Btukfyl

I currently have this string called'json' in C "{"method":"emptyFood","params":"08:00 09:00 10:00"}"

and trying to extract the times into 3 different variables.

for(int i = 0; i<5; i++)

    {

        ts1[i] = json[i+32];

    }    



    for(int j = 0; j<5; j++)

    {

        ts2[j] = json[j+38];

    }



    for (int k = 0; k<5; k++)

    {

        ts3[k] = json[k+44];

    }



    printf("time1 = %sn", ts1);

    printf("time2 = %sn", ts2);

    printf("time3 = %sn", ts3);

If I print them all at the same time, I get strange outputs like this

time1 = 08:009:010:00

time2 = 09:010:00

time3 = 10:00

However if I print them one by one, it prints out the correct time for each print statement.

What am I doing wrong?

Strings in C are null-terminated. This means that a string ends with a null byte. (A byte in a string has the type char.) When you copy the bytes of a substring to a new array of bytes, you're copying the “useful” content of the string, but to get a complete string, you also need to write the null terminator. For example:

for(int i = 0; i<5; i++)

{

    ts1[i] = json[i+32];

}

ts1[5] = 0;

(Don't confuse the null byte 0, which can also be written '', with the digit '0'.)

A more idiomatic way of writing this would be

memcpy(ts1, json + 32, 5);

ts1[5] = 0;

A better-designed function to copy a substring is strlcpy but unfortunately it is not part of standard C so many platforms don't have it. Note that strncpy doesn't do the job well because it wouldn't write a null byte in your case.

Note that you need 6 bytes to store a 5-byte string, i.e. you must declare char ts1[6] or char *ts1 = malloc(6) (or with a larger number).

The reason it seemed to work when you printed the strings one by one is that there must have been a null byte immediately after the string in memory. This is just a coincidence and might not happen if the function was called from different place, if the code before it had worked on different data, if compiled with a different compiler, etc. Given the output you got with the three variables, what must have happened is that the compiler happened to put ts1, ts2 and ts3 and something else which happened to start with a null byte consecutively in memory. So when you tried to print ts1, the string at that address ran over ts1, ts2 and ts3 to finally stop at that null byte afterward. Having a null byte was just a stroke of luck, or maybe a stroke of unluck since it hid the bug about the missing null terminator.