Uninitialized value was create by a heap allocation if i don't do memset

What im interested in is this.

char *assign_value = (char*)malloc(10 * sizeof(char));
if(strlen(assign_value) == 0) {
    strcpy(assign_value, "A");
} else {
    strcat(assign_value, "A");
}

Basically in the example above i will get error that Uninitialized value was create by a heap allocation. But if i do following thing and insert memset() in between malloc() and if statement i wont see same error. I would like to hear some sugestions, is this the right way, if not, what is right thing to do?

char *assign_value = (char*)malloc(10 * sizeof(char));
memset(assign_value, 0, sizeof(assign_value));
if(strlen(assign_value) == 0) {
    strcpy(assign_value, "A");
} else {
    strcat(assign_value, "A");
}

Thanks!

Solution

The problem comes from calling strlen(assign_value) when assign_value is uninitialized, as returned by malloc(10).

Here are three ways to fix the problem:

you can manually set all bytes of the array with memset(assign_value, 0, 10);. Note that your call is incorrect as sizeof(assign_value) evaluated to the size of the pointer, not the size of the array.
you can allocate the array with calloc(10, sizeof(char)). calloc returns tha address of a block of memory initialized to all bits zero, which has the same effect as calling memset, but is potentially more efficient.
you can set the initial byte to '\0' to make the array an empty string, valid for use with both strcpy and strcat.

Using calloc() instead of malloc() is a good habit to avoid unpredictable behavior if you fail to initialize any or all of the allocated data before use.

Note that your code could be simplified radically: if the destination string is indeed empty, calling strcat() is equivalent to strcpy(). You can just write:

char *assign_value = calloc(10, sizeof(char));
...
strcat(assign_value, "A");

Furthermore, you should verify that strcat() will not cause a buffer overflow by checking the length of the string already stored in the buffer:

char *assign_value = calloc(10, sizeof(char));
...
if (strlen(assign_value) < 10 - 1) {
    strcat(assign_value, "A");
} else {
    // handle the error: not enough space in assign_value
}