Patial copy of string array

Question

Can I do something like this, to copy string array content from one to another?

char **words = (char **)malloc(sizeof(char*) * lines_allocated); 
char **childWords = (char **)malloc(sizeof(char*) * len1); 
int i;
for(i = startIndex; i < endIndex; i++) {
    childWords[i] = words[i];       
}

You are copying the pointers to the string, not the strings themselves. — ajay, Feb 17 '14 at 06:06

score 3 · Answer 1 · edited May 23 '17 at 10:25

You are copying pointers to the strings, not the strings themselves. You should change your for loop to

for(i = startIndex; i < endIndex; i++) {
    childWords[i] = strdup(words[i]);  // creates a new copy of the string       
}

strdup duplicates the string by allocating enough memory to copy the string passed to it, i.e., it returns a pointer to a new string which is a duplicate of the string pointed to by words[i]. The pointer can be passed to free. It's declared in the string.h header file.

Please note that you should not cast the result of malloc. There's no benefit and it can lead to bugs if you forget to include the stdlib.h header file. Also strdup is a part of the POSIX standard and under the hood, it effectively does the same thing as malloc and then strcpy. Read more here What does strdup do?

score 2 · Accepted Answer · answered Feb 17 '14 at 05:18

2

What you are doing is duplicating the pointers to the lines of text, so that you will end up with two arrays of char*'s which point to the same physical memory containing the actual words.

The problem with this is that if you change the memory through 'words' then 'childWords' would change because they point to the same memory.

The safest way to do this would be to get the string length of each 'word' and allocate new memory, copying the string over so that 'childWords' points to different memory.

answered Feb 17 '14 at 05:18

Chris

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Do I need to allocate new memory for each "word", allocating "childWord" from the beginning wouldn't be the same? – Lisa Feb 17 '14 at 05:25
You would need to allocate new memory for "childWord[i]" based on length word[i]. E.G. childWord[i] = new char[strlen(word[i])+1]; This creates a new piece of memory the same size as the original word[i] and childWord[i] now points to it. Now you can copy original word[i] into this new childWord[i] using memcpy/strcpy/or whatever other method you choose. – Chris Feb 17 '14 at 05:35

score 1 · Answer 3 · answered Feb 17 '14 at 05:16

1

Why not use memcpy() or bcopy() instead? Also, I don't believe you need to cast the result from malloc().

answered Feb 17 '14 at 05:16

James McPherson

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Patial copy of string array

3 Answers3