Flexible Array Members in a structure in C

Flexible Array Member(FAM) is a feature introduced in the C99 standard of the C programming language.

• For the structures in C programming language from C99 standard onwards, we can declare an array without a dimension and whose size is flexible in nature.
• Such an array inside the structure should preferably be declared as the last member of the structure and its size is variable(can be changed at runtime).
• The structure must contain at least one more named member in addition to the flexible array member.

What must be the size of the structure below?

The size of structure is = 4 + 4 + 4 + 0 = 12

In the above code snippet, the size i.e. length of array "stud_name" isn't fixed and is a FAM. The memory allocation using flexible array members(as per C99 standards) for the above example can be done as:

struct student *s = malloc( sizeof(*s) + sizeof(char [strlen(stud_name)]) );

Note: While using flexible array members in structures some convention regarding the actual size of the member is defined. The convention is that the flexible array member does not contribute to the size of the structure itself. Instead, the size of the structure is determined by the other data members plus any memory allocated dynamically for the flexible array.

In the above example, the convention is that the member "stud_name" has a size of the number of characters stored.

For Example, Consider the following structure:

Input : id = 15, name = "Kartik" 
Output : Student_id : 15
 Stud_Name : Kartik
 Name_Length: 6
Allocated_Struct_size: 18

Memory allocation of the above structure: 

struct student *s = 
 malloc( sizeof(*s) + sizeof(char [strlen("Kartik")]));

Its structure representation is equal to: 

Example

The below C program shows the implementation of flexible array members.

Student_id : 523
Stud_Name : Cherry
Name_Length: 6
Allocated_Struct_size: 19

Student_id : 535
Stud_Name : Sanjayulsha
Name_Length: 11
Allocated_Struct_size: 24

Size of Struct student: 12
Size of Struct pointer: 8

Output

Student_id : 523
Stud_Name : SanjayKanna
Name_Length: 11
Allocated_Struct_size: 23

Student_id : 535
Stud_Name : Cherry
Name_Length: 6
Allocated_Struct_size: 18

Size of Struct student: 12
Size of Struct pointer: 8

Important Points

1. Adjacent memory locations are used to store structure members in memory.
2. In previous standards of the C programming language, we were able to declare a zero-size array member in place of a flexible array member. The GCC compiler with C89 standard considers it as zero size array.