Question

I am developing a routing protocol on a Linux OS (ubuntu 12.04) in C language. My question is that I need to save the routing table on an external file, so that if the program is closed or the computer is shut down, the program should be able to fetch the routing table from the file when it is restarted.

What would be the easiest solution for it? I would appreciate if you could explain it a little bit. If it would help you to answer, I save the table struct routing user_list[40] here below: the smaller index has a higher priority in the table.

struct routing {
   int hop_distance;
   char senderID[16]; // 192.168.001.122
   char gateway[16];
};

struct routing user_list[40] = { [0] = {0, {0}, {0}, {0} } };

Thank you in advance.

Answer 1

man fread

size_t fread(void *ptr, size_t size, size_t nmemb, FILE *stream);
size_t fwrite(const void *ptr, size_t size, size_t nmemb, FILE *stream);
...
RETURN VALUE
fread()  and  fwrite() return the number of items successfully read or written (i.e., not the number of charac‐
ters).  If an error occurs, or the end-of-file is reached, the return value is a short item count (or zero).

the easiest solution

int nb_written = fwrite(&user_list, sizeof(struct routing), 40, myfile);
int nb_read = fread(&user_list, sizeof(struct routing), 40, myfile);

As indicated in the comments:

• the file generated will not be portable due to int type which is dependent of architecture.
• the returned values must be checked to handle io errors