attribute method:

#include <stdio.h>

struct packed
{
    char a;
    int b;
} __attribute__((packed));

struct not_packed
{
    char a;
    int b;
};

int main(void)
{
    printf("Packed:     %zu\n", sizeof(struct packed));
    printf("Not Packed: %zu\n", sizeof(struct not_packed));
    return 0;
}

Output:

$ make example && ./example
cc     example.c   -o example
Packed:     5
Not Packed: 8

pragma pack method:

#include <stdio.h>

#pragma pack(1)
struct packed
{
    char a;
    int b;
};
#pragma pack()

struct not_packed
{
    char a;
    int b;
};

int main(void)
{
    printf("Packed:     %zu\n", sizeof(struct packed));
    printf("Not Packed: %zu\n", sizeof(struct not_packed));
    return 0;
}

Output:

$ make example && ./example
cc     example.c   -o example
Packed:     5
Not Packed: 8

-fpack-struct[=n]

Without a value specified, pack all structure members together without holes. When a value is specified (which must be a small power of two), pack structuremembers according to this value, representing the maximum alignment (that is, objects with default alignment requirements larger than this will be outputpotentially unaligned at the next fitting location.

Warning: the -fpack-struct switch causes GCC to generate code that is not binary compatible with code generated without thatswitch. Additionally, it makes the code suboptimal. Use it to conform to a non-default application binary interface.