How to add struct to an array of structs in C?

Question

Ok I have a struct defined as such...

typedef struct
{
    enum COMMAND command;
    enum CMD_SOURCE source;
    CHAR parameters[16];
} focuserCommand;

I am trying to make a function that will add an instance of this type of struct to an array of focuserCommands. The array is defined like this...

extern focuserCommand CommandBuffer[CMD_BUFFER_SIZE];

the function I am trying to write should take a pointer to a focuserCommand and add it to CommandBuffer. I am implementing CommandBuffer as a FIFO ring buffer so I know i need to move the tail forwared so that my other functions can see that the buffer contains data. CmdBuffHead and CmdBuffTail represent the read and write pointers of the buffer. Writes are added to the tail, reads are made from the head.

void AddCmdToBfr( focuserCommand * cmd )
{
// What goes here to add the struct pointed to by cmd to 
    //   element CmdBuffTail of the buffer?

    CmdBuffTail++;
}

Jonathan Leffler · Answer 1 · 2011-01-11T15:23:42.517

Given that you have the variable CmdBuffTail pointing at the next element of CommandBuffer to be written to, then:

void AddCmdToBfr(focusCommand *cmd)
{
    assert(CmdBuffTail >= 0 && CmdBuffTail < CMD_BUFFER_SIZE);
    CommandBuffer[CmdBuffTail++] = *cmd;
    if (CmdBuffTail >= CMD_BUFFER_SIZE)
        CmdBuffTail = 0;
}

This preserves the invariant that you need - that the index is in the range 0..CMD_BUFFER_SIZE-1.

You may also need a separate index (CmdBuffHead perhaps) to identify where to read from. If so, you would probably need to alter that value in AddCmdToBfr() if CmdBuffTail if catches up with it.

After fixing a goof copying the incoming command, it occurs to me that you need to think carefully about the memory management. In this case, your focususerCommand structure is simple enough (unless the typedef CHAR hides a pointer - in which case, slap wrists; don't hide pointers) that you can simply copy it with impunity. If you can't copy the structure simply (because it contains pointers to allocated memory), then you need to ensure you understand who owns the data to make sure that (a) any allocated memory is released, and (b) any allocated memory is released just once. This would mean, for example, that instead of just copying the new command over the old, you would have to first release the memory allocated to the old entry. Think in terms of C++ copy constructors and destructors - in C.

i tried this but I get a compiler error at the line `CommandBuffer[CmdBuffTail++] = cmd;` saying incompatible types in assignment. Is it because cmd is a pointer to a struct and not the struct itself? — PICyourBrain, Jan 11 '11 at 15:03
@Jordan: my mistake - I forgot to dereference (copy) the `cmd` variable; I fixed it to `CommandBuffer[CmdBuffTail] = *cmd;`. Apologies. — Jonathan Leffler, Jan 11 '11 at 15:06

How to add struct to an array of structs in C?

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