Sending float Windows <-> Linux

Denis Ivanov · 2012-07-05T07:40:56

[color=#000000][font=verdana, arial, sans-serif]I send a float equal to 4.000 from Windows to an Amazon EC2 Linux server, but the server reads it as 8.47522e+11.[/font] [color=#000000][font=verdana, arial, sans-serif]The server sends back 4.000 but Windows reads it as 0.0.[/font] [color=#000000][font=verdana, arial, sans-serif]I tried changing the byte order using this on Windows:[/font] [color=#000000][font=verdana, arial, sans-serif][background=rgb(242, 242, 242)]void ReverseFloat(float* f) { unsigned char* uc = (unsigned char*)f; unsigned char temp = uc[0]; uc[0] = uc[3]; uc[3] = temp; temp = uc[1]; uc[1] = uc[2]; uc[2] = temp; }[/background][/font] [color=#000000][font=verdana, arial, sans-serif]But the server now gets 8.47518e+11 and Windows gets 0.0.[/font] [color=#000000][font=verdana, arial, sans-serif]The struct is packed and it still gives me the wrong result.[/font] [color=#000000][font=verdana, arial, sans-serif][background=rgb(242, 242, 242)]#if !defined( _SERVER ) #pragma pack(push, 1) #endif typedef struct { unsigned char type; float version; char username[16]; char password[16]; } #if defined( _SERVER ) __attribute__ ((packed)) #endif LoginPacket; #if !defined( _SERVER ) #pragma pack(pop) #endif[/background][/font] The client sends 37 bytes and server receives 37 bytes. The server sends back this: #if !defined( _SERVER ) #pragma pack(push, 1) #endif typedef struct { unsigned char type; float version; } #if defined( _SERVER ) __attribute__ ((packed)) #endif WrongVersionPacket; #if !defined( _SERVER ) #pragma pack(pop) #endif The server says it sends 8 bytes for some reason and the client receives 8. I don't know why it's 8... should be 5. Is a float 7 bytes on Amazon Linux?

Networking and Multiplayer Programming

Started by polyfrag July 03, 2012 04:22 PM

10 comments, last by Tribad 11 years, 9 months ago

hplus0603

11,916

July 05, 2012 05:09 AM

The size of char in C and C++ is well defined to be the smallest addressable unit of memory. Unless you're running on some exotic DSP architecture, where everything is a full word in size, a char is 8 bits. And sizeof(char) is defined to be 1 -- that's the definition of what "sizeof" is expressed in.
If you believe that the only change you made was changing from "char" to "uint8_t" and it suddenly changed struct alignment, then I suggest you look a lot deeper for the real cause and solution.

enum Bool { True, False, FileNotFound };

Tribad

981

July 05, 2012 07:40 AM

Sign extent?
A signed data type is extended into a larger format, e.g. from byte to int or int to long, by copying the sign (MSB) bit into the new upper bits of the new format. This is valid for all all integer datatypes.

Sending float Windows <-> Linux

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Sending float Windows <-> Linux

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