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Wiki MarkupThe C99 standard \[ [ISO/IEC 9899:1999|AA. Bibliography#ISO/IEC 9899-1999] \] introduces flexible array members into the language. While flexible array members are a useful addition, they need to be understood and used with care.

The following is an example of a structure that contains a flexible array member:

...

struct flexArrayStruct flexStruct;
size_t array_size;
size_t i;

/* Initialize array_size */

/* Initialize structure */
flexStruct.num = 0;

for (i = 0; i < array_size; i++) {
  flexStruct.data[i] = 0;
}

Wiki MarkupThe problem with this code is that the {{flexArrayStruct}} does not actually reserve space for the integer array data; it can't because the size hasn't been specified. Consequently, while initializing the {{num}} member to zero is allowed, attempting to write even one value into data (that is, {{data\[0\]}}) is likely to overwrite memory outside of the object's bounds.

Compliant Solution (Storage Allocation)

...

struct flexArrayStruct *flexStruct;
size_t array_size;
size_t i;

/* Initialize array_size */

/* Dynamically allocate memory for the structure */
flexStruct = (struct flexArrayStruct *)malloc(
  sizeof(struct flexArrayStruct) + sizeof(int) * array_size
);
if (flexStruct == NULL) {
  /* Handle malloc failure */
}

/* Initialize structure */
flexStruct->num = 0;

for (i = 0; i < array_size; i++) {
  flexStruct->data[i] = 0;
}

...

The {{data\[\]}} member of {{flexStruct}} can now be accessed as described in C99, Section 6.7.2.1, paragraph 16.

Noncompliant Code Example (Copying)

...

ISO/IEC 9899:1999

Bibliography

Wiki Markup\[[JTC1/SC22/WG14 N791|http://www.open-std.org/jtc1/sc22/wg14/www/docs/n791.htm]\]

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MEM32-C. Detect and handle memory allocation errors      08. Memory Management (MEM)