Can this FindString function be optimized further, in terms of speed?

int FindStr(char* str, int strsize, char* fstr, int from)
{
    for(int i=from, j=0; i<strsize; i++)
    {
        if(str[i]==fstr[j])
            j++;
        else
            {i-=j; j=0;}

        if(fstr[j]=='\0')
            return i-j+1;
    }

    return -1;
}

The function searches for a string fstr in str and returns its index in str if found, otherwise, it will return -1. It's also possible to specify where to start searching in the string.

My question is, can I optimize this function further? Also, do you see any potential problems in this function?

Some comments to add to the first two of @200_success:

-
strstr does a good job. Why reinvent?

-
sizes are often passed as size_t rather than int

-
modifying the loop variable within the loop is generally considered bad
practice

-
i -= j executes on every loop unless there is a match. Mostly in this
case j is zero so the line has no effect, but it still executes

-
if fstr is an empty string it returns the wrong result (1)

-
add some spaces around operators and after if, for

StackExchange Code Review Q#38230, answer score: 4