Comments on: Safe replacement for gets

By: James

James — Mon, 10 Aug 2009 21:12:30 +0000

I strongly recommend you #define LINEBUFFERLEN 100
and then replace every instance of 100 with LINEBUFFERLEN so that you (or another programmer) don’t change your buffer length, forget to change it in every single location and end up with another stack overflow just like the one you were trying to fix in the first place.

By: mirabilos

mirabilos — Mon, 10 Aug 2009 08:16:13 +0000

Uhm… fgetln(3) has been part of the BSDs for ages (there are GNU/Linux implementations too, e.g. in libbsd in Debian), and getline(3) from glibc has made it into SUSv3 (POSIX) 2008. So what’s the point?

By: Jon

Jon — Mon, 10 Aug 2009 05:10:51 +0000

@Jon
*smacks head against wall*
length = 0;
finished = 1;

I’m going to sleep now!

By: Jon

Jon — Mon, 10 Aug 2009 05:10:17 +0000

@Jon
err… the above should read
{
length = 1;
finished = 0;
}

Brain failure while typing.

By: Jon

Jon — Mon, 10 Aug 2009 05:09:31 +0000

@Wesley

Your fix isn’t quite right:
if (fgets(buffer, 100, stdin)) {

fgets returns buffer on success, so your logic is backwards. Also, the EOF case isn’t an error, but normal operation. Something like:

if (!fgets(buf, 100, stdin))
{
if (feof(stdin))
length = finished = 0;
else
error = … blah;
}
else
length = strlen(buf);

By: Wesley

Wesley — Sun, 09 Aug 2009 19:48:57 +0000

@Justin why is that bad? realloc behaves just like malloc when the pointer to the memory block is NULL according to this link

By: Wesley

Wesley — Sun, 09 Aug 2009 19:43:06 +0000

@Jon Okay. I modified the example code again. I hope that it's less error-prone now. Thanks for your tips.

By: Justin

Justin — Sun, 09 Aug 2009 18:32:24 +0000

Trace through the execution of that function in the case where the user enters a string < 100 chars. You call realloc even when you don't need to.

Then you do

temp = (char *)realloc(out, count * 99 + length + 1);

where count is 0 so that becomes

temp = (char *)realloc(out, 0 * 99 + length + 1);

so you are calling

temp = (char *)realloc(out, length + 1);

By: Jon

Jon — Sun, 09 Aug 2009 18:03:26 +0000

Check the return value of fgets! It will return NULL on an EOF read (which happens all the time… user types and then hits ctrl-D, or input is piped to you). In your example, you’ll continue the loop, using the last contents of buffer. Over and over and over, with no end.

Check the return value of getLine! Your docstring indicates it can return NULL, but then you go ahead and try to use it without checking for that.

Beginning C coders do this sort of thing all the time. It is a very fast, very powerful language, and consequently it will allow you to load the gun and blow your foot right off.

Side note: you can gain all sorts of little speed-ups by thinking like a computer:
buffer[length - 1] = ”;
–length;
becomes:
buffer[--length] = ”;

By: Wesley

Wesley — Sun, 09 Aug 2009 17:28:56 +0000

@vart You are absolutely right of course. I have modified the example code slightly so that it shouldn't crash when realloc fails. Thanks!