Copying string array into another string array - C

Question

I have a struct that has an array of strings (char **args). I need to be able to copy an array of strings (char *input[32]) into that element of the struct. For example:

Thing s;
s.args = input; //assuming input already has some strings in it

When I try to do that, the next time s.args = input is called, it completely overwrites the old input. How can I achieve this functionality in the appropriate manner?

EDIT

This is what the struct looks like.

typedef struct{
char **args;
} Thing;

Then in my function, I have declared:

char *args[512];
.....
args[count] = string //string is a char *

Then finally, I want to do:

s.args = input.

You mean that you want to extend the current array of strings with more strings? — zneak
– zneak, Commented Sep 18, 2012 at 2:26

paddy · Accepted Answer · 2012-09-18 22:49:07Z

5

You are not copying it. You are actually just setting the pointer. Effectively you have this:

char **args;
char *other[32];
args = other;

You need to actually copy the array - for that you need to allocate memory for it:

s.args = malloc( 32 * sizeof(char*) );

for( i = 0; i < 32; i++ ) s.args[i] = input[i];

That is a shallow copy - it will copy your string pointers but not duplicate them. If you change the contents of a string in input, that change will be reflected in s.args. To copy the strings, you must do this:

for( i = 0; i < 32; i++ ) s.args[i] = strdup(input[i]);

Since you have allocated memory, then before you overwrite s.args again (and also when your program is finished) you need to free what you allocated. This includes the strings (if you called strdup);

if( s.args != NULL ) {
    // Only do the loop if you did a deep copy.
    for( i = 0; i < 32; i++ ) free(s.args[i]);

    // Free the array itself
    free(s.args);
}

edited Sep 18, 2012 at 22:49

answered Sep 18, 2012 at 2:27

paddy

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7 Comments

paddy Over a year ago

Oops, I misread one important fact in the question. Have edited to correct my answer.

Andrew Backes Over a year ago

So if I use strdup, the order of what I have to do is like this?: 1) allocat memory 2) free the the allocated space 3) copy with s.args[i] = strdup(input[i]) ?

paddy Over a year ago

Yes. Allocate the array first, then strdup each string into it. When you're done, free the strings first, then the array.

paddy Over a year ago

Oh, you edited your comment while I was answering the old one. No. You only free pointers that you have allocated (eg malloc or strdup). After you allocate the array, its contents will be undefined. You don't go and free those values until after you have initialised them.

Jens Gustedt Over a year ago

please don't cast the return of malloc: stackoverflow.com/questions/605845/…

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