memory allocation error with scanf - c

Question

I have this simple code, and can't figure out how to allocate memory for scanf
char* string= (char*) malloc (sizeof(char));
printf("insert string: \n");
scanf("%s", string);
free(string);
doesn't matter how many chars my string is, it's an error. I want to use malloc for the char*, any way to set memory for scanf.

Answer 1

You are just request 1 byte. You need to allocate more than that if you want store more than just the 0 terminator in the string:
char* string= malloc (256); //256 bytes, arbitrary value...
I removed:
sizeof(char) because it's always guaranteed to be 1.
(char*) because casting the return value of malloc() is needless.
I would also recommend using fgets() instead of scanf() to prevent overflowing the buffer. The same could be done with scanf() by specifying the length in the format string. But I personally prefer using fgets() and parsing using sscanf() if necessary.

Answer 2

Try
char buf[256];
scanf( "%s" , buf);
Delete your malloc.
You're allocating a tiny piece of memory to what could be a variable sized array of string. By default an array is just a pointer to buf[0] which is what string will give it. You'll need a buffer to capture the initial read and then allocate the proper memory afterwards.
Afterwards you can do string = length(buf) * sizeof(char) <- pseudocode
to assign the correct length of the string.

Answer 3

As #Blue Moon correctly pointed out, you are just allocating memory to store one character, and the only string a single character can store is an empty string or a string without characters. In C, all strings are null terminated, and the 1 byte of memory you have allocated can accommodate only a null character, i.e. '\0'
In order to store more characters in your string, say a hundred characters, you can do the following:
#define MAX_STRING_SIZE 102
char* string= (char*) malloc (sizeof(char) * MAX_STRING_SIZE);
memset(string, '\0', MAX_STRING_SIZE);
printf("Insert string: \n");
scanf("%101s", string);
if (strlen(string) == (MAX_STRING_SIZE - 1))
{
printf("The user entered a string larger than 100 characters!");
free(string);
/****Error handling***/
}
/*****Do something with string****/
free(string)
The above snippet is a nice little trick to find out if the user entered a valid number of characters or not. If you wanted the user to enter a maximum of 100 character and your program is always running in an ideal scenario(where the user listens to you all the time and never enters more than 100 characters) you just need one extra byte to store the null character. Hence, you need space for 101 characters. But you can add space for another character, and allocate 102 bytes instead to find out the cases when the user enters a string longer than 100 characters.
How this works: Say the user enters 100+ characters. The scanf function can read a maximum of 101 characters. So, we can use the strlen function to check if the number of characters input by the user are equal to 101 or not, which will help us to validate the input string.