Question

Buffer* b_create(int init_capacity, char inc_factor,char o_mode){

Buffer* buffer=NULL;    /*local pointer to the buffer structure on heap*/ 

if (o_mode=='f' || inc_factor==0){
    o_mode=0;
    inc_factor=0;
}else if (o_mode=='f' && inc_factor!=0){
    o_mode=0;
    inc_factor=0;
}else if (o_mode=='a' && 1<=inc_factor<=255){
    o_mode=1;
    inc_factor=inc_factor;
}else if (o_mode=='m' && 1<=inc_factor<=100){
    o_mode=-1;
    inc_factor=inc_factor;
}
else{
    buffer=NULL;
    return buffer;
}

    //throw a more meaningful error??

    //cleanup :: how about i just call the ca_destroy() ??
    /*
    free(buffer->ca_head);
    buffer->ca_head=NULL;
    free(buffer);
    buffer=NULL;
*/

buffer = (Buffer*)malloc(sizeof(Buffer));
buffer->ca_head=(char*)malloc(sizeof(char)*init_capacity);
buffer->mode=o_mode;
buffer->inc_factor=inc_factor;

buffer->capacity=init_capacity;
buffer->addc_offset=0;
buffer->mark_offset=0;
buffer->r_flag=0;

return buffer;

}

I have the above method. I am trying to get it to work. First of all, I check and set a couple of variables. If none of the cases apply, then I am setting the buffer to NULL and attempting to return the value to the calling function.

else{
    buffer=NULL;
    return buffer;
}

However after that return, the function continues and goes ahead and creates the buffer structure anyways. I was hoping that the function would exit once it sees a return statement. Why does it continue executing ??

Answer 1

This condition

1<=inc_factor<=255

is always true. It is parsed as (1 <= inc_factor) <= 255, and 1 <= inc_factor evaluates to either 0 or 1. So if o_mode is one of 'a' or 'm', one of the earlier conditions holds, even if inc_factor is 0 or negative.

Looking at the condition-chain, we find:

if (o_mode=='f' || inc_factor==0){
    o_mode=0;
    inc_factor=0;
}else if (o_mode=='f' && inc_factor!=0){ // This is never true, because if o_mode == 'f',
    o_mode=0;                            // the first one already was true
    inc_factor=0;
}else if (o_mode=='a' && 1<=inc_factor<=255){ // This is just o_mode == 'a'
    o_mode=1;
    inc_factor=inc_factor;
}else if (o_mode=='m' && 1<=inc_factor<=100){ // This is just o_mode == 'm'
    o_mode=-1;
    inc_factor=inc_factor;
}
else{ // This is only reached if o_mode is none of 'f', 'a', 'm' and inc_factor is != 0
    buffer=NULL;
    return buffer;
}

but if o_mode is none of 'f', 'a', 'm' and inc_factor != 0, then the final else is reached and NULL is returned. There is no implementation broken enough to ignore a return statement.