how do i use sscanf() method in C

Question

i use sscanf in this code

while(totalByte < sizeof(rdata)) 
{   
    read(client_sockfd, &buffer[totalByte],1); 
    printf("bytes[%d] : %x \n", totalByte, buffer[++totalByte]);
}

use this code and i got a result like this

client send 1 1 +

bytes[0] : 0  
bytes[1] : 0  
bytes[2] : 0  
bytes[3] : 1  
bytes[4] : 0  
bytes[5] : 0
bytes[6] : 0  
bytes[7] : 1  
bytes[8] : 2b 
bytes[9] : 0  
bytes[10] : 0  
bytes[11] : 0 
bytes[12] : 0  
bytes[13] : 0  
bytes[14] : 0  
bytes[15] : 0  
bytes[16] : 0  
bytes[17] : 0 
bytes[18] : 0  
bytes[19] : 0 

got a result

then i use sscanf method

sscanf(buffer,"%d%d%c" ,&rdata.left_num, &rdata.right_num, rdata.op); 
printf("%d %c %d \n", ntohl(rdata.left_num),rdata.op,ntohl(rdata.right_num));

but when print rdata(structure)'s value , get a 0 value(init value).

0 0

i know sscanf method split a string and insert a value

Is there a any misunderstood for me?

this is what i used structure

struct cal_data 
{  
    int left_num;  
    int right_num;
    char op;  
    int result;  
    int error;  
}; 

Answer 1

I don't think this is doing what you want it to:

printf("bytes[%d] : %x \n",totalByte, buffer[++totalByte]);

If that is working, then it's working by coincidence. You don't want to rely on coincidence, rather than logic, do you? Read this question for more information on the undefined behaviour here. Change it to this, and avoid omitting sequence points in order to cram logic together in the future:

printf("bytes[%d] : %x \n",totalByte, (unsigned int) buffer[totalByte]);
totalByte++;

then i use sscanf method sscanf(buffer,"%d%d%c" ,&rdata.left_num, &rdata.right_num, rdata.op);

Where is your error checking? Like most standard C functions, your code should be checking the return value of sscanf to ensure that it extracted the amount of information you want. How can you be sure sscanf successfully processed two decimal digit sequences and a character? Use this:

int n = sscanf(buffer,"%d%d%c" ,&rdata.left_num, &rdata.right_num, rdata.op);
if (n == 3) {
    /* sscanf extracted and assigned three values;
     * One for each of the format specifiers,
     * and variables you passed in. Success! */
    printf("%d %c %d \n", ntohl(rdata.left_num),rdata.op,ntohl(rdata.right_num));
}
else {
    /* sscanf failed to extract some values from the string,
     * because the string wasn't correctly formatted */
    puts("Invalid input to sscanf");
}

... and now you'll see the problem! sscanf failed!

As others have indicated, a string terminates at the first '\0' (or 0) byte. The pointer you're passing to sscanf points to an empty string. sscanf can't extract any of the information you want from an empty string.

Answer 2

sscanf like many other C library functions stops iterating a string when it reaches a 0 (same as '\0'). The first element of your bytes array is 0. Hence as far as sscanf is concerned, your input string used for sscanf is "". To see another example of this behaviour, add the following code after your while loop.

printf("%s\n", bytes);

This will not print anything (will print an empty string), because printf like sscanf will consider byte[0] to be the end of the string.

You should probably use something like this to read data into your structure - http://c-faq.com/stdio/extconform.html

Answer 3

According to function description (http://www.cplusplus.com/reference/cstdio/sscanf/):

int sscanf ( const char * s, const char * format, ...);

The first argument should be:

C string that the function processes as its source to retrieve the data.

But as you print out each byte as "hexadecimal integer":

printf("bytes[%d] : %x \n", totalByte, buffer[++totalByte]);

The value of the first byte is 0. So if you treat it as a string, it is an empty string.