Thread: C++ - Get current position of file pointer.

Hello algorism,

Windows 10 - 32 bit
gcc version 5.3.0 (i686-posix-dwarf-rev0, Built by MinGW-W64 project)

Compiling from within Qt Creator 4.2.0

*.pro file
CONFIG += console
CONFIG -= app_bundle
CONFIG -= qt
CONFIG += C++11

SOURCES += test.cpp

Thanks for your reply.
I've corrected the printf's, getting the same results.
No warnings or errors from the compiler
I'm only reading about 60 bytes into the structure.
If you still want compileable code, I can do this tomorrow, Please let me know.
I'm getting as far as shown OK, now I want to save the pointer for later use,
I need to get its postion.

Regards

Code:

    long size = ftell (in2);
    printf("pos - %Ix - %lu\n", size, size);

    fpos_t pos;
    fgetpos (in2, &pos);
    printf("pos %I64d\n", pos);

Code:

typedef struct {
    unsigned char jmpCode[3];                 // 3
    char           oem[8];                    // 8
    unsigned short sector_size;               // 2
    unsigned char  sectors_per_cluster;       // 1
    unsigned short reserved_sectors;          // 2
    unsigned short root_dir_entries;          // 2
    unsigned short total_sectors_short;       // 2 
    unsigned char  media_descriptor;          // 1
    unsigned short fat_size_sectors;          // 2
    unsigned short sectors_per_track;         // 2
    unsigned short number_of_heads;           // 2
    unsigned long  hidden_sectors;            // 4
    unsigned long  total_sectors_long;        // 4
    unsigned long  sectors_per_fat;           // 4
    unsigned short flags;                     // 2
    unsigned short fat32_version;             // 2
    unsigned long  root_directory_cluster_no; // 4
    unsigned short file_info;                 // 2
    unsigned short backup_root;               // 2
    char           reserved[12];              // 12
    unsigned char  drive_number;              // 1
    unsigned char  unused;                    // 1
    char           extended_sig;              // 1
    unsigned long  serial;                    // 4
    
//*** I'm only reading this far - about 60 bytes ***
    
    char           volume_label[11];          // 11
    char           fat_name[8];               // 8
    char           boot_code[448];            // 448
    unsigned short boot_sector_signature;     // 2
} __attribute((packed)) myStructure;

A better question, to begin with, might be why are you using C file I/O functions in C++?

Are you familiar with C++ iostreams? Here is a brief example, in the context of your problem:

Code:

std::ifstream infile("/dev/usb/whatever", std::ios::binary); // open the file
infile.seekg(0); // reset to beginning of file
infile.read(pt2, sizeof(myStructure)); // read the structure
auto position = infile.tellg(); // get the current position

For further reading on the subject, look up a C++ binary file I/O tutorial on google.

Hello,

Elkvis
Are you familiar with C++ iostreams? Here is a brief example, in the context of your problem:

Not yet, but playing. Thanks.

Regards

Code:

int main() 
{
    int count = 0;

    Fat32BootRecord pt2;

    char buff[sizeof(Fat32BootRecord) + 1]; 
    
    std::vector <int> posList = { (sizeof(pt2.jmpCode)), (sizeof(pt2.oem)) };
                         // and the rest of structure
    
    std::vector <string> sType = { "Jmpcode", "OEM" };
                         // and the rest of structure 
    
    std::stringdrLetter = "J:"; // GET THE RIGHT DEVICE    
    std::stringdevice = "\\\\.\\" + drLetter; 
    
    std::ifstream infile(device, std::ios::in | std::ios::binary);
    
    if (!infile)
        {
             cerr << "Device '" + drLetter + "' could not be opened!" << endl;
             exit(-1);
        }

    infile.seekg(0); // reset to beginning of file
    infile.read(buff, sizeof(Fat32BootRecord)); // read the structure 

    for (int j = 0; j < posList.size(); j++)
    {
        printf("%s - %i\n", sType[j].c_str(), posList[j]);
        for(int i = count; i <  posList[j] + count; i++) 
        {
            printf("%i - %c - %d - %x\n",
                 i, buff[i], buff[i], buff[i]);
        }
        count = count + posList[j];
    } 
    exit(0);
}

Code:

Jmp code - 3
     0 - ë - -21 - ffffffeb
     1 - X - 88 - 58
     2 - � - -112 - ffffff90
 OEM - 8
     3 - M - 77 - 4d
     4 - S - 83 - 53
     5 - D - 68 - 44
     6 - O - 79 - 4f
     7 - S - 83 - 53
     8 - 5 - 53 - 35
     9 - . - 46 - 2e
     10 - 0 - 48 - 30