When fgets is dangerous (with the size less than buffer size)?

Question

FILE * file = fopen( “in.txt”, “r” );
char line[80];
if( NULL != file ) {
    fgets( line, 40, file );
    fclose( file );
}

Is there anything dangerous in the above code, what i saw here was it got the maximum length 40 string to the buffer line then close the file.

Are you sure that you understand `fgets()`? Did you read `fgets()` [Manual](https://linux.die.net/man/3/fgets)? What exactly is `40` in your `fgets()`call? — Michi, Apr 16 '18 at 19:02
@Jean-FrançoisFabre oh okay, it's just a question from a review and i want to make sure if i miss sth or not. — Thai, Apr 16 '18 at 19:03
@Michi keep calm, i just want to make sure, 40 is just 40, that's the point of the question, whether the size used in fgets less than the buffer size. — Thai, Apr 16 '18 at 19:06
For `fgets`, in your example, unless you have a _special_ reason not to, the idiomatic [recommended] call is: `fgets(line,sizeof(line),file)`. Using a shorter length is okay [just wasteful]. Using a longer length is undefined behavior [and will probably segfault] — Craig Estey, Apr 16 '18 at 19:06
@CraigEstey it segfaults only if some line is longer than the buffer length. — Jean-François Fabre, Apr 16 '18 at 19:17
the posted code does not compile! it is missing the needed #include statement for the needed header file. When asking a question about a run time problem, as this question is doing, always post a [mcve]. — user3629249, Apr 17 '18 at 05:36
@Jean-FrançoisFabre, Having a line in the file that is longer than the length given to `fgets()` will NOT cause a seg fault event. — user3629249, Apr 17 '18 at 05:38

score 0 · Accepted Answer · answered Apr 16 '18 at 22:59

You may be confusing fgets() and gets(). See Why is the gets function so dangerous that it should not be used?

As long as the size param matches the buffer size, fgets is safe. In your case it would be normal to do fgets( line, sizeof(line), file ); Note: fgets() reads at most 1 less than "size" and always 0 terminates.

When fgets is dangerous (with the size less than buffer size)?

1 Answers1