Assignment to a struct array failed with "error: initializer element is not constant"

Question

When I tried to initialize a global array that contains structure elements, I got the " error: initializer element is not constant"

a.h

#define MAX_MSG_NUM 20
#define NAME_SIZE   15
#define MAX_QUE_NUM  10

typedef struct {
   int         index;
   int         tid;
   int         front;
   int         rear;
   char        name[NAME_SIZE];
   char        msgbuf[MAX_MSG_NUM];
} THREAD;

typedef enum {
   I1        = 0,
   I2        = 1,
   I3        = 2,
   I4        = 3
} DMTHREAD;

a.c

THREAD   a[MAX_MSG_NUM];
THREAD   b[MAX_MSG_NUM];
THREAD   c[MAX_MSG_NUM];

THREAD   T[MAX_QUE_NUM] = {
   {I1,   0, 0, 0, "CONFIG1",      a[MAX_MSG_NUM]},
   {I2,   0, 0, 0, "CONFIG2",      b[MAX_MSG_NUM]},
   {I3,   0, 0, 0, "CONFIG3",       c[MAX_MSG_NUM]},
   0
};

GCC compiler.

bash-3.2$ gcc -g a.h a.c
a.c:8: error: initializer element is not constant
a.c:8: error: (near initialization for 'T[0].msgbuf[0]')
a.c:9: error: initializer element is not constant
a.c:9: error: (near initialization for 'T[1].msgbuf[0]')
a.c:10: error: initializer element is not constant
a.c:10: error: (near initialization for 'T[2].msgbuf[0]')

Thanks for help.

Enlightened by your answers, I changed the code and it works fine now.

MSG   T[MAX_QUE_NUM] = {
           {I1,   0, 0, 0, "CONFIG1",      {0}},
           {I2,   0, 0, 0, "CONFIG2",      {0}},
           {I3,   0, 0, 0, "CONFIG3",      {0}}, 
           0,
    }

Thank you for your answers.

Asaf · Accepted Answer · 2012-02-28 08:23:34Z

3

a[MAX_MSG_NUM] is indeed non-constant. It also denotes a Thread instance, which happens to be outside the array you defined earlier, and does not match the type you should pass there (char[]).

So it is wrong on so many levels.

answered Feb 28, 2012 at 8:23

Asaf

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1 Comment

Joe.Z Over a year ago

You are right, I posted wrong code here, sorry. Thread was supposed to be replaced by a self-defined type MSG.

Coren · Accepted Answer · 2012-02-28 08:21:28Z

2

You're trying to assign a value that isn't knowable to the compiler at compile time but knows at run time - this is why it's causing the error.

try doing it in main for example:

main(){
  THREAD   T[MAX_QUE_NUM] = {
     {I1,   0, 0, 0, "CONFIG1",      a[MAX_MSG_NUM]},
     {I2,   0, 0, 0, "CONFIG2",      b[MAX_MSG_NUM]},
     {I3,   0, 0, 0, "CONFIG3",       c[MAX_MSG_NUM]},
     0
  };
}

answered Feb 28, 2012 at 8:21

Coren

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Comments

ouah · Accepted Answer · 2012-02-28 08:22:44Z

2

In C, objects with static storage duration such as objects declared at file scope can only be initialized with constant expressions.

For example (declared at file scope):

char bla[] = {1, 2, 3, 4};  // correct
int a = 1;
char blop[] = {a, a + 1};   // incorrect, a is not a constant in C

answered Feb 28, 2012 at 8:22

ouah

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