c trimming newline character when reading input from file

Question

Is this a safe way to trim the newline character off of a line after reading it in from a file?

while ( fgets(buffer, 1024, fp) != NULL )
{
    buffer[strlen(buffer)-1] = '\0';
    fprintf (stderr, "%s\n", buffer);
}

It doesn't give me any seg faults but could it cause problems down the road? Should I do something like this instead?

while ( fgets(buffer, 1024, fp) != NULL )
{
    tmp = (char *) malloc(strlen(buffer));
    strncpy(tmp, buffer, strlen(buffer) - 1);
    tmp[strlen(buffer)-1] = '\0';
    fprintf (stderr, "%s\n", tmp);
}

Answer 1

There won't be a trailing newline if the line doesn't fit into the buffer, so your approach could delete a valid character. So to be safe, you should:

int len = strlen(buffer);
if (len > 0 && buffer[len-1] == '\n')
    buffer[len-1] = 0;

Of course, that will not work on a Mac or in Windows (which use \r and \r\n respectively).

Answer 2

I see no real problems with your first option -- if you own buffer, you're allowed to modify it. Just a couple of minor issues:

• fgets may not actually return a newline if the last line of the file does not end with one (edit: or if the line doesn't fit into the buffer, as pointed out by Aaron) -- so you should check that there is really a newline there.

• Also, I would check that strlen(buffer)>0 before accessing buffer[strlen(buffer)-1].

Note that your second option has a bug -- it allocates one byte too few (you need an extra byte for the null character).

Answer 3

Assuming...
1) that buffer is a 1024 char array and...
2) that the buffer you are reading doesn't have a line that is 1024 characters long and then a \n.

3) that you are in UNIX and \n is the terminating character. \r\n in Windows and \r on Macs. So in other words, your code isn't really portable... But if that's not an issue then no worries.

if 2 is a possibility then you have a few issues...
A) You will be nulling out a letter instead of a \n.
B) The strlen() will most likely not be < 1024 which will lead to modifying memory outside of your buffer. Memory corruption in other words.

The first solution is fine otherwise.

The second solution is unnecessary and you'll find that dynamic memory allocation is extremely slow in comparison.

Answer 4

Depending on what you're reading there, and how processing does continue, you could make sure what you're going to trim off is actually a whitespace using isspace() from <ctype.h>.

Answer 5

Use strchr() to search the buffer for a newline:

if (fgets(buffer, sizeof buffer, stdin))
{
  char *c = strchr(buffer, '\n');
  if (c)
    *c = 0;
  ...
}