Question

I’m reading a buffer of type char from a microcontroller device like this:

char data_buffer[DATA_LEN];
FILE *aq_dev = fopen(argv[1], "r");

fread(data_buffer, 1, DATA_LEN, aq_dev);

fclose(aq_dev);

In this buffer blob there are unsigned 16bit integer numbers, each distributed over two entries in the buffer. For example there is

data_buffer[10] = 0x07
data_buffer[11] = 0xc3

which I would like to read out as 0x07c3 which equals 1987. Despite the fact that I originally thought I could just access the buffer with a uint16_t pointer and directly read the value, I have to do some strange bitmasking:

int value = (buffer[10] << 8) + buffer[11];

gives a wrong value beyond 35000 while a seemingly useless bitmasking like

int value = ((buffer[10] & 0xff) << 8) + (buffer[11] & 0xff);

returns the correct value.

How can an entry of an array of char be bigger than one byte?
According to my limits.h (Linux, x86_64) there is #define CHAR_BIT 8.

Can someone please tell me what is wrong with my code? Thanks in advance!

Answer 1

Your problem is that char is signed on your platform. So if the high bit of buffer[11] is set, it will be a negative value, and when you use it in an expression it will first be promoted to a (negative) integer, effectively propagating the high bit of the char into all higher bit positions.

buffer[11]   (char)                0xc3 -61
             (int)           0xffffffc3 -61      // sign-extended

The reason that &-ing with 0xff works is that the char value is promoted to int before the & operation is performed:

buffer[11]   (char)                0xc3 -61
             (int)           0xffffffc3 -61      // sign-extended
    & 0xff   (int)           0x000000c3 195

The fix is to cast to unsigned char before performing the bit manipulation:

int value = (((unsigned char) buffer[10]) << 8) | ((unsigned char) buffer[11]);

buffer[11]   (char)                0xc3 -61
             (unsigned char)       0xc3 195
             (int)           0x000000c3 195

It’s probably easier just to make buffer an array of unsigned char.