c - strtok causing segfault but not when step through code

Question

I am new to C and I am trying to split a date/time string into separate variables. However, when I step through the code in gdb line by line, it works, however, when I let it run through normally without breakpoints it seg faults and I can't see why.

Below is the code:

char * dateTimeString = "2011/04/16 00:00";
char dateVar[11];
char timeVar[6];

if (splitTimeAndDateString(dateVar, timeVar, dateTimeString))
{
    exit(1);
}

printf("Date: %sTime: %s
", dateVar, timeVar);

Below is the function

int splitTimeAndDateString(char date[11], char time[6], char * dateString)
{
    char *token;
    token = strtok(dateString, " ");
    int i = 0;
    while (token != NULL)
    {
        if (i == 0)
        {
            strcpy(date, token);
        }
        else if (i == 1)
        {
            strcpy(time, token);
        }
        else
        {
            printf("Overrun date time string
");
            return 1;
        }
        token = strtok(NULL, " ");
        i++;
    }
    return 0;
}

Thanks for any help you can provide.

Answer 1

The strtok() function modifies string that you wants to parse, and replace all delimiters with nul symbol.

Read: char * strtok ( char * str, const char * delimiters );

str
C string to truncate.
Notice that the contents of this string are modified and broken into smaller strings (tokens). Alternativelly, a null pointer may be specified, in which case the function continues scanning where a previous successful call to the function ended.

In your code:

 strtok(dateString, " "); 
           ^
           |  is a constant string literal 

dateString points to "2011/04/16 00:00" a constant string literal, and by using strtok() your code trying to write on read-only memory - that is illegal and this caused segmentation fault.

Read this linked answer for diagram to understand: how strtok() works?

Edit:

@: char * strtok ( char * str, const char * delimiters ); In given code example, str is an array, not constant string literal. Its declaration:

char str[] ="- This, a sample string.";

Here str[] is an nul terminated array of chars, that initialized with string and its length is equals to size of the assigned string. You can change the content of str[] e.g. str[i] = 'A' is a valid operation.

Whereas in your code:

char * dateTimeString = "2011/04/16 00:00"; 

dateTimeString is pointer to string literal that is not modifiable e.g dateTimeString[i] = 'A' is an illegal operation this time.