Question

I’m trying to write more efficient code in a C program, and I need some help getting my pointers and assignments correct. I’ve shown two methods below, each using the following declarations and strncpy:

int kk, arraysize;
char person_name[100] = "";
char * array_person_name, * array_param;
...
strncpy(person_name, "John Smith", 100);
arraysize = <this value is downloaded from database>;
...

Method A (rectangular array):

array_person_name = malloc( sizeof(char) * arraysize *100 );
array_param = malloc( sizeof(char) * arraysize * 2 );
for (kk = 0; kk < arraysize; kk++) {
    strncpy(array_person_name[kk], person_name, 100);
    strncpy(array_param[kk], "bt", 2);
}

Method B (ragged array):

for (kk = 0; kk < arraysize; kk++) {
array_person_name[kk] = &person_name;
array_param[kk] = "bt";
}

Notice that the arrays I’m trying to create place the same value into each element of the array. Method A is an (rectangular) array of arraysize elements, each element itself being an array of 100 characters. Method B attempts not to waste storage space by creating an (ragged) array of arraysize elements, where each element is a pointer-to-char.

QUESTION 1: Am I allocating memory (e.g. malloc) correctly in Method A?

QUESTION 2: Does the syntax look correct for Method B?

QUESTION 3: How do I allocate memory for the arrays in method B?

QUESTION 4: Am I correct that Method B is generally preferred?

Answer 1

You are pretty far off here. 1:yes, 2:no, 3:no, 4:yes. I’m not going to do it all, but here are a few hints.

You need space to store the strings and space to store pointers to the strings (the latter isn’t strictly necessary for Method A). The first will have type char*, the second will have type char**.

For Method A, you are allocating the string storage correctly, but you need to allocate the storage for the string pointers correctly (hint: you need arraysize instances of a char* pointer). It then gets initialized to pointers which differ from each other by 100 characters.

For Method B, there is no easy way of allocating space to store the strings, as you don’t know how much space you’ll need. You could iterate through all the strings once just to count their length, or do one malloc per string, or use a fixed size chunk and allocate more when you run out.

Method B uses the same string storage pointer array as Method A. You need to assign the string pointers into the array once you know where they will go.