A function to scan user input as string

Question

I know that multiple functions are already available. However, I thought of writing my own because I wanted to learn the logic (and also because I thought there wasn't enough confusion :P). Please review the function I wrote and suggest me efficient changes.

Without further ado, here I go:

scan(string,size)
char** string;
size_t size;
{
    string[0]=(char*)malloc(sizeof(char));
    char keystroke=' ';
    while((keystroke=getc(stdin))!='\n') {
        string[0][size++]=keystroke;
        string[0]=(char*)realloc(string[0],size+1);
    }
    string[0][size]='\0';
    return size;
}

Answer 1

Your code is horrible:

• You don't check for end-of-file, which will lead to endless loops
• You don't check for failure of malloc or realloc
• You call realloc way too often, which makes the code slow for large lines of input
• You cast the result of malloc
• You use the outdated style of function definition from the 1980s
• The variable string is not a string but points to an array of pointers to strings, which is confusing
• The #include for size_t is missing
• Using [0] instead of * is extremely confusing
• There is no need to initialize keystroke to a space character
• The parameter size is useless since the only possible value that makes sense for it is 0
• The return type of the function is implicitly int, which is obsolete
• The returned value is of type size_t, which doesn't fit into an int and additionally differs in signedness

Answer 2

Code does not handle end-of-file or rare input error well as there is no attempt to detect EOF.

Perhaps something like

// char keystroke=' ';
int keystroke=' ';
while((keystroke=getc(stdin))!='\n' && keystroke!= EOF) {
    string[0][size++]=keystroke;
    string[0]=(char*)realloc(string[0],size+1);
}
if (keystroke == EOF && size == 0) {
   free(string[0]);
   string[0] = NULL;
   return -1;  // Maybe use (size_t)-1 to indicate EOF.
}

Fuller example code