Thread: Directly create dynamic string from stdin

Is it possible to create a dynamic char array on the fly directly from stdin? I do not want to create a fixed length array before hand and then copy contents of it into a malloc created array.

Code:

//[1]
char line[MAX1];
gets(line);

//[2]
char line[MAX1];
fgets(line,MAX1-1,stdin);

//[3]
char *str = (char *)malloc(sizeof(char)*(strlen(line)));

I could do either [1](buffer overflow problem) or [2] and then goto [3]. But both will have a problem if the input is more than the size MAX1(use defined).

Is it possible to do something of the effect of readLine() method of BufferedReader class in Java or the Console.readLine in .NET? Is it possible to peek into stdin and see the size of the input and then creat an array of the same exact size?

If you are willing to use a non-standard function, then the POSIX standard getline function will do what you want (remember to call free when done).

If not, what you can do is to call fgets in a loop (reading into a fixed size array) and append the string read to your result string which is resized with realloc. When fgets returns a null pointer or when the string read has a newline character, you terminate the loop.

laserlight Yes!

I have even tried these:-
[1]first read into linked list of chars and then combine them into a single string. But then, I read somewhere that malloc and family of statement are costly to process; and what I do may leave a lot of fragmented memory(no holes but fragmented). So..
[2]I then created a linked list of char arrays, meaning each element of the list was a constant length array of char. This reduced the calls to malloc by 1/n compared to the previous method where n is the size of the linked list element which is an array.

Oh, I missed the requirement that stated that you "do not want to create a fixed length array before hand and then copy contents of it into a malloc created array".

Originally Posted by laughing_man

[1]first read into linked list of chars and then combine them into a single string. But then, I read somewhere that malloc and family of statement are costly to process; and what I do may leave a lot of fragmented memory(no holes but fragmented).

If you are going to combine them into a single string, then why not just copy the contents of the fixed length array into a "malloc created array"?

Originally Posted by laughing_man

[2]I then created a linked list of char arrays, meaning each element of the list was a constant length array of char. This reduced the calls to malloc by 1/n compared to the previous method where n is the size of the linked list element which is an array.

Then you have a linked list of char arrays, not a single string.

What kind of input do you expect to read from stdin? The approach that I would reach for will have as many calls to malloc as there are calls to fgets. If the string is going to be so long that this is a concern, then you might want to double (or increased by some other factor greater than 1) the capacity of the array each time you need to expand it... or maybe you should not be reading everything at once but rather should process the input in parts.

Originally Posted by laughing_man

I forgot to mention that I am working on Windows XP. Is that "POSIX standard getline function " available in win32.

It depends on your compiler and the libraries that it comes with, e.g., it is possible that the MinGW port of gcc supports it.

Originally Posted by laughing_man

in the last method (linked list of char arrays), in the end I would combine them into one dynamically created char array.

That would work. It is just a variant of what I suggested except that you complicate it by involving linked lists. Maybe the linked lists will make it more efficient, maybe they won't, but I do know that they mean that at one point you will have allocated twice as much memory as you need for the final string (just before you combine into one char array).

Originally Posted by laughing_man

I am using PellesC and it has that getline function you spoke about. But it seems to be similar to fgets, in that I have to mention th size there too and provide a char array to. Is there something that would just create an array and just return the pointer to me.

As I wrote in post #2, getline will do what you want. However, I assumed that you would have read the manual entry on getline.