解析字符串会打印垃圾

Question

I have this problem parsing a string, maybe you can help.

I am not from a C field, so please try to be patient with the stupid things I do.

I have this :

#include <stdio.h>
#include <string.h>

char DataIn[65] = "WV,000B,404C494748543B.";

int main()
{
    char *p = DataIn;

    puts(DataIn);

    for (int count = 0;count<2 ; ++count) {
        p = strstr(p, ",");
        if (!p )
            break;
        p++;            
    }

    char *endPointer = strchr(p, '.');
    *endPointer = '\0';

    puts(p);
}

The output I want from this example is: 404C494748543B (which can have different length).

When I print p , usually its ok, but sometimes, only once in a while , I get this :

404C494748543B
X
@LIGHT;
��
@LIGHT;

    ��03

Which includes some previous data + garbage .

Is there something wrong with the way I extract the data ?

Answer 1

When things work most of the time, but sometimes you get garbage, it is undefined behavior. This means that DataIn points to invalid memory - something that has been de-allocated, or is allocated on the stack of a function that has finished working.

The best way to diagnose and fix this problem is to run your code through a memory profiler, such as valgrind .

Note: There are two situations when your loop ends:

• count reaches 2 (normal exit), or
• p is set to NULL (on a break)

The code searching for endPointer , however, assumes that p is not NULL without any checks, which means your program may crash when the loop exits on a break .