How to write file that is converted to hex to a new file in C-CodePudding

I have an image in jpeg format that I converted into a hex string using an online tool. I copied that hex string and saved in a text file called imgHex.txt. How can I convert the hex value inside imgHex.txt back to its original form (jpeg).

I tried to open imgHex.txt using "r" parameter and then open a file test.jpeg with the "wb" parameter. And the write the contents of imgHex.txt to test.jpeg but with no success.

FILE* f;
    FILE* img;
    fopen_s(&f, "C:\\Users\\name\\Documents\\imgHex.txt", "r");
    fopen_s(&img, "C:\\Users\\name\\Documents\\test.jpeg", "wb");

    char b[512];
    while (fgets(b, 512, f) != NULL)
    {
        fputs(b, img);
    }

CodePudding user response：

Assuming that your imgHex.txt DOES NOT start with 0x but rather only consists of the hexadecimal digits, then this is a very simple operation. (If you file does start with 0x, just skip past the first two bytes of the file).

First, you need to understand that each hexadecimal digit represents 4 bits, i.e. A = 1010. This means you need to read two digits at a time so that you have one byte, i.e. A2 = 10100010. Once you read each two digit pair, you convert this textual representation of a byte into its actual value. Then you just write that value into your output file. Repeat until you reach EOF for imgHex.txt.

(Credits to @Weather Vane for actual code)

unsigned val; 
while(fscanf(f, "%2x", &val) == 1) {
    fputc(val, img)
}

This code succinctly reads two hexadecimal digits, "%2x" from the file handle f, and puts the resulting value into val. It makes sure that one byte was read from f, fscanf(...) == 1, in order to make sure no errors occurred. Then it writes the single byte to the file handle img, via fputc(val,img).

This will stop since once we reach the end of the imgHex.txt file, fscanf(...) will not return 1, but rather EOF (commonly this is -1, but depends on implementation).