Question

I have a binary value stored in a char in C, I want transform this byte into signed int in C.
Currently I have something like this:

char a = 0xff;
int b = a; 
printf("value of b: %d\n", b);

The result in standard output will be “255”, the desired output is “-1”.

Answer 1

Replace:

char a = 0xff

by

signed char a = 0xff;  // or more explicit: = -1

to have printf prints -1.

If you don’t want to change the type of a, as @veer added in the comments you can simply cast a to (signed char) before assigning its value to b.

Note that in both cases, this integer conversion is implementation-defined but this is the commonly seen implementation-defined behavior.