CODEWITHSUNDEEP
chex
askingtoomuch
askingtoomuch

Reputation: 537

Convert hexadecimal from socket to decimal

In a server, I get the length of an image data first, then the image data via TCP socket. How can I convert the length (in hexadecimal) to decimal, so that I know how much image data I should read? (eg. 0x00 0x00 0x17 0xF0 to 6128 bytes)

char len[4];
char buf[1024];
int lengthbytes = 0;
int databytes = 0;
int readbytes = 0;

// receive the length of image data
lengthbytes = recv(clientSocket, len, sizeof(len), 0);

// how to convert binary hex data to length in bytes

// get all image data 
while ( readbytes < ??? ) {

    databytes = recv(clientSocket, buf, sizeof(buf), 0);

    FILE *pFile;
    pFile = fopen("image.jpg","wb");
    fwrite(buf, 1, sizeof(buf), pFile);

    readbytes += databytes;
}

fclose(pFile);

EDITED: This is the working one.

typedef unsigned __int32 uint32_t; // Required as I'm using Visual Studio 2005
uint32_t len;
char buf[1024];
int lengthbytes = 0;
int databytes = 0;
int readbytes = 0;

FILE *pFile;
pFile = fopen("new.jpg","wb");

// receive the length of image data
lengthbytes = recv(clientSocket, (char *)&len, sizeof(len), 0);

// using networkd endians to convert hexadecimal data to length in bytes
len = ntohl(len);

// get all image data 
while ( readbytes < len ) {
databytes = recv(clientSocket, buf, sizeof(buf), 0);
fwrite(buf, 1, sizeof(buf), pFile);
readbytes += databytes;
}

fclose(pFile);

Upvotes: 0

Views: 1419

Answers (1)

Some programmer dude
Some programmer dude

Reputation: 409404

If you zero-terminate the number, so it becomes a string (assuming you send the digits as characters), you can use strtoul.

If you send it as a binary 32-bit number, you already have it as you need it. You should just use a different data-type for it: uint32_t:

uint32_t len;

/* Read the value */
recv(clientSocket, (char *) &len, sizeof(len));

/* Convert from network byte-order */
len = ntohl(len);

When designing a binary protocol you should always use the standard fixed-size data types, like uint32_t in the example above, and always send all non-textual data in network byte-order. This will make the protocol more portable between platforms. However, you don't have to convert the actual image data as that should already be in a platform-independent format, or just plain data bytes which doesn't have any byte-ordering issues.

Upvotes: 3

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