C programming: Replace an inner string using strcpy?

Question

I've copied an HTML file into an array using the following code:

fseek(board, 0, SEEK_END);
long int size = ftell(board);
rewind(board);
char *sourcecode = calloc(size+1, sizeof(char));
fread(sourcecode, 1, size, board);

Now my goal is to replace a certain comment in the array with the already defined char string 'king'. E.g.

< html code>< !comment>< more html code>

to

< html code>king< more html code>

Im using the following code:

    find_pointer = strstr(sourcecode, text2find);
    strcpy(find_pointer, king);
    printf("%s", sourcecode);

where text2find = "< !comment>";

however when I print, it is evident that all my characters past 'king' have been erased.. as if it automatically added a terminating character. How can i fix this so < more html code> remains in place?

EDIT::::: I used strncpy and set a number of characters such that the terminating character was not added. is this the best method?

skyel · Accepted Answer · 2012-06-04 00:02:43Z

4

You basically can't do that, unless the stuff you want to replace is exactly the same size. In which case you can use either memcpy or strncpy.

If the sizes are different, you could try something along the lines of:

char *buffer = malloc(size); // size should be big enough to store the whole final html code
find_pointer = strstr(sourcecode, text2find);
len = find_pointer - sourcecode;
memcpy (buffer, sourcecode, len);
memcpy (buffer + len, "king", 4);
memcpy (buffer + len + 4, find_pointer + 4, strlen(sourcecode) - len - strlen(text2find)); 
free(sourcecode);
sourcecode = buffer;

edited Jun 4, 2012 at 0:02

answered Jun 3, 2012 at 23:56

skyel

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Comments

Daniel Fischer · Accepted Answer · 2012-06-03 23:55:10Z

3

Well, strcpy adds a 0-terminator. So although the remainder of the string remains in place, the standard string handling functions don't see it anymore because they stop at the 0-terminator. You can either manually overwrite it with a space or use memcpy instead of strcpy.

answered Jun 3, 2012 at 23:55

Daniel Fischer

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2 Comments

Daniel Fischer Over a year ago

Yes, or that. Whichever you prefer.

tobyodavies Over a year ago

This misses the much bigger problem that this strategy will not work unless both the search and replacement strings are the same length. Which, in this instance, they are not.

Sergey Kalinichenko · Accepted Answer · 2012-06-04 00:07:42Z

1

Replacing characters in a C string is painful, because you perform manipulations at a very low level, compared to, say, C++. You literally need to work out an algorithm for it!

First, observe that in-place replacement is not always possible: if the substring that you are replacing is shorter than the replacement, you would need to allocate more memory. It is easier to allocate the memory for the result either way, so you may proceed as follows:

Find the length of the string after the replacement. For that, you'd need to find the beginning and the end of the comment you're replacing, and do the math.
Next, you allocate a new chunk of memory for the result, and memcpy the source up to the replacement point into it.
Now you copy the replacement string, and finally the ending portion of the source into the result
Finally, you free the buffer of the source string, and return the result.

answered Jun 4, 2012 at 0:07

Sergey Kalinichenko

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1 Comment

Nth.gol Over a year ago

thank you. skyel said something similar and posted some code. cheers

Greg E. · Accepted Answer · 2012-06-04 00:00:46Z

0

First, you should use strncpy (or, better yet, strlcpy if available) because it actually performs bounds checking (i.e., it copies only a specified, supplied number of characters). Otherwise you may end up attempting to copy memory past the end of the destination string, resulting in undefined and potentially destructive behavior. Second, even if you were to use a function like strncpy or memcpy to avoid copying the terminating null character, your destination string would not be properly formatted because the string that you are trying to overwrite has a different length than the string that you're attempting to copy.

answered Jun 4, 2012 at 0:00

Greg E.

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1 Comment

Nth.gol Over a year ago

correct, it so happens that in html I just added a few spaces to make them the same length

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