从二进制文件中读取十六进制

Question

i have been constantly looking for an appropriate answer that would give me the "ahha" moment, but could not find one yet, but i have a binary file in which many other files are in, these files have hexadecimal references to the location to where the file is.
Say for instance, if i open up a hex editor and look at this binary file at offset 0x500, then from 0x500 to 0x507 would be the postition of that particular file within the binary file. then from 0x508 to 0x50F would be the actual file size (or block size, or chunk size or whatever you want to call it) and then the next 32 bytes is the actual file name.
Here is the C script, i know its wrong and thats why i am here asking for help to get me going :p

int main (){

FILE* in = fopen("file", "rb");

        int location[0x08];
        int size[0x08];
        int name[0x20];
        int fileLocation;
        int fileSize;
        int fileName;

        int buffer[0x08];
        int entryCount;

        fseek(in, 0x08, SEEK_SET);
        entryCount = fread(buffer, sizeof(buffer), 1, in);


        for ( int x = 0; x < 25; ++x) {

            fileLocation = fread(location, sizeof(location), 1, in);
            fileSize = fread(size, sizeof(size), 1, in);
            fileName = fread(name, sizeof(name), 1, in);
            // extract data based on references//


    }
}

I made a script in perl that does this great, but trying to migrate it to C and it is a little bit confusing to me ;0
any help on how i could possibly migrate this to C would be very helpful, thanks for looking here is the script in perl:

my $fileLocation = '';
my $fileSize     = '';
my $fileName     = '';
my $file         = '';
my $chunk        = '';
my $exit         = '';

seek( $infile, 0x10, 0 ) or die "cannot seek $infile: $!";



until ($exit) {
    read( $infile, $fileLocation, 0x08 );
    read( $infile, $fileSize,     0x08 );
    read( $infile, $fileName,     0x20 );

    if ( $fileLocation =~ 'terminating reference' ) {
        last;
    }

    $fileLocation =~    s/(.)/sprintf("%02x",ord($1))/egs;
    $fileSize =~        s/(.)/sprintf("%02x",ord($1))/egs;
    $fileName =~        s/\0+$//;

    if ( $fileLocation =~ 'terminating reference' ) {
        last;
    }

    open( $file, '>', "extracted/$fileName" ) or die "Cannot open $fileName $!";
    binmode($file);
    sysseek( $infile, hex($fileLocation), 0 );
    sysread( $infile, $chunk, hex($fileSize) );
    syswrite( $file, $chunk );

    $fileLocation = '';
    $fileSize     = '';
};

this will read 8 bytes then 8 more bytes then 32 bytes ad then it will extract the data based of the variables $fileLocation, $fileSize, and $filename over and over until i come to the terminating reference, then will exit

Answer 1

Since the data is stored inside the file in hexadecimal characters , you (1) need to read char s, not int s (thanks @user3121023), (2) convert hex to integer, then (3) use the resulting value.

Also, the return result of fread is not what you think it is! It returns the number of bytes read, and so you can check if the operation was successful.

read_buffer needs to be your maximum read length (32 bytes) plus 1 for a terminating 0. (Granted: only necessary if you want to printf the read data. But it's sensible to print the data, to make sure you are doing everything right.)

char read_buffer[0x21];

for ( int x = 0; x < 25; ++x)
{
     if (fread (read_buffer, 1,8, in) < 8) break;
     read_buffer[8] = 0;
     fileLocation = strtol(read_buffer, NULL, 16);
     printf ("location: %s -> %08Xh\n", read_buffer, fileLocation);

     if (fread (read_buffer, 1,8, in) < 8) break;
     read_buffer[8] = 0;
     fileSize = strtol(read_buffer, NULL, 16);
     printf ("size: %s -> %08Xh\n", read_buffer, fileSize);

     if (fread (read_buffer, 1,0x20, in) < 0x20) break;
     read_buffer[0x20] = 0;
     printf ("name: %s\n", read_buffer);
     // extract data based on references//
}

I omitted the step of copying read_buffer into name , because for this your name declaration is too small. Since you want name as a C-string (regardless of how it is saved in the original file), you need to declare it as at least

char name[21];

Answer 2

I think you could declare your variables on a 32 bit system as

unsigned long location;
unsigned long size;
unsigned long entry;
char name[0x20];

on a 64 bit system you would use unsigned int instead of unsigned long

then read the data

fseek(in, 0x08, SEEK_SET);
entryCount = fread(entry, sizeof(entry), 1, in);
printf ( "entry %lu\n", entry);

for ( int x = 0; x < 25; ++x) {

    fileLocation = fread(&location, sizeof(location), 1, in);
    printf ( "location %lu\n", location);
    fileSize = fread(&size, sizeof(size), 1, in);
    printf ( "size %lu\n", size);
    fileName = fread(name, sizeof(name), 1, in);
    printf ( "name %s\n", name);
    // extract data based on references//
}