C++ - Get current position of file pointer.

Hello,

I'm trying to get the file pointer position.
The result I'm getting is 4096 when I know its less.
I understand that the 4096 is probably the cluster size.
I know I can count the read sizes, just seems a bit clumsy,
How do I get the absolute position of the file pointer
when I've finished my reads, not always to the strucure end?

Regards

Code:

myStructure pt2[1]; FILE * in2 = fopen(tmp.c_str(), "r"); fseek(in2, 0x00, SEEK_SET); // go to start fread(pt2, sizeof(myStructure), 1, in2); //structure ...various reads // all OK long size = ftell (in2); printf("pos - %lx - %u\n", size, size); // prints "pos - 1000 - 4096" fpos_t pos; fgetpos (in2, &pos); printf("pos - %lx\n", pos); // prints "pos - 1000"

What OS?
Are you compiling with full warnings? On gcc use -Wall -W -pedantic.

What does your structure look like?
How do you know that the position isn't 4096?
Are you taking account of padding in your calculations?

You aren't using the correct format to print a long. It should be "%lu".

An fpos_t is not necessarily a long or even an integer (it could be a struct or array). It's an opaque object and not meant to necessarily be printable at all.

You probably need to post a complete runnable program to get help.

So how'd that work out for you?

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;

fpos_t

Object containing information to specify a position within a file
This type of object is used to specify a position within a file. An object of this type is capable of specifying uniquely any position within a file.
The information in fpos_t objects is usually filled by a call to fgetpos, which takes a pointer to an object of this type as argument.
The content of an fpos_t object is not meant to be read directly, but only to be used as an argument in a call to fsetpos.

*************

It looks like I can save the position without knowing the actual position.
Thanks for your time.

Regards

Your original code shows that you forgot to open the file in binary mode (rb).

You say you only read up to a certain point in the struct, but your original code has an fread that reads an entire struct.

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

Hello.

algorism
Opening (rb) makes a lot of difference (my mistake).
fread whole structure (again my mistake), I was printing upto a
certain point in the structure.

Elkvis
I'm opening a USB device as a file, don't know how to do this in C++.

Regards

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,

Quote:

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