Failed to lookup symbol 'testCallBack': error code 127

Hello creator, I would like to ask why the error was reported only after callback. My English is not very good, so I rely on translation to communicate with you

#ifdef __cplusplus
extern "C"
{
#endif
int sum(int a, int b);

/** Subtracts 2 integers. */
int subtract(int *a, int b);

/** Multiplies 2 integers, returns pointer to an integer,. */
int *multiply(int a, int b);

/** Divides 2 integers, returns pointer to a float. */
float *divide(int a, int b);

/** Divides 2 floats, returns a pointer to double. */
double *dividePrecision(float *a, float *b);

static int g_jCount = 0;
int callDart(void (*callback)());

#ifdef __cplusplus
}
#endif