Problems when using fgets and strcat

Question

I am having some problems while using fgets and strcat (I tried to replace the fgets into scanf and the program worked). It seems the strcat is not working.

char name[256];
char text[256];

fgets(name,250,stdin);
strcat(name,".txt");
printf("%s\n",name);

The output is (in separate lines):

d
.txt

Take a look at [my answer to another question](http://stackoverflow.com/a/36341266/434551). While the questions are different, the solution is exactly the same. — R Sahu, Mar 31 '16 at 20:57
If `strcat` weren't working, you wouldn't see `.txt` in the output — M.M, Mar 31 '16 at 21:00

Carey Gregory · Accepted Answer · 2016-03-31T21:08:26.053

2

You have to remove the newline from the input. Like this:

fgets(name, 250, stdin);
char *p = strchr(name, '\n');
if (p)
    *p = '\0';
strcat(name, ".txt");

Obviously you'd want to add some error checking there but this demonstrates the idea.

edited Mar 31 '16 at 21:08

answered Mar 31 '16 at 21:03

Carey Gregory

6,836
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1

This assumes a newline is in `name` which may not always be the case. Maybe use `strchr`. – Fiddling Bits Mar 31 '16 at 21:06

score 1 · Answer 2 · answered Mar 31 '16 at 21:10

1

because '\n' is also consumed in your fgets function, it will be displayed like that. You can use scanf instead for simplicity:

int main() {
   char name[256];
   char text[256];

   scanf("%250s",name);
   strcat(name,".txt");
   printf("%s\n",name);
}

answered Mar 31 '16 at 21:10

Pooya

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Note: `scanf("%s")` only reads until the next space, see http://stackoverflow.com/questions/1247989/how-do-you-allow-spaces-to-be-entered-using-scanf for details. – Roland Illig Mar 31 '16 at 21:23
@RolandIllig that is right, since the example shows something like a filename "d.txt" it is not viable to catch the space and it doesn't require extra functions to drop the final "\n" either. However if the code's intention is really for getting a hole string including space, tabs ,... then you are right – Pooya Mar 31 '16 at 21:26

score 1 · Answer 3 · answered Mar 31 '16 at 21:16

Using the rarely-used function strcspn, this becomes simple and error-prone:

if (fgets(name, 250, stdin) != NULL) {
  strcpy(name + strcspn(name, "\n"), ".txt");
  ...
}

The strcspn function counts the number of characters that are not \n; the c in strcspn means complement. Therefore, no matter whether or not the name contains a newline or not, the .txt will always be added at the correct position.

Problems when using fgets and strcat

3 Answers3