Appending a data to file in c

Question

I require to add a string before 45byte in an existing file. I tried using fseek as bellow.

int main()
{
 FILE *fp;
 char str[] = "test";     

 fp = fopen(FILEPATH,"a");
 fseek(fp,-45, SEEK_END);                
 fprintf(fp,"%s",str);
 fclose(fp);     
 return(0);
}

I expected that this code will add "test" in before 45 char from EOF. But it just appends "test" to the EOF.

Please help me to find the solution.

This is continuation of my previous question
http://stackoverflow.com/questions/3262886/append-item-to-a-file-before-last-line-in-c

Answer 1

Don't open it as append (a) if you plan to write at arbitrary positions; it will force all writes to the end of the file. You can use r+ to read or write anywhere.

Answer 2

Open it with mode r+ (if it already exists) or a+ (if it doesn't exist and you want to create it). Since you're seeking to 45 bytes before the end of file, I'm assuming it already exists.

fp = fopen(FILEPATH,"r+");

The rest of your code is fine. Also note that this will not insert the text, but will overwrite whatever is currently at that position in the file.

ie, if your file looks like this:

xxxxxxxxxxxxxxxxxxxxxxxxxx
xxxxxxxxxxxxxxxxxxxxxxxxxx

Then after running this code, it will look like this:

xxxxxxxtestxxxxxxxxxxxxxxx
xxxxxxxxxxxxxxxxxxxxxxxxxx

If you really want to insert and not overwrite, then you need to read all the text from SEEK_END-45 to EOF into memory, write test and then write the text back

Answer 3

To avoid platform-specific configurations, always explicitely indicate the binary or text mode in your fopen() call.

This will save you hours of desperations if you port your code one day.