Thread: How to set an 32 bit allocation to 31 bit for address, 1 bit for flag?

I am using the following code to read in the characters of a file and put them into an array. Thus, each array element contains a character from the file

Code:

unsigned char *str;
str = (unsigned char*)malloc(filelength*sizeof(unsigned char));
fread(str, sizeof(unsigned char), filelength, file);

But how do I just assign 31 bits of str as the address location and the MSB 1 bit as a flag bit?

This flag bit is used to indicate if this array element is read before. If this array element is read before, then the flag bit is set to 1.

I think it is possible to assign an integer to every array element as the flag bit, but that will be a waste of space, using 1 byte as a flag bit.

Any suggestions or help! Thanks!