将带符号的char数组传递给期望const char指针的函数会导致数组值出现乱码

Question

I cant seem to wrap my head around what is happening here:

I have a function that calculates the difference between two dates in seconds. Then it gets bitshifted around to produce some output:

void GetUTC(unsigned char buffer[5])
{
    struct tm str_time;
    time_t start;
    time_t current = time(NULL);
    str_time.tm_year = 2010 - 1900;
    str_time.tm_mon = 1;
    str_time.tm_mday = 1;
    str_time.tm_hour = 0;
    str_time.tm_min = 0;
    str_time.tm_sec = 0;
    str_time.tm_isdst = 0;
    start = mktime(&str_time);

    printf("%s\n",ctime(&start));
    printf("%s\n\n",ctime(&current));

    uint32_t someInt = difftime(current,start);
    buffer[0] = 2;
    buffer[1] = (someInt & 0xff);
    someInt = someInt >> 8;
    buffer[2] = (someInt & 0xff);
    someInt = someInt >> 8;
    buffer[3] = someInt;
    someInt = someInt >> 8;
    buffer[4] = (10 & 0xff);
}

I feed it this array:

    unsigned char toWrite[5];

Then call it

    GetUTC(toWrite);

All goes well. Now I have a function I am trying to feed the new array into, which takes these parameters:

void gatt_write_attribute(GDBusProxy *proxy, const char *arg)

And I call it with:

gatt_write_attribute(someProxy, toWrite);

However the array I passed to the function gatt_write_attribute shows that it's the jumbled garbage: $2\\20023\\n23 instead of my expected value of toWrite. (numbers differ since it has to do with time):

[0]: 2
[1]: 23
[2]: 54
[3]: 128
[4]: 10

I tried adding a terminating \\0 to the end of toWrite , but it did not change anything. I tried casting it to a const char pointer and that didn't work either.

I feel like I'm missing a very simple detail. Could someone explain why I cant pass this char array to the gatt_write_attribute function?

Answer 1

As noted by Ian Abbott and Chux in the comments to my original question, I was indeed being silly and was not realizing that my debugger was displaying the thing being pointed to as an array of characters instead of numbers. This is what caused the garbled values.

I should have probably allowed myself a bit more sleep in-between work hours.

Answer 2

You need to pass the array to the first function as a pointer. Otherwise it just gets copied to the stack and the function writes to the new copy. The garbage is probably there because it has not been initialised.