函数return char []代替int

Question

I need some help with my C function. I'm trying to get the current CPU time and return it as a char[] for use later on. The problem that I'm getting is that I don't have my pointers/dereferences in the right place and I'm getting compiler warnings/errors. What changes do I need to make to get this to work correctly?

Function:

char *get_time()
{
    char time_char[10];
    time_t rawtime;
    struct tm * timeinfo;

    time ( &rawtime );
    timeinfo = localtime ( &rawtime );
    int time = timeinfo->tm_yday*1000000 + timeinfo->tm_hour*10000 + timeinfo->tm_min*100 + timeinfo->tm_sec;

    sprintf(time_char, "%d", time);
    return *time_char;
}

Calling function with char time_char[] = get_time();

What I had before I moved the code to a function and worked as I needed it to was this:

char time_char[10];
time_t rawtime;
struct tm * timeinfo;

time ( &rawtime );
timeinfo = localtime ( &rawtime );
int time = timeinfo->tm_yday*1000000 + timeinfo->tm_hour*10000 + timeinfo->tm_min*100 + timeinfo->tm_sec;

sprintf(time_char, "%d", time);

And then I used time_char as needed.

Answer 1

The array is allocated in the function's call-frame and is deallocated when the function returns.

You'll either need to pass-in the array by pointer (as keltar comments) in which case you don't need to return anything and the function can be marked void , or allocate memory with malloc and return the pointer.

Answer 2

Change your prototype to:

char *get_time(char *time);

Allocate the appropriate memory to time in the calling function , then call free(time); when you are done using it (also from the calling function).

Also, your return statement should simply be:

return time;

Answer 3

First, you can't return a pointer to an automatic local variable. Second, to return a pointer you do not need a dereference operator.

Answer 4

Although in general this is not advisable, sometimes it is useful to return a character array, as follows:

typedef struct char10 {char str[10];} char10;

char10 get_time() {
    char10 time_char;
    // ...
    sprintf(time_char.str, "%d", time);
    return time_char;
}

One downside is that, for example, gcc gives an unnecessary warning message for code such as:

    printf("%s\n", get_time().str);

which you can avoid via the ugly:

    printf("%s\n", &get_time().str[0]);