I have the program in c
int main() {
char str[1];
gets(str);
return 0;
}
I have tried to give the input '\0' to cause the str[0] to represent the null character but it's actually store the character \.
Is there an input that will cause that?. Thanks.
A NUL character cannot be inside a C string, since by definition it is ending a C string.
(The NUL character is '\0' -practically, a zero byte- and is of course different of the NULL pointer).
And your str is probably too small. Consider declaring it with char str[32]; and clearing it with memset(str, 0, sizeof(str)); before reading into it.
So what your user would input is not a string, but simply a sequence of bytes (you might read character by character a stream of bytes using getchar). Hence you cannot use fgets for reading (of course, forget about gets, it is obsolete, dangerous, and removed from the C11 standard. Read n1570 to check).
A different question is what should your user do to enter such a NUL character on the keyboard. Such details are operating system specific (on some systems there might be no way to type a NUL character, on other systems you might have to press several keys simultaneously, including the Ctrl key). You could consider some redirection from some binary file containing NUL bytes.
It is surprising (and very unusual) that you want to enter a NUL; I am guessing that you are mis-undestanding your homework.
Perhaps you want to handle the empty input case. For that, you need to explicitly detect the end-of-file condition (e.g. when getchar gives EOF, which is not a character). Beware that feof is only valid after some (failed) read operation.
Be sure to compile with all warnings and debug info (with GCC compiler, use gcc -Wall -Wextra -g). Read How to debug small programs
How to receive a null character in gets function from the user input?
The obsolete, since c11, function gets() reads a line of text and then saves it (without the '\n') with an appended null character insuring a string.
gets()
The gets function reads characters from the input stream pointed to by stdin, into the array pointed to by s, until end-of-file is encountered or a new-line character is read. Any new-line character is discarded, and a null character is written immediately after the last character read into the array. C99 §7.19.7.7 2
line
A text stream is an ordered sequence of characters composed into lines, each line consisting of zero or more characters plus a terminating new-line character. Whether the last line requires a terminating new-line character is implementation-defined. C99dr §7.19.2 2
string
A string is a contiguous sequence of characters terminated by and including the first null character. C99dr §§7.1.1 1
Is there an input that will cause that?
Yes. Entering a null character on various keyboards is not easy. Sometimes a Ctrl Shift 2 (Ctrl @) will work. Some keyboards lack or obscure any way to input a null character. Usually a re-directed input from a file that contains a null character into stdin will suffice.
Entering \ 0 Enter is not entering a null character.
An important consideration is that with gets() or fgets(), the input of a null character is not special in its reception. A following Enter ('\n') or end-of-file-signal is still needed. After which a null character is appended.
With gets(), the size of the input buffer needs to be at least 2 to read in any input more than an empty line of just Enter, so char str[1]; is too small to save an input of a null character with the appended null character.
If the input was Enter, gets(str) would save no input in str[0] and then append a null character to str[0].
If the input was Ctrl @, Enter, gets(str) would save the user inputted null character to str[0] and then append a null character to str[1].
Let us try an example with fgets().
int main(void) {
// Form a file of 1 byte
FILE *f = fopen("zero.txt", "w");
if (f == NULL) {
return EXIT_FAILURE;
}
fputc('\0', f);
fclose(f);
f = fopen("zero.txt", "r");
if (f == NULL) {
return EXIT_FAILURE;
}
char buf[80];
memset(buf, 42, sizeof buf);
fgets(buf, sizeof buf, f);
for (int i = 0; i < 5; i++) {
printf("%d\n", buf[i]);
}
fclose(f);
return 0;
}
Output
0 // from the file zero.txt
0 // the appended null character
42
42
42
Text processing functions like fgets(), scanfs(), fscanfs() and the obsolete gets() having to cope with a user inputed null character is a corner case. Such input is either a mistake or, more likely, a hacker attempt to break the program.
when we declare char str[1],we give instruction to compiler that str is string having 1 byte,'\' is also one charactor. string defination also states that char array ending with '\0' is called string.
As per your declaration we can give only one char which '\'.
