Dereferencing type-punned pointer will break strict-aliasing rules

Question

I used the following piece of code to read data from files as part of a larger program.

double data_read(FILE *stream,int code) {
        char data[8];
              


        

Answer 1

It looks a lot as if you really want to use fread:

int data;
fread(&data, sizeof(data), 1, stream);

That said, if you do want to go the route of reading chars, then reinterpreting them as an int, the safe way to do it in C (but not in C++) is to use a union:

union
{
    char theChars[4];
    int theInt;
} myunion;

for(int i=0; i<4; i++)
    myunion.theChars[i] = fgetc(stream);
return myunion.theInt;

I'm not sure why the length of data in your original code is 3. I assume you wanted 4 bytes; at least I don't know of any systems where an int is 3 bytes.

Note that both your code and mine are highly non-portable.

Edit: If you want to read ints of various lengths from a file, portably, try something like this:

unsigned result=0;
for(int i=0; i<4; i++)
    result = (result << 8) | fgetc(stream);

(Note: In a real program, you would additionally want to test the return value of fgetc() against EOF.)

This reads a 4-byte unsigned from the file in little-endian format, regardless of what the endianness of the system is. It should work on just about any system where an unsigned is at least 4 bytes.

If you want to be endian-neutral, don't use pointers or unions; use bit-shifts instead.