Bitwise left shift is turning value of my variable negative

Question

so I'm trying to shift these values left to store all this data into a 64 bit value. Unfortunately the numbers turn negative right at the first shift, what's causing this? Isn't it only suppose to store the first 41 bits of mil_t time into x? Also why would the remainder of serial and userid be zero?

long int gen_id(){
     printf("Calling gen_id...\n");

    unsigned long int mil_t = 1623638363132;
    unsigned long int serial = 10000;
    unsigned int rowId = 5;
    unsigned int userId = 30000;

    printf("Original mil_t: %ld\n", mil_t);
    unsigned long int x = mil_t << 41;
    printf("GEN_ID | MIL_T shift left 41: %ld\n", x);
    unsigned long int tbusMod = userId % serial;
    printf("GEN_ID | tbusMod = userId mod serial: %ld\n", tbusMod);

    x += tbusMod << (64 - 41 - 13);
    printf("GEN_ID | x1 : %ld\n", x);
    x += (rowId % 1024);
    printf("GEN_ID | x2 : %ld\n", x);

    return x;
}

OUTPUT:

Original mil_t: 1623638647191

GEN_ID | MIL_T shift left 41: -4136565053832822784

GEN_ID | tbusMod = userId mod serial: 0 

GEN_ID | x1 : -4136565053832822784

GEN_ID | x2 : -4136565053832822779 

FINAL: -4136565053832822779 

TOTAL BYTES: 68

Answer 1

Your program causes undefined behaviour by using the incorrect format specifier. %ld is only for long int (and not unsigned long int).

Instead use %lu to display x and tbusMod.

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30000 divided by 10000 gives quotient 3, remainder 0 .