第一次尝试用英文写博客，先写篇超级简单的总结，有问题还望指正。

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If you are writing program by C programming language, you may need to care about how many memories to allocate, and when you should deallocate them. That is a heavy work for programmers, but at the same time it is very interesting. You will recognize that when defining a float variable, how many memories we need to allocate? Why a function can modify extern variables by passing the pointer of them?

As for me, I love dynamic programming language, such as Ruby, JavaScript, Python for creating things quickly. But sometimes I need system programming language like C. In my opinion, comparing to some modern system language C programming language is easy enough. In this article, I want to talk about the memory allocation for struct and union data structure when using C programming language.

How many memories I need?

As we know struct and union are the composite data structure. They are combined by some basic data type, like float, double, long, pointer, etc. So if you want to calculate how many memories we need to allocate for a composite data structure, you should know the basic data type first.

Different data type may takes different memory spaces. We also measure the memory costs by byte instead of bit in machine underlayer. 1 byte usually equals to 8 bits and 1 byte also represented by char data type in C programming language. You can use this simple program to valid it.

#include <stdio.h>

int main() {
  unsigned long size = sizeof(char);
  char * unit = size > 1 ? "bytes" : "byte";
  printf("Char data type take %ld %s.", size, unit);
}

The result is

Char data type take 1 byte.

Also you can check others data type like long, int, double and float.

#include <stdio.h>

char * unit(size) {
  return size > 1 ? "bytes" : "byte";
}

int main() {
  unsigned long c_size = sizeof(char);
  unsigned long i_size = sizeof(int);
  unsigned long l_size = sizeof(long);
  unsigned long f_size = sizeof(float);
  unsigned long d_size = sizeof(double);

  printf("Char data type takes %ld %s\n", c_size, unit(c_size));
  printf("Int data type takes %ld %s\n", i_size, unit(i_size));
  printf("Long data type takes %ld %s\n", l_size, unit(l_size));
  printf("Float data type takes %ld %s\n", f_size, unit(f_size));
  printf("Double data type takes %ld %s\n", d_size, unit(d_size));
}

The result is

Char data type takes 1 byte
Int data type takes 4 bytes
Long data type takes 8 bytes
Float data type takes 4 bytes
Double data type takes 8 bytes

PS: I ran the program in the 64-bit machine/system, some data type may have different memory costs in different machine/system. But don't worry about that now. Today, most of time we are using the 64-bit operating system.

Composite data type

We know that different data type may takes different memory spaces. Sometimes same data type in C may have different memory cost when running in different system/machine. If we alway use the 64-bit machine, how many memories will the composite data structure takes? Different composite data structure have different rule.

1. struct

First of all, let's focus on the struct composite data type. In short, machine must allocate enough memory to store all the data fields which included in struct. I will show you some example, After that you will know how to calculate the memory cost in struct data structure. So, How may memories we need for the below data structure?

struct Info1 {
  int a; // 4 bytes
  long b; // 8 bytes
  char c; // 1 byte
  double d; // 8 bytes
};

The answer is 32 bytes. Why? In fact, 21 bytes is enough to store all the data fields, why 32 bytes? This situation is called as Data Alignment. Because of that we have some wasted memory spaces. To avoiding wasted we can change the order of the data fields.

struct Info2 {
  int a; // 4 bytes
  char c; // 1 byte
  long b; // 8 bytes
  double d; // 8 bytes
};

Now we just need to allocate 24 bytes, just have 3 bytes wasted memory spaces. Let's valid that by this program.

#include <stdio.h>

struct Info1 {
  int a; // 4 bytes
  long b; // 8 bytes
  char c; // 1 byte
  double d; // 8 bytes
};

struct Info2 {
  int a; // 4 bytes
  char c; // 1 byte
  long b; // 8 bytes
  double d; // 8 bytes
};

int main() {
  printf("Info1 need %ld bytes\n", sizeof(struct Info1));
  printf("Info2 need %ld bytes\n", sizeof(struct Info2));
}

we will get the result

Info1 need 32 bytes
Info2 need 24 bytes

In this situation it uses 8 bytes aligned, because the longest field in our struct is 8 bytes. Sometimes machine may take others aligned strategy. Let me show you more example.

...

struct Info3 {
  char a; // 1 byte
  char b; // 1 byte
  char c; // 1 byte
};

struct Info4 {
  char a; // 1 byte
  char b; // 1 byte
  short c; // 2 bytes
};

struct Info5 {
  char a; // 1 byte
  short b; // 1 byte
  int c; // 4 bytes
};

...

What about the struct struct Info3, struct Info4, struct Info5? the result is

Info3 need 3 bytes
Info4 need 4 bytes
Info5 need 8 bytes

1. All fields in struct Info3 are 1 byte cost, it is 1 byte aligned. Totally, machine just need to allocate 3 bytes.
2. The longest field in struct Info4 is short c, it will takes 2 bytes. So machine will use 2 bytes aligned strategy. The field char a, char b can store in the first 2-bytes space, and the short c will store in the second one. Totally, machine need to allocate 4 bytes.
3. Similar to struct Info4, the longest field in struct Info5 is int c, it will take 4 bytes. So machine will use 4 bytes aligned strategy. The field char a, short b can store in the first 4-bytes space, and int c will store in the second one. Finally, machine need to allocate 8 bytes.

That is all about struct data structure memory allocation. So easy, right? Next, let's go to the union composite data type.

2. union

union is another important composite data structure in C programming language, it is similar to struct, except the memory allocation strategy. In short, The memories we need for union is decided by the longest field in it. As an example, let's create union Info6, it's fields are same as struct Info1

union Info6 {
  int a; // 4 bytes
  long b; // 8 bytes
  char c; // 1 byte
  double d; // 8 bytes
};

The result is

Info6 need 8 bytes

We just need 8 bytes now. Comparing to struct Info1, it saves too many memories. The longest field in this union is 8 bytes, all fields will share the same memory spaces. You can image that we just store what we want to store in it, and then tell the machine how to read them. All data in it just the binary, so reading them in the different way may get the different result. Let's valid them by a program.

....

union Info6 {
  int a;
  long b;
  char c;
  double d;
};

int main() {
  union Info6 info;
  info.b = 0xffffffffffffffff; // Set all bits are 1
  printf("Read by double is %lf.\n", info.d);
  printf("Read by int is %d.\n", info.a);
  printf("Read by long is %ld.\n", info.b);
  printf("Read by char is %d.\n", info.c);
}

The result is

Read by double is nan.
Read by int is -1.
Read by long is -1.
Read by char is -1.

As the conclusion above, we need to allocate 8 bytes for union Info6. First of all, we define an variable info, and then set all bits in it are 1.

1111111111111111111111111111111111111111111111111111111111111111

If we recognize this bit strings as double type we will got

1 11111111111 1111111111111111111111111111111111111111111111111111

Following the IEEE775 standard, it is the schema of NaN. If we recognize this as int type we just need to calculate the low 32-bits

11111111111111111111111111111111

The value of this bit strings is -1. If we recognize this as long type, the bit strings we need to calculate will be the entire 64-bits

1111111111111111111111111111111111111111111111111111111111111111

The value of this bit strings is -1. If we recognize this as char type we just need to calculate the low 8-bits

11111111

So the value is the same -1.

3. conclusion

Both struct and union are composite data structure but have different memory allocation strategy. In summary, struct need to store all the fields in the limited memory spaces as possible as it can. union will share the memory spaces between all fields, so sometimes you need an extra field to target that which type you would like to use.

"Hard" Example

Finally, let's practice what we have learned so far. How many memories we need for this data structure?

struct HardExample {
  union {
    int a; // 4 bytes
    long b; // 8 bytes
  } union1;

  union {
    int c; // 4 bytes
    double d; // 8 bytes
  } union2;

  struct {
    long e; // 8 bytes
    char f; // 1 byte
    double g; // 8 bytes
    int h; // 4 bytes
  } struct1;
};

It looks hard, but easy in fact, right? The memories required for union1 and union2 is decided by the longest fields in them. The longest field in union1 is long type, 8-bytes cost. The longest field in union2 is long type, 8-bytes cost. So each of them takes 8 bytes memory spaces. The longest field in struct1 is double or long, it will be 8-bytes data alignment. So it need 8 + (1 + 7) + 8 + (4 + 4) = 32 bytes. Totally, we need 32 + 8 + 8 = 48 bytes for struct HardExample.

HardExample need 48 bytes

End

Thank you for your reading. I think that is all what I want to talk about. Is it clear enough for you? If not, please contact me and give me some advice.