c - Incomplete Type is not allowed, 2 structs -

i know there million of these posts, i'm not sure how fix this. because elua_adc_ch_state defined above used again? lines err marked.

typedef struct  {   // status bit flags   volatile u8     op_pending: 1, // there pending conversion?                   blocking: 1, // in blocking or non-blocking mode? (0 - blocking, 1 - nonblocking)                   freerunning: 1, // if true, don't stop when we've acquired requested number of samples                   smooth_ready: 1, // has smoothing filter warmed (i.e. smoothlen samples collected)                   value_fresh: 1; // whether value pointed value_ptr fresh    unsigned        id;    u8              logsmoothlen;   volatile u16    smoothidx;   volatile u32    smoothsum;   u16             *smoothbuf;    volatile u16    reqsamples;   volatile u16    *value_ptr; } elua_adc_ch_state;  typedef struct {   elua_adc_ch_state   *ch_state[ num_adc ]; *** <----   volatile u16        sample_buf[ num_adc ]; *** <----   volatile u8         clocked: 1,                       force_reseq: 1,                       skip_cycle: 1,                       running: 1; // whether or not sequence running   volatile u32        ch_active; // bits represent whether channel should converted on device   volatile u32        last_ch_active; // keep copy of old configuration   unsigned            timer_id, seq_id; // timer bound device, sequencer device id   volatile u8         seq_ctr, seq_len; } elua_adc_dev_state; 

all of u16, u8, num_adc assignments defined , included. i'm not sure why second struct fails when first fine... no circular header dependencies either.

thanks

most issue variable length arrays flexible array members @ top of structure, not portable. rules on vlas flexible array members require (§6.7.2.1p3) @ end of structure definition:

typedef struct{    int foo;    unsigned int bar;    int foobar [] } barfoo; 

i highly recommend not using vlas flexible array members exist in ansi c99 , optional in iso c11 (see §6.10.8.3) implemented more widely. the ansi standard not seem specify behavior using vlas in structures in c11 it's undefined behavior.

update:

per casey's comment below defined flexible array members (§6.7.2.1p18) have many cases of undefined behavior (§6.7.2.1p21) in standard , should avoided.