What is the difference between `cc -std=c99` and `

问题:

Given the following program:

/*  Find the sum of all the multiples of 3 or 5 below 1000. */

#include <stdio.h>

unsigned long int method_one(const unsigned long int n);

int
main(int argc, char *argv[])
{
        unsigned long int sum = method_one(1000000000);
        if (sum != 0) {
                printf("Sum: %lu\n", sum);
        } else {
                printf("Error: Unsigned Integer Wrapping.\n");
        }

        return 0;
}

unsigned long int
method_one(const unsigned long int n)
{
        unsigned long int i;
        unsigned long int sum = 0;
        for (i=1; i!=n; ++i) {
                if (!(i % 3) || !(i % 5)) {
                        unsigned long int tmp_sum = sum;
                        sum += i;
                        if (sum < tmp_sum)
                                return 0;
                }
        }

        return sum;
}

On a Mac OS system (Xcode 3.2.3) if I use cc for compilation using the -std=c99 flag everything seems just right:

nietzsche:problem_1 robert$ cc -std=c99 problem_1.c -o problem_1
nietzsche:problem_1 robert$ ./problem_1 
Sum: 233333333166666668

However, if I use c99 to compile it this is what happens:

nietzsche:problem_1 robert$ c99 problem_1.c -o problem_1
nietzsche:problem_1 robert$ ./problem_1 
Error: Unsigned Integer Wrapping.

Can you please explain this behavior?

回答1:

c99 is a wrapper of gcc. It exists because POSIX requires it. c99 will generate a 32-bit (i386) binary by default.

cc is a symlink to gcc, so it takes whatever default configuration gcc has. gcc produces a binary in native architecture by default, which is x86_64.

unsigned long is 32-bit long on i386 on OS X, and 64-bit long on x86_64. Therefore, c99 will have a "Unsigned Integer Wrapping", which cc -std=c99 does not.

You could force c99 to generate a 64-bit binary on OS X by the -W 64 flag.

c99 -W 64 proble1.c -o problem_1

(Note: by gcc I mean the actual gcc binary like i686-apple-darwin10-gcc-4.2.1.)

回答2:

Under Mac OS X, cc is symlink to gcc (defaults to 64 bit), and c99 is not (defaults to 32bit).

/usr/bin/cc -> gcc-4.2

And they use different default byte-sizes for data types.

/** sizeof.c
 */
#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv)
{
   printf("sizeof(unsigned long int)==%d\n", (int)sizeof(unsigned long int));

   return EXIT_SUCCESS;
}

cc -std=c99 sizeof.c
./a.out
sizeof(unsigned long int)==8


c99 sizeof.c
./a.out
sizeof(unsigned long int)==4

Quite simply, you are overflowing (aka wrapping) your integer variable when using the c99 compiler.

.PMCD.