strncpy function does not work properly

Question

what i am trying to do is asking user to enter something in format like: cd directory. Then I store "cd" in a string and "directory" in another string. Here is my code:

void main()
{
   char buf[50], cdstr[2], dirstr[50];

   printf("Enter something: ");
   fgets(buf, sizeof(buf), stdin);

   //store cd in cdstr
   strncpy(cdstr, buf, 2);
   printf("cdstr: %s(test chars)\n", cdstr);

   //store directory in dirstr
   strncpy(dirstr, buf+3, sizeof(buf)-3);
   printf("dirstr: %s(test chars)\n", dirstr);
}

The output is below with the input: cd pathname

cdstr: cdcd pathname  //incorrect answer
(test chars)          //an extra "\n"
dirstr: pathname      //correct answer
(test chars)          //an extra "\n"

That's why?

Hint: look for the data type of `buf` and analyze `buf+3`. Also, check for null when using `strncpy()` — Sourav Ghosh, Nov 19 '14 at 14:34
Because `strncpy` does not append an end-of-string character (`\0`) to the characters copied. Besides, you did not reserve room for `\0` in `cdstr`. — Ruud Helderman, Nov 19 '14 at 14:34
As for the extra `\n`, that's because `fgets` always captures a trailing `\n`. This is all common knowledge; please take some time to study the documentation of both `strncpy` and `fgets`. — Ruud Helderman, Nov 19 '14 at 14:41
http://stackoverflow.com/questions/1453876/why-does-strncpy-not-null-terminate — Lundin, Nov 19 '14 at 14:47
Also please note that strncpy shouldn't be used, because it is an unsafe and mostly obsolete function. http://stackoverflow.com/questions/2114896/why-is-strlcpy-and-strlcat-considered-to-be-insecure — Lundin, Nov 19 '14 at 14:48

syntagma · Accepted Answer · 2014-11-19T14:43:57.347

1

This is because after doing strncpy(cdstr, buf, 2), you don't have a NULL-terminated string in the cdstr char array. You can fix it by changing cdstr length to 3 and adding: cdstr[2] = '\0':

void main()
{
   char buf[50], cdstr[3], dirstr[50]={0};

   printf("Enter something: ");
   fgets(buf, sizeof(buf), stdin);

   //store cd in cdstr
   strncpy(cdstr, buf, 2);
   cdstr[2] = '\0';
   printf("cdstr: %s(test chars)\n", cdstr);

   //store directory in dirstr
   strncpy(dirstr, buf+3, sizeof(buf)-3);
   printf("dirstr: %s(test chars)\n", dirstr);
}

edited Nov 19 '14 at 14:43

answered Nov 19 '14 at 14:36

syntagma

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Now the thing is, after I add '\0' to cdstr, there is a messy code appears between cdstr and (test chars). It is like: cd?(test chars). – user3368506 Nov 19 '14 at 14:48
Are you sure, did you compile it? – syntagma Nov 19 '14 at 14:49

strncpy function does not work properly

1 Answers1