Question

I’m new to C, so feel free to correct mistakes.

I have some code that somewhat goes like this:

// some variables declared here like int array_size
char* cmd = (char*)malloc(array_size*sizeof(char));
for(;;){
    // code here sets cmd to some string
    free(cmd);
    array_size = 10;
    cmd = (char*)malloc(array_size*sizeof(char));
    // print 1
    printf(cmd);
    printf("%d\n", strlen(cmd));

    // repeat above for some time and then break

}

So I do the loop for a while and see what it prints. What I expected was every time the string would be empty and the length would be 0. However, that is not the case. Apparently sometimes malloc gets memory with junk and prints that out and that memory with junk has a length != 0. So I was thinking about solving this by setting all char in a new char string to ‘\0’ when malloc returns; however, I’m pretty sure I just did something wrong. Why is it even after I free the string and do a whole new malloc that my string comes with junk unlike the first malloc? What am I doing wrong?

Answer 1

malloc just allocated the memory and nothing more. It has no promises about what is in the memory. Specifically, it does not initialize memory. If you want allocated memory to be zeroed out, you can either do it manually with memset or simply call calloc (which is essentially malloc with zeroing out of memory).