How to scan IP Address with fscanf

Question

how to scan IP Adress with scanf in c?

I already user:

scanf(" %d ", &ip_adress)

Should i use

%d.%d.%d

or there is a easier way.

This is my function for reading information from a file:

void read_tree_from(FILE *in, PC **root){
    char name[MAX_NAME];
    int ip_adress;
    int speed;
    while(fscanf(in, "%s %d %d", name, &ip_adress, &speed) != EOF){
        PC *new = create_new_item(name, ip_adress, speed);
        add_to_tree(new, root);
    }
}

But when i run the program it works fine but in output file its mess...

Answer 1

You could use format specifier %hhu.%hhu.%hhu.%hhu, which reads in unsigned 8 bit values, and you could directly read into the 4 bytes of your int-representation of an ip-address, respectively:

int main () {

    const char* testLine = "aName 192.168.112.54 100";

    char name[100];
    int speed;

    volatile uint32_t ipAddress=0;
    unsigned char* ipAddressBytes = (unsigned char*)&ipAddress;


    if (sscanf(testLine, "%s %hhu.%hhu.%hhu.%hhu %d", name, ipAddressBytes+3, ipAddressBytes+2,ipAddressBytes+1,ipAddressBytes+0, &speed) == 6) {
        printf("%s %08X %d\n",name, ipAddress, speed);
    }

    return 0;   
}

Output:

aName C0A87036 100

Format %hhu reads in an integral value and maps it to 8 bits; hence, it does not check if the input concerning the IP-address is valid in the sense that 1025.168.112.54 would not be valid. However, it avoids technical overflows in that any part exceeding 8 bits is simply ignored. So before mentioned input would yield 01A87036.

Note the volatile-specifier, which tells the compiler that it shall not optimize access to this variable; otherwise, the compiler might assume that - since the value is not obviously changed - the variable is unchanged / not initialized.

Note further that you have to take care whether your environment is of big endian or little endian architecture.

Hope it helps.

Answer 2

How to scan IP Adress with fscanf

With line data, fgets() has many advantages, including better error recovery than fscanf(), so a fgets/sscanf() solution

void read_tree_from(FILE *in, PC **root){
  char buffer [MAX_NAME + 50];
  // Read a line of input
  if (fgets(buffer, sizeof buffer, in)) {
    int ip_address;
    int speed;
    unsigned char octet[4];
    int n = 0;
    int namelen; 

    // This approach uses %n to record the scan offset - if it got that far 
    // %*s%n to scan over the name, all tht is needed is how long it is
    // %hhu to scan an `unsigned char`
    // " ." to scan spaces and then a '.'
    sscanf(buffer, "%*s%n %hhu .%hhu .%hhu .%hhu %d %n",
      &namelen, &octet[0], &octet[1], &octet[2], &octet[3], &speed, &n);

    // When `n > 0`, the complete scan occurred
    // buffer[n] != '\0' detects extra junk after the speed
    if (n == 0 || buffer[n] != '\0') {
      // failed scan
      return;
    }

    // Check data for validity 
    if (namelen >= MAX_NAME) {
      // name too long
      return;
    }

    buffer[namelen] = '\0';  // \0 terminate the name

    // form IP address.  I'd recommend `uint32_t` rather than `int`
    // The `1u*` insures code deals with unsigned promotions and shifts.  No sign extend
    ip_address = 1u*octet[0] << 24 | 1u*octet[1] << 16 | 1u*octet[2] << 8 | octet[3];

    PC *new = create_new_item(buffer, ip_adress, speed);
    add_to_tree(new, root);
    }
  }
}

or there is a easier way (?)

Yes, if code does not care about detecting ill-form data, many short-cuts are available. Production code checks for bad data. Robust checking would take the buffer and look for numeric values out of range, (maybe with strtoul), etc.

Answer 3

I tried this the first time but it didnt work probably bcs i had some error somewhere else in the code and i tought this was the part that making the eror.But it worked when i fixed some mistakes in my code.

while(fscanf(in, "%d %s %s %d", tmp_number, tmp_name, tmp_ip, &tmp_speed) != EOF){ RACUNAR *new = create_new_item(tmp_number, tmp_name, tmp_ip, tmp_speed); add_to_tree(new, root);