#pragma pack（）调用有什么作用？

Question

I was given the following structs:

struct s2 {
int f0;
char f1;
unsigned long long f2;
unsigned long f3;
short f4;
long f5;
};

#pragma pack (1)
struct s2_packed {
int f0;
char f1;
unsigned long long f2;
unsigned long f3;
short f4;
long f5;
};
#pragma pack ()

struct s3 {
unsigned short f0;
int f1;
unsigned int f2;
int f3;
unsigned short f4;
char f5;
};

I understand s2_packed should be packed now, but is s3 packed? What does the call #pragma pack () do?

UPDATE

I just examined the bytes in s3 and they are indeed packed.. Not sure whats going on here, any ideas?

UPDATE

Here is my makefile I was given:

all: 
 gcc -m32 -g -O -Wall struct.c main.c -o struct

clang:
 clang -m32 -g -O -Wall struct.c main.c -o struct

strict:
 gcc -m32 -g -O -Wall -Werror struct.c main.c -o struct

clang-strict:
 clang -m32 -g -fsanitize=undefined -O -Wall -Werror struct.c main.c -o struct

clean:
 rm -f struct

Answer 1

#pragma pack ()

with gcc and some other compilers restores the default packing of the implementation.

Documentation for gcc of this #pragma is here: http://gcc.gnu.org/onlinedocs/gcc/Structure_002dPacking-Pragmas.html