string replace in file using C

Question

I haven't yet implemented this, I'm still in the thinking stage, but I have to go through a file and replace a certain string with another string. For example,

<img src="/images/logo.gif" ...

should become

<img src="/proxy/www.example.com/images/logo.gif" ...

Any advice on how I can approach this? Perhaps there exist some "string replace" C functions that would do this for me that I don't know about...?

Right now, if I had to write this function myself, I would give it as parameters the file, string to replace, replacement string. Then I would manually go through the file and look for an occurrence of the string and recreate it. This, however, seems very inefficient. Are there better ways to do this?

Thanks, Hristo

Answer 1

No, there is no function in C that replaces a string throughout a file. You must implement it yourself.

That said, what you're showing us is HTML, and HTML is tricky, because it's hierarchical. Are you required to correctly parse it? Because if you are, the task is much more difficult. Seeing that it's homework, I doubt it, so you might do enough by:

1. open the file and load it to memory (assuming it isn't too large - if it is, you can write into a temporary file and move it onto the original one after you've finished)
2. continuously use strstr to find the anchor string you need to start replacing
3. replace
4. repeat 2 and 3 until finished with file
5. write file back

Answer 2

Since this is homework, I won't give you an answer but I'll point out a classic issue that trips people up.

In C, it's easiest to read a fixed byte count (you can try to do line by line but if a line is too long, that reverts to reading a fixed number of bytes). If the string you are trying to replace ends up getting split between one buffer and a second buffer:

buf1 -> "...<img src=\"/ima"
buf2 -> "ges/logo.gif\"..."

you won't be able to do a simple search replace in memory.

Answer 3

Since it's homework I'm going with the assumption that the string can not span multiple lines. If this assumption is correct (and barring the complications with "replacing text in HTML") then:

1 Read the next line

2 Replace string and write line (to another file)

3 If not at end, goto #1

4 Win \o/

Or perhaps the teacher wants something else shrug

Answer 4

Are you try strcpy function for this,

Assign the url in one string and replace it by strcpy function.

Answer 5

First of all, C is an awesome language, but is one of the most painful languages to do this type of operation in. Just had to say it.

Can you safely assume that the contents of the entire file can fit in memory? If so:

allocate buffer big enough to hold file contents
read entire file into buffer
inputPtr = 0

while(inputPtr < size of buffer) {
    replacePosition = strstr(inputPtr, stringToReplace);
    if (replacePosition != NULL)
        writeUntil = replacePosition - 1
    else
        writeUntil = end of buffer

    write out buffer from inputPtr to writeUntil inclusive (could be 0 bytes)

    if (replacePosition == NULL) break

    write out the replacement string

    inputPtr = replacePosition + strlen(stringToReplace)
}

Answer 6

You should investigate the sed command. See what it does and do something similar.

It works as a filter, so when using it to replace something in a file what you often do is capture the output into a file and then replace the old file with the new file.