C Segfaults when trying to free memory that no longer needs to be used -

i working on circular buffer program in c. buffer has able expand , shrink depending on conditions checked each time buffer updated new quote (the buffer contains stock quotes). however, when try expand buffer , free old (smaller) buffer, "invalid next size" errors gilbc.

my expand function follows:

cbuf* expandbuffer(cbuf *cb_ptr){       int newsize;     newsize = (cb_ptr->maxsize * 2) -1;     cbuf *tempbuffer = malloc(sizeof(cbuf) + newsize * sizeof(quote));     tempbuffer ->maxsize = cb_ptr->maxsize * 2;     tempbuffer ->start = cb_ptr->start;     tempbuffer ->end = cb_ptr->end;     tempbuffer ->freeslots = tempbuffer->maxsize - cb_ptr->maxsize;      int x;     int counter;     counter = 0;      for(x = cb_ptr->end; x < cb_ptr->maxsize; x ++){          tempbuffer->quotebuffer[counter].time = cb_ptr->quotebuffer[x].time;         tempbuffer->quotebuffer[counter].rate = cb_ptr->quotebuffer[x].rate;         counter ++;     }     int y;     for(y = 0; y < cb_ptr->start; y ++){          tempbuffer->quotebuffer[counter].time = cb_ptr->quotebuffer[y].time;         tempbuffer->quotebuffer[counter].rate = cb_ptr->quotebuffer[y].rate;         counter++;     }      tempbuffer->start = cb_ptr->maxsize;     tempbuffer->end = 0;     free(cb_ptr);     return tempbuffer;  } 

and update function follows:

void cbuf_update(cbuf *cb_ptr, unsigned int time, double rate){      *code*      if(cb_ptr->freeslots == 0){         printf("\n\nexpanding circular buffer!\n\n");         *cb_ptr = *(expandbuffer(cb_ptr));       }         *more code here edited out since doesnt pertain question. } 

and main:

int main(){      cbuf *cb1 ;     cb1 = cbuf_init() ;     cbuf_update(cb1, 60, 1.291) ;     cbuf_update(cb1, 63, 1.287) ;     cbuf_update(cb1, 63, 1.231) ;     cbuf_update(cb1, 69, 1.229) ;     cbuf_update(cb1, 72, 1.247) ;     cbuf_update(cb1,361,1.291);     cbuf_update(cb1, 411, 1.291) ;     cbuf_update(cb1, 412, 1.281) ;     cbuf_update(cb1, 413, 1.292) ;     cbuf_update(cb1, 414, 1.284) ;     cbuf_update(cb1, 414, 1.290) ;     cbuf_update(cb1, 511, 1.241) ;     cbuf_update(cb1, 512, 1.251) ;     cbuf_update(cb1, 513, 1.232) ;     cbuf_update(cb1, 514, 1.202) ;     cbuf_update(cb1, 517, 1.119) ;     cbuf_update(cb1, 551, 1.080) ;     cbuf_update(cb1, 552, 1.081) ;     cbuf_update(cb1, 553, 1.079) ;     cbuf_update(cb1, 554, 1.088) ;     cbuf_update(cb1, 561, 1.072) ;     cbuf_update(cb1, 562, 1.113) ;     cbuf_update(cb1, 563, 1.091) ;     cbuf_update(cb1, 564, 1.092) ;     cbuf_update(cb1, 571, 1.089) ;     cbuf_update(cb1, 572, 1.073) ;     cbuf_update(cb1, 573, 1.061) ;     cbuf_update(cb1, 574, 1.111) ;     cbuf_update(cb1, 581, 1.119) ;     cbuf_update(cb1, 582, 1.123) ;     cbuf_update(cb1, 583, 1.151) ;     return 0; } 

this segfaults @ line:

cbuf *tempbuffer = malloc(sizeof(cbuf) + newsize * sizeof(quote)); 

in expandbuffer after has expanded once successfully. if need expand buffer more once, segfault on line.

however, if comment out:

    free(cb_ptr); 

i no longer segfaults. however, cb_ptr (the "old" buffer being replaced new, larger buffer) must freed since no longer in use. confused why freeing , returning new, larger buffer cb_ptr in cbuf_update cause segfaults.

the problem this:

*cb_ptr = *(expandbuffer(cb_ptr)); 

in expandbuffer free pointer, dereference free'd pointer in assignment. causes undefined behavior, can (and in case definitely) lead crash.

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i suggest instead pass pointer reference (i.e. pointer pointer) cbuf_update function:

void cbuf_update(cbuf **cb_ptr, unsigned int time, double rate) {     /* ... */      if((*cb_ptr)->freeslots == 0)     {         printf("\n\nexpanding circular buffer!\n\n");         *cb_ptr = expandbuffer(*cb_ptr);     }      /* ... */ } 

call new function address-of operator pointer:

cbuf_update(&cb1, 60, 1.291) ;