string replace in file using C

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

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)
}

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.

Are you try strcpy function for this,

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

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.

You can use following program to search & replace string in a file.

int main()

{

.....

replaceIPAddress( "System.cfg", "172.16.116.157", "127.0.0.1");

.....

}


void replaceIPAddress( char * confFileName, char *text_to_find , char *text_to_replace  )

{

FILE *input = fopen(confFileName, "r");

FILE *output = fopen("temp.txt", "w");

char buffer[512];


while (fgets(buffer, sizeof(buffer), input) != NULL)
{
    char *pos = strstr(buffer, text_to_find);
    if (pos != NULL)
    {
        /* Allocate memory for temporary buffer */
        char *temp = calloc(
                strlen(buffer) - strlen(text_to_find) + strlen(text_to_replace) + 1, 1);

        /* Copy the text before the text to replace */
        memcpy(temp, buffer, pos - buffer);

        /* Copy in the replacement text */
        memcpy(temp + (pos - buffer), text_to_replace, strlen(text_to_replace));

        /* Copy the remaining text from after the replace text */
        memcpy(temp + (pos - buffer) + strlen(text_to_replace),
                pos + strlen(text_to_find),
                1 + strlen(buffer) - ((pos - buffer) + strlen(text_to_find)));

        fputs(temp, output);

        free(temp);
    }
    else
        fputs(buffer, output);
}

fclose(output);
fclose(input);

/* Rename the temporary file to the original file */
rename("temp.txt", confFileName);

}