Thread: Modifying live input

I currently have a program that does the following:

It does a vfork() then executes another binary. This new binary takes its input on descriptors 0 and descriptors 1.

Meanwhile, back in my program, I open two files and I use dup2() to redirect the first file to descriptor 0 and the second file to descriptor 1.

This works very well.

What I'm trying to do now is to modify the data as it's being read from file #2. I basically need to add a line of text before the first bare \n that's being found and then send the data on its way to descriptor 1. What is the easiest/fastest way to do this ?

My only thoughts so far were to read the file, do the modification, write back to the file. Then open it back again and do the dup2() but it seems a very inefficient way to do things.

I'd probably use fgets() to read a line at a time, into a char array. Then use strcat() to add the string you want to add, to the target line. Then continue writing out the char array, to the new (updated) file.

If you imagine that you have no table of contents or index, and you had to do this job yourself, by hand; you'll probably have a pretty efficient algorithm to design your program around.

Be sure to include string.h to let the string functions above, work.

So basically you're saying to:

1. open the file, do my change, save the file
2. re-open the file, do the dup2()

There's no way to do the string modification on the fly ?

Originally Posted by Adak

Not quite.

I'm saying get your input, put it into a string, then add your change string with strcat().

Now output the data twice, once to the new original file, and once to the dup of that new file. There's no problem having two files opened for output at the same time, is there?

Sorry but I don't think I'm following.

I can tell you that once the file is read/modified, I no longer need it - there is actually code to remove it after it was read once.

I also have to use fread() instead of fgets() and I have to add more data in the middle of the buffer.

Finally, how do I print to descriptor 1 ? Should I eliminate dup2() altogether and do a fprintf(1, "%s", buffer) for example ?

fgets() isn't necessary, it was just an example. You have a string from the file. You can use strcat() or other logic, to make the changes you need to make.

Post your code and give a small example of the input you have, and the output you want. Then these general idea's can be replaced with solutions instead of posting around in circles for lack of specifics.

Here's the current code. It's meant to deal with e-mail messages and I want to add a newline in the headers of the message. The headers in an e-mail are from the first line and until the first line that only contains a single '\n'. I would want to add my custom line right before that single '\n'

Current code starts here:

Code:

  /* open the message read only*/
  if ( (fd = open(message_name, O_RDONLY)) == -1 ) {
    exit_clean(EXIT_400);
  }

  /* open the address read only */
  if ( (fd_per = open(addr_name, O_RDONLY)) == -1 ) {
    exit_clean(EXIT_400);
  }

  /* set the standard input to be the new file */
  if ( fd_move(1,fd_per)  == -1 ) {
    exit_clean(EXIT_400);
  }

if ( pipe(pim) != 0 ) return(-1);

  switch(pid = vfork()) {
    case -1:
      close(pim[0]);
      close(pim[1]);
      exit_clean(EXIT_400);
    case 0:
      close(pim[1]);
      dup2(pim[0],0);
      execl(queue, queue, 0);
      _exit(-1);
  }
  close(pim[0]);

  while( (ret = read(fd, buffer, sizeof(buffer))) > 0 ) {
    if ( write(pim[1], buffer,ret) == -1 ) {
      exit_clean(EXIT_400);
    }
  }
  close(pim[1]);
  close(fd);

So it's between the read and the write that my modification should take place.

The code is very clear - and I get it that you need to make your changes after you read in the data, and before you write the data back out.

Good one!

Show an example of the header, and the line you want to add, one time. Should be very simple.

I see no mention of what you want to add, in your code. Do you have this in a string, someplace in your code?

Originally Posted by Adak

The code is very clear - and I get it that you need to make your changes after you read in the data, and before you write the data back out.

Good one!

Show an example of the header, and the line you want to add, one time. Should be very simple.

I see no mention of what you want to add, in your code. Do you have this in a string, someplace in your code?

Glad I made myself clear. Here's an example of the data being contained in the message file.

Code:

Received: by sphinx (mbox mlande) (with Cubic Circle's cucipop (v1.31 1998/05/13) Wed Aug 20 19:41:38 2003) 
X-From_: [email protected] Wed Aug 20 19:40:22 2003 
Return-Path: <[email protected]> 
Received: from psmtp.com (exprod5mx37.postini.com [12.158.34.194]) by sphinx.got.net (8.12.3/8.12.3/Debian-6.3) with SMTP id for <[email protected]>; Wed, 20 Aug 2003 19:40:05 -0700 
Message-Id: <[email protected]> 
Received: from source ([69.9.251.177]) by exprod5mx37.postini.com ([12.158.34.245]) with SMTP; Wed, 20 Aug 2003 21:40:05 CDT 
From: <[email protected]>
To: <[email protected]>
Subject: whatever

Start of message here

Like I mentioned before, it's that new line that marks the separation between headers and body. The line I wish to add right before the newline is something like: X-Test: hello world

Thanks for your help by the way!

The buffer variable doesn't contain just the header and it doesn't end with a newline necessarily.

The buffer reads 2048 bytes at a time from the entire e-mail file. So it might read half the header first, then the other half + half the message.

I have to search for that first newline in each buffer read, and insert my line right before. The newline (\n) must not be replaced. I simply want to add something before.

Ah! Then use strchr() to get the location of the newline.

Code:

 #include <string.h>
 #include <stdio.h>

int main(void)
 {
    char string[15];
    char *ptr, c = 'r';

    strcpy(string, "This is a string");
    ptr = strchr(string, c);
    if (ptr)
       printf("The character %c is at position: %d\n", c, ptr-string);
    else
       printf("The character was not found\n");
    return 0;
 }

Originally Posted by Adak

Ah! Then use strchr() to get the location of the newline.

Code:

 #include <string.h>
 #include <stdio.h>

int main(void)
 {
    char string[15];
    char *ptr, c = 'r';

    strcpy(string, "This is a string");
    ptr = strchr(string, c);
    if (ptr)
       printf("The character %c is at position: %d\n", c, ptr-string);
    else
       printf("The character was not found\n");
    return 0;
 }

and then I use strcat() to append my line there ?

Finally, how do I search for a 'line' containing just \n on it ? I'm asking because each 'line' ends with a \n or \r\n and just searching for a \n is not enough. I'm assuming I'd have to do a search for \n\n ?

This is an example of one way to insert a string of char's, into the middle of another string.

I believe this is what you want:

Code:

 #include <stdio.h>
 #include <string.h> 

 int shift(char *dest, char *src, int len_dest, int len_src, int size);

 int main(void)
 {
   size_t size;
   int len_dest, len_src, ip;
   //dest[] must be oversized to handle it's current char's + the char's
   //that will be inserted.
   char dest[68] = "abcdefghijklmnopqrstuvwxyz0123456789";
   char *src = "******************************";
   len_dest = strlen(dest);
   len_src = strlen(src);
   size = sizeof(dest);

   printf("\n\n\ndestination prior to memmove:\n%s", dest);
   ip = shift(dest, src, len_dest, len_src, size);
   memmove(dest+ip, src, len_src);
   printf("\ndestination after memmove:\n%s", dest);
   ip=getchar();
   return 0;
 }
int shift(char *dest, char *src, int len_dest, int len_src, int size) {
  int i, j, ipoint;
  char *iptr;
  if(size < (len_dest + len_src))
    printf("\n Destination buffer is too small");
  else {
    //find the insertion point:
    iptr = strstr(dest, "defghij"); //
    ipoint = iptr-dest;

    //shift the char's after the insertion point
    for(i=len_dest,j=len_dest+len_src;i>=ipoint;i--, j--) {
      dest[j] = dest[i];
    }
    /*After the shift, the string may not view correctly, until after
      the memmove(). Shifted letters may not appear. They are there ;)
    */   
  }
  return ipoint;
}