Question

I have a code, but have a warning when it was compiling, this was the warning:

1_redis.c: In function \342\200\230main\342\200\231:
1_redis.c:131:23: warning: assignment makes integer from pointer without a cast
 [enabled by default]

it says assignment makes integer from pointer without a cast, but cFile and lQueryData are all char* type, why?

#define MAX_LINE_NUM 8000000
#define EACH_THREAD_NUM 10000


long i,posF,posDB;
for (i=0;i<DB_NUM;i++) { lQueryPerDB[i] = 0; }
char *lQueryData = (char *)malloc(DB_NUM*MAX_LINE_NUM*sizeof(char));

lLineCount = lFileLen / lLineLen;
for (i=0;i<lLineCount;i++) {
    posF = i * lLineLen;
    iDB = get_DB(cFile[posF]);
    posDB = iDB * MAX_LINE_NUM + lQueryPerDB[iDB];
    lQueryData[posDB] = &cFile[posF];                   // this line have warning!!!!
    lQueryPerDB[iDB]++;
}

Answer 1

If cFile is a char *, indexing it like cFile[posF] is the same as doing *(cFile + posF), or “give me the value of whatever is posF places from the start of the array.” Your address-of operator (&) is unnecessary, and if it weren’t, it would probably be the opposite of what you wanted (you want dereference — * — which is automatically done for you with the subscript notation).

As other people have suggested, the correct code is most likely:

lQueryData[posDB] = cFile[posF];

The reason you get the particular warning you do is because using &cFile[posF] gets the address of (i.e., a pointer to) the character in cFile at posF. However, when you then try to assign it to a place in the lQueryData array, it has to first cast that (internally) into a scalar type. A pointer is just a number that indexes into memory, and thus when used for anything but “pointing” to things, it’s just an integer. Therefore, what you have is making an integer, from a pointer, without an explicit cast.

You might consider compiling with clang instead of (what I presume is) GCC if you have access to it, it has much more accessible error messages. For instance, the error message for your code above is: warning: incompatible pointer to integer conversion assigning to ‘char’ with an expression of type ‘char *’; remove & (with a little visual pointer to the offending “&” sign).