is there a function like sizeof that can be used at run time not compile time?

[Onceler]

I am trying to make a program that reads several (lets say eight) bytes from a file into an array but what if there is only a few bytes left (lets say three)?
Is an array the best option for doing this and if so how do I do a sizeof to determine how many bytes are left in the file.
I am using C not C++ but is there a way to do it in either language and which one should I be using? I don't like OOP.
I will be reading from the file into long long int or hex.

[Chervil]

I presume you mean "only a few bytes left" in the file.

You could take a look at fread() where you can request a certain number of bytes to be read, and the return value tells you how many bytes were actually read.

http://www.cplusplus.com/reference/cstdio/fread/

[Disch]

The fread return value approach is one way.

Also if you want to know the size of a file, the typical way with the std library is to seek to the end, do a tell, then seek back to the beginning:

int filesize;
FILE* f = fopen("yourfile","rb");

fseek(f,0,SEEK_END);  // seek to the end
filesize = ftell(f);   // 'tell' to find out our current position (ie:  the file size)
fseek(f,0,SEEK_SET);  // seek back to beginning

//... 

Of course this will only work properly if the file is open in binary mode, since text mode translates line breaks as they're read, messing with the file size.

[Onceler]

Thank you so much for the replies, I will study this.

[Lowest0ne]

When you say "array", it makes me think you are reading your data into a char array and then converting the array into a number. You should be able to go directly to the number:

int x;
int result = fread(&x, sizeof(x), 1, myFile);

At least, a similar approach can be done with C++ "std::ifstream.read()".

I also wonder about your variable lengths of file. Typically, the data you read from a file (in binary mode particularly) is data that you wrote in a specific pattern. To know when that pattern breaks, you can put an unsigned before the pattern to signify the amount of data in the pattern.

4 Bytes :  Amount of People
  (one for each Item)
  4 Bytes : Id of person
  4 Bytes : length of name
    1 Byte   : one for each letter in name
  4 Bytes : Age of person
4 Bytes : Amount of Other things //... 

[Onceler]

I would like to keep it long long int avoiding the array if possible and the filesize would not be known at compile time.

[Chervil]

It might help to clarify the question if you could provide a sample of the file data, and state how it should be interpreted by the program,