/* Remove a trailing '\n' if necessary. */
size_t length = strlen(line);
if (line[length - 1] == '\n')
{
line[--length] = '\0';
return length;
}
else
{
/* Swallow any unread characters in the line. */
fscanf(fp, "%*[^\n]");
fgetc(fp); /* Swallow the trailing '\n'. */
return -1;
}
}
}
Ugh!
I personally prefer using C.B. Falconer’s public-domain ggets/fggets functions, which have gets-like semantics and dynamically allocate buffers large enough to accomodate the entire line.
Additional reading: Getting Interactive Input in C
