The wonderful use of 0 length array in C language

1、0 Length array

      0 Length array means that the length of the array is 0, such as int a[0], It does not take up memory space after compilation , namely sizeof(a) = 0.

      C99 The regulation can be Variable length array  int len ; int array[len]; In other words, the size of the array is specified only when the program runs . It is common to input data for initialization .

      GCC Directly supports zero length arrays ,int a[0]; A characteristic of zero length arrays is that they do not occupy memory space .0 Length arrays are rarely used alone , It is generally used in structures , It does not occupy the length of the structure , Used to allocate contiguous memory .

2、0 The use of length array in structure

     0 Length arrays are often used in structures , The following code is a structure with 0 Length array . This structure sizeof(struct rndis_query_msg) = 7 A word =28 Byte length ,OIDInputBuffer[0] Unoccupied space in the structure .

/* Remote NDIS Query Message */
struct rndis_query_msg
{
    rt_uint32_t MessageType;
    rt_uint32_t MessageLength;
    rt_uint32_t RequestId;
    rt_uint32_t Oid;
    rt_uint32_t InformationBufferLength;
    rt_uint32_t InformationBufferOffset;
    rt_uint32_t Reserved;
    rt_uint8_t  OIDInputBuffer[0];
};
typedef struct rndis_query_msg* rndis_query_msg_t;

      OIDInputBuffer It is mainly used to access structure member variables when allocating memory Reserved Continuous memory usage later . For example, to apply for a struct rndis_query_msg Memory space occupied by structure , also OIDInputBuffer The memory pointed to has 10 Bytes of space can be used . The code can be written as follows .

rndis_query_msg_t prndis = NULL;

prndis = malloc(sizeof(struct rndis_query_msg) + 10);

if(prndis)
{
    for(i = 0; i < 10; i++)
    {
        prndis->OIDInputBufferp[i] = i;
    }

}

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3、 Supported compilers

     0 The length array is written in IAR compiler ,GCC Supported in compiler . But in keil Compiler is not supported by default , Even if the C99 The setting of is not easy to use .keil In the software, you need to add... In the compilation options --gcc Can only be compiled through .