Mistake using scanf - c++

Question

could you say me what is the mistake in my following code?
char* line="";
printf("Write the line.\n");
scanf("%s",line);
printf(line,"\n");
I'm trying to get a line as an input from the console.But everytime while using "scanf" the program crashes. I don't want to use any std, I totally want to avoid using cin or cout. I'm just trying to learn how to tak a full line as an input using scanf().
Thank you.

Answer 1

You need to allocate the space for the input string as sscanf() cannot do that itself:
char line[1024];
printf("Write the line.\n");
scanf("%s",line);
printf(line,"\n");
However this is dangerous as it's possible to overflow the buffer and is therefore a security concern. Use std::string instead:
std::string line;
std::cout << "Write the line." << std::endl;
std::cin >> line;
std::cout << line << std::endl;
or:
std::getline (std::cin, line);

Answer 2

Space not allocated for line You need to do something like
char *line = malloc();
or
Char line[SOME_VALUE];
Currently line is a poor pointer pointing at a string literal. And overwriting a string literal can result in undefined behaviour.

Answer 3

scanf() doesn't match lines.
%s matches a single word.
#include <stdio.h>
int main() {
char word[101];
scanf("%100s", word);
printf("word <%s>\n", word);
return 0;
}
input:
this is a test
output:
word <this>
to match the line use %100[^\n"] which means 100 char's that aren't newline.
#include <stdio.h>
int main() {
char word[101];
scanf("%100[^\n]", word);
printf("word <%s>\n", word);
return 0;
}

Answer 4

You are trying to change a string literal, which in C results in Undefined behavior, and in C++ is trying to write into a const memory.
To overcome it, you might want to allocate a char[] and assign it to line - or if it is C++ - use std::string and avoid a lot of pain.

Answer 5

You should allocate enough memory for line:
char line[100];
for example.

Answer 6

The %s conversion specifier in a scanf call expects its corresponding argument to point to a writable buffer of type char [N] where N is large enough to hold the input.
You've initialized line to point to the string literal "". There are two problems with this. First is that attempting to modify the contents of a string literal results in undefined behavior. The language definition doesn't specify how string literals are stored; it only specifies their lifetime and visibility, and some platforms stick them in a read-only memory segment while others put them in a writable data segment. Therefore, attempting to modify the contents of a string literal on one platform may crash outright due to an access violation, while the same thing on another platform may work fine. The language definition doesn't mandate what should happen when you try to modify a string literal; in fact, it explicitly leaves that behavior undefined, so that the compiler is free to handle the situation any way it wants to. In general, it's best to always assume that string literals are unwritable.
The other problem is that the array containing the string literal is only sized to hold 1 character, the 0 terminator. Remember that C-style strings are stored as simple arrays of char, and arrays don't automatically grow when you add more characters.
You will need to either declared line as an array of char or allocate the memory dynamically:
char line[MAX_INPUT_LEN];
or
char *line = malloc(INITIAL_INPUT_LEN);
The virtue of allocating the memory dynamically is that you can resize the buffer as necessary.
For safety's sake, you should specify the maximum number of characters to read; if your buffer is sized to hold 21 characters, then write your scanf call as
scanf("%20s", line);
If there are more characters in the input stream than what line can hold, scanf will write those extra characters to the memory following line, potentially clobbering something important. Buffer overflows are a common malware exploit and should be avoided.
Also, %s won't get you the full line; it'll read up to the next whitespace character, even with the field width specifier. You'll either need to use a different conversion specifier like %[^\n] or use fgets() instead.

Answer 7

The pointer line which is supposed to point to the start of the character array that will hold the string read is actually pointing to a string literal (empty string) whose contents are not modifiable. This leads to an undefined behaviour manifested as a crash in your case.
To fix this change the definition to:
char line[MAX]; // set suitable value for MAX
and read atmost MAX-1 number of characters into line.

Answer 8

Change:
char* line="";
to
char line[max_length_of_line_you_expect];

Answer 9

scanf is trying to write more characters than the reserved by line. Try reserving more characters than the line you expect, as been pointed out by the answers above.