Detecting new line in C - c

Question

My code goes something like this:
char k[1000];
while(1){
scanf("%s",&k);
if(k[0] == '\n'){
exit(0);}
/* Do some processing on k */
memset(k,0,1000);
}
My intention is to process user input per normal and terminate when user inputs empty string or new line. This doesn't seem to work.
Could you guys help on what went wrong?
On related note, I also want to terminate if it is the end of file, how should I do it for EoF?
Thank you in advance for all the help.

Answer 1

First off -- don't use scanf for user input. It is a minefield of subtle issues just waiting to bite new C programmers, instead use a line-oriented input function like fgets or POSIX getline. Both read up to (and including) the trailing '\n' every time (as long as you provide a buffer of sufficient size for fgets -- otherwise it just keep reading blocks of characters of its buffer size until it encounters a '\n' or EOF)
So to read user input until an empty-string or EOF is encountered, you could simply do something like the following:
#include <stdio.h>
#include <string.h>
#define MAXC 1000
int main (void) {
char k[MAXC] = "";
for (;;) { /* loop until empty-string of EOF */
printf ("input: "); /* prompt for input */
if (fgets (k, MAXC, stdin)) { /* read line (MAXC chars max) */
if (*k == '\n') { /* test for empty-string */
fprintf (stderr, "empty-string! bye.\n");
break;
}
size_t l = strlen (k); /* get length of string */
if (l && k[l - 1] == '\n') /* check if last char is '\n' */
k[--l] = 0; /* overwrite with nul-terminator */
printf ("got input: %s\n", k);
}
else { /* got EOF */
fprintf (stderr, "EOF -- bye.\n");
break;
}
}
return 0;
}
Example Use/Output
>bin\fgets_user_input.exe
input: this
got input: this
input: is some
got input: is some
input: input
got input: input
input:
empty-string! bye.
>bin\fgets_user_input.exe
input: this is more
got input: this is more
input: ^Z
EOF -- bye.
>bin\fgets_user_input_cl.exe
input: it works the same
got input: it works the same
input: compiled by gcc
got input: compiled by gcc
input: or by cl.exe (VS)
got input: or by cl.exe (VS)
input:
empty-string! bye.
(note: for Linux Ctrl+d generates the EOF, I just happened to be on windoze above)

Answer 2

Like ever so often, the problem here is inappropriate usage of scanf(). scanf() is not for reading input but for parsing it and the format-string tells it how to parse.
In your case, %s is looking for a sequence of non-whitespace characters (IOW, a word) and it skips any leading whitespace. \n (newline) is just a whitespace character, so it is always skipped -- your scanf() will just wait for more input until it can parse %s.
For more information on scanf() pitfalls, I recommend you my beginners' guide away from scanf(). As a rule of thumb, with interactive input (which is the default), scanf() is almost always wrong.
There's another huge problem with scanf("%s", ...): It will happily overflow any buffer you provide it, as long as the input contains non-whitespace characters, just like gets() which was even removed from C for exactly that reason: Buffer overflows are extremely dangerous! Therefore always use a field-width, in your case scanf("%999s", ...). This parses a maximum of 999 characters, leaving one for the necessary 0 byte terminating a string.
But now for how to do it correctly: There are several functions in C that are indeed for reading input and one of them is for reading a line of input: fgets(). In your code, it would look like this:
char k[1000];
while(fgets(k, 1000, stdin)){
if(k[0] == '\n'){
exit(0);
}
/* Do some processing on k */
memset(k,0,1000);
}
I used your original code here, still some further remarks:
It would be better to define a macro instead of using the magic number 1000, e.g. #define INPUTSIZE 1000 and use this instead, like char k[INPUTSIZE];, fgets(k, INPUTSIZE, stdin) etc.
Clearing the whole array is not needed, so to avoid unnecessary work, replace the memset() with just k[0] = '\0'; or similar. A C string ends at the first 0 byte, so this is enough to make k hold an empty string. If your program does nothing more than shown here, you could even get rid of this completely, as the next fgets() call overwrites the array anyways (or returns NULL on error, which would stop the loop).
Also note that fgets() reads the whole line including the newline character at the end, so keep this in mind when processing the contents of k.

Answer 3

This one is guaranteed to give everything except newlines (and EOFs) for you:
char k[1000];
scanf("%[^\n]", k);
And when it returns, the next character is guaranteed to be either a newline, or non-existent at all (EOF reached). Get it like this:
int next_char = getcgar();
if (next_char == EOF){
your_eof_process();
}
else if (nexr_char == '\n'){
your_newline_process();
}
Personally, I would do it using only getchar():
char k[1000];
int ind, tempc;
for (ind = 0; ind < sizeof k; ind ++){
tempc = getchar();
if (tempc == '\n'){
// Some stuff
}
else if (tempc == EOF){
// Other stuff
}
else {
k[ind] = tempc;
}
}
k[sizeof(k)-1] = '\0';