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円';
 return size;
}

• Vogel612: Please do not update the code in your question to incorporate feedback from answers, doing so goes against the Question + Answer style of Code Review. This is not a forum where you should keep the most updated version in your question. Please see what you may and may not do after receiving answers .

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• You use the outdated style of function definition from the 1980s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• The return type of the function is implicitly int, which is obsolete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

• chux: @d4rk4ng31 "why would I check for EOF?" --> to know when stdin has no more data to provide.
• L. F.: @d4rk4ng31 what if the input ends before you can read a '\n'?
• chux: @d4rk4ng31, When stdin is closed, getc(stdin) returns EOF - that is the true end of input. Your code, instead of stopping, goes into an infinite loop. '\n' is only the end of a line.
• chux: @d4rk4ng31 It does not wait, it loops, consuming memory until an allocation failed and then UB of attempting to de-reference a NULL. Loop should quit on either '\n' and NULL.
• chux: @d4rk4ng31 Because you have not tested it in enough different cases.

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

• kesarling: any particular reason to use int keystroke over char keystroke ?
• chux: @d4rk4ng31 int getc(stdin) returns type int and typically 257 different values. To save in char loses information. Using char, cannot distinguish EOF from a valid character.