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Flexible array members are a special type of array where the last element of a struct structure with more than one named member has an incomplete array type; that is, the size of the array is not specified explicitly within the structstructure.

If a dynamically sized structure is needed, flexible array members should be used.

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In the following non-compliant code, an array of size 1 is declared, but when the struct structure itself is instantiated, the size computed for malloc() is modified to take into account the full size of the dynamic array.

struct flexArrayStruct {
     int num;
     int data[1];
};

/* ... */
/* Space is allocated for the struct */
struct flexArrayStruct *structP = malloc(sizeof(struct flexArrayStruct) + sizeof(int) * (ARRAY_SIZE - 1));
if (!structP) {
     /* handle malloc failure */
}
structP->num = SOME_NUMBER;

/* Access data[] as if it had been allocated as data[ARRAY_SIZE] */
for (i = 0; i < ARRAY_SIZE; i++) {
  structP->data[i] = i;
}

However, in thethis abovenon-compliant code example, the only member that is guaranteed to be valid, by strict C99 definition, is {{structP->data\[0\]}}. ThusConsequently, for all {{i > 0}}, the results of the assignment are undefined.

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struct flexArrayStruct{
  int num;
  int data[];
};

/* ... */
/* Space is allocated for the struct */
struct flexArrayStruct *structP = malloc(sizeof(struct flexArrayStruct) + sizeof(int) * ARRAY_SIZE);
if (!structP) {
     /* handle malloc failure */
}

structP->num = SOME_NUMBER;

/* Access data[] as if it had been allocated as data[ARRAY_SIZE] */
for (i = 0; i < ARRAY_SIZE; i++) {
  structP->data[i] = i;
}

TheThis priorcompliant methodsolution allows the structstructure to be treated as if it had declared the member {{data\[\]}} to be {{data\[ARRAY_SIZE\]}} in a waymanner that conforms to the C99 standard.

However, some restrictions do apply. :

1. The incomplete array type must be the last element within the

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1. structure.
2. You

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1. cannot have an array of

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1. structure if the

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1. structure contains flexible array members

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1. .
2. Structures that contain a flexible array member cannot be used as a member in the middle of another

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1. structure.

Risk Assessment

Although the non-compliant method approach results in undefined behavior, it will does work under most architectures.

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