Passing a structure as a char buffer to a proc entry

Question

I want to pass a character buffer to a proc entry (say /proc/my_file) from my program. This character buffer contains my structure's elements which is of the form:

struct my_table { char src_ip[4]; char dest_ip[4]; int out_flag; }my_t;

I assign the elements of my_table and copy its contents to an unsigned char buffer as follows:

memcpy(buffer, &my_t, sizeof(struct my_table));

Then I write the buffer's contents into a proc entry created by me (by the name my_file) as:

write(fd, buffer, sizeof(buffer));

where fd is the file descriptor returned by open() after opening the /proc/my_file with O_WRONLY | O_APPEND flags.

What I couldn't understand is that I can only see the first character string i.e my_t.src_ip in this case to be written to /proc/my_file (did a

cat /proc/my_file

to check the content written) and subsequently I observed that the write() operation to /proc/my_file ends as soon as it encounters a null character in the buffer content.

Can I know why does this happen and how to solve this problem of writing a structure's contents into a /proc entry?

Edit: SSCCE for my question: The structure:

struct my_iptable { 
    char protocol[5];                   // to check whether the protocol mentioned is tcp/udp/icmp 
    char src_ip[16];                    // source ip address
    char dest_ip[16];                   // destination ip address
    char src_net_mask[16];              // source net mask
    char dest_net_mask[16];             // destination net mask
    int src_port;                       // source port number
    int dest_port;                      // destination port number
    char action[8];                     // either block or unblock
    int delete_rule;                    // gets the rule number to be deleted
    int print_flag;                     // is set if we are asked to print the rules that have been set         
    int out_flag;                       // is set if the packet is outbound, else set to 0;
};

The assignment of my_ipt to null:

struct my_iptable my_ipt; memset(&my_ipt, '\0', sizeof(struct my_iptable));

I have assigned my_ipt's fields properly.

The copying to buffer and writing to proc part:

unsigned char write_buf[sizeof(struct my_iptable)];     
    memcpy(write_buf, &my_ipt, sizeof(struct my_iptable));
int proc_fp = open("/proc/minifw", O_WRONLY | O_APPEND);
    if(proc_fp < 0) {
        printf("Couldn't open /proc/minifw for writing\n");
        exit(EXIT_FAILURE);}

   if(write(proc_fp, write_buf, sizeof(struct my_iptable)) == -1) {
        printf("There was an error writing to minifw buffer\n");
        exit(EXIT_FAILURE);
    }

I hope this gives you appropriate info on what I want to understand.

Thanks!

Whu not write(fd, &my_t, sizeof(struct my_table)) directly ?.. — xtof pernod
Hi @xtofpernaud, I did that too, but couldn't get what I expected. Also, I have declared my buffer as unsigned char buffer[sizeof(struct my_table)] — freax
I think you'll need to produce an SSCCE if you want more specific answers to this question. — autistic
@modifiablelvalue, have edited my question as you have asked. — freax

MOHAMED MOHAMED · Accepted Answer · 2013-03-28T13:17:24

use sizeof(struct my_table) instead

write(fd, buffer, sizeof(struct my_table));

If your buffer is defined as pointer:

struct my_table *buffer;

then the size of buffer will be equal to the size of pointer (4 for 32-bit systems and 8 for 64-bit systems) and not the real size of the struct my_table

Passing a structure as a char buffer to a proc entry

1 Answers