Question

gcc 4.4.4 c89

I have always done the following when using structs to hide the elements in the implementation file.

port.h header file

struct port_tag;
struct port_tag* open_ports(size_t port_id);
void close_ports(struct port_tag *port);

port.c implementation file

#include "port.h"
typedef struct port_tag {
    size_t port_id;
} port_t;

port_t* open_ports(size_t port_id)
{
    port_t *port = malloc(sizeof *port);
    if(port == NULL) {
        return NULL;
    }
    port->port_id = port_id;
    return port;
}

void close_ports(port_t *port)
{
    if(port != NULL) {
        free(port);
    }
}

driver.c driver file

#include "port.h"
int main(void)
{
    size_t i = 0;
    struct port_tag *port = NULL;

    for(i = 0; i < 5; i++) {
        port = open_ports(i);

        if(port == NULL) {
            fprintf(stderr, "Port [ %d ] failed to open\n", i);
        }
        close_ports(port);
    }
    return 0;
}

In the above code it is clear that the tag name is port_tag and the actual typedef’ed name is port_t.

However, I am re-engineering some one code. And I have found they have used a different method, that I have never seen before. I have a few questions about their method.

channel.h header file

typedef struct channel_t channel_t;
channel_t* open_channels(size_t channel_id);
void close_channels(channel_t *channel);

channel.c implementation file

#include "channel.h"
struct channel_t {
    size_t channel_id;
};

channel_t* open_channels(size_t channel_id)
{
    channel_t *channel = malloc(sizeof *channel);

    if(channel == NULL) {
        return NULL;
    }
    channel->channel_id = channel_id;
    return channel;
}

void close_channels(channel_t *channel)
{
    if(channel != NULL) {
        free(channel);
    }
}

driver.c driver file

#include "channel.h"   
int main(void)
{
    size_t i = 0;
    channel_t *channel = NULL;

    for(i = 0; i < 5; i++) {
        channel = open_channels(i);

        if(channel == NULL) {
            fprintf(stderr, "Channel [ %zu ] failed to open\n", i);
        }

        close_channels(channel);
    }
    return 0;
}

1) When they have declared the typedef’ed struct which is the tag name or the name of the
struct itself?

typedef struct channel_t channel_t;

2) In the implementation file shouldn’t the name of the struct follow the last curly brace?

struct channel_t <-- tag name {
    size_t channel_id;
} <-- itsn't this the name of the typedef'ed struct;

Many thanks for any advice,

Answer 1

1. The struct type is struct channel_t, and the new typedef is channel_t.

That means it can now be used as:

channel_t some_instance;

2. They’re creating a typedef elsewhere, not here. So all:

struct channel_t {
    size_t channel_id;
};

does is define a struct type with tag channel_t. No typedef is required when defining a struct. It can be used as, e.g.:

struct channel_t some_instance;

As you’ve seen, the two syntaxes are basically equivalent in this case.