Question

All this is probably a real simple one but I am missing something and hope you can help. Ok this is my issue as simple as I can put it.

I am returning a buffer from readfile after using a USB device. This all works ok and I can out put the buffer fine by using a loop like so

for (long i=0; i<sizeof(buffer); i++)  //for all chars in string
{
    unsigned char c = buffer[i];

    switch (Format)
    {
        case 2:         //hex
            printf("%02x",c);
        break;
        case 1:         //asc
            printf("%c",c);
        break;
    }  //end of switch format
}

When I use the text (%c) version I can see the data in the buffer in my screen the I way I expected it. However my issue is when I come to read it using sscanf. I use strstr to search some key in the buffer and use sscanf to retrieve its data. However, sscanf fails. What could be the problem?

Below is an example of the code I am using to scan the buffer and it works fine with this standalone version. Buffer section in the above code can’t be read. Even though I can see it with printf.

#include <stdio.h> 
#include <string.h> 
#include <windows.h> 

int main () 
{ 
    // in my application this comes from the handle and readfile
    char buffer[255]="CODE-12345.MEP-12453.PRD-222.CODE-12355" ;    
    //
    int i; 
    int codes[256];  
    char *pos = buffer;  
    size_t current = 0;  
    //  
    while ((pos=strstr(pos, "PRD")) != NULL) {  
        if (sscanf(pos, "PRD - %d", codes+current))  
            ++current;  
        pos += 4;  
    }  

    for (i=0; i<current; i++) 
        printf("%d\n", codes[i]); 
    system("pause");
    return 0; 
} 

Thanks

Answer 1

The problem is that, your ReadFile is giving you non-printable characters before the data you are interested in, specifically with a '\0' in the beginning. Since strings in C are NUL-terminated, all standard functions assume there is nothing in the buffer.

I don’t know what it is exactly that you are reading, but perhaps you are reading a message that contains a header? In such a case you should skip the header first.

Blindly trying to solve the problem, you can skip the bad characters manually, assuming they are all in the beginning.

First of all, let’s make sure the buffer is always NUL-terminated:

char buffer[1000 + 1];    // +1 in case it read all 1000 characters
ReadFile(h,buffer,0x224,&read,NULL);
buffer[read] = '\0';

Then, we know that there are read number of bytes filled by ReadFile. We first need to go back from that to find out where the good data start. Then, we need to go further back and find the first place where the data is not interesting. Note that, I am assuming in the end of the message, there are no printable characters. If there are, then this gets more complicated. In such a case, it is better if you write your own strstr that doesn’t terminate on '\0', but reads up to a given length.

So instead of

char *pos = buffer;

We do

// strip away the bad part in the end
for (; read > 0; --read)
    if (buffer[read - 1] >= ' ' && buffer[read - 1] <= 126)
        break;
buffer[read] = '\0';
// find where the good data start
int good_position;
for (good_position = read; good_position > 0; --good_position)
    if (buffer[good_position - 1] < ' ' || buffer[good_position - 1] > 126)
        break;
char *pos = buffer + good_position;

The rest can remain the same.

Note: I am going from the back of the array, because assuming the beginning is a header, then it may contain data that might be interpreted as printable characters. On the other hand, in the end it may be all zeros or something.