CGI excutable program problem in Linux

Hello experts,

I have a simple CGI function that runs correctly if I am using a console but it doesn't work when it is trigger by a web page submit button.

The function below will change username to "user2" and password to "pass2" on a web page, and redirect the user to another web page.

When execute on a console it changes the username and password and print "Location: /ssi/access.shtml" on the screen.  However, it will not change the username and password but will redirect the user to the new page if it is trigger by a web page submit button.

/**********************************************************************************/
#include <stdio.h>            /* Standard input/output definitions */
#include <stdlib.h>
#include <string.h>            /* String function definitions */
#include <errno.h>
#include <unistd.h>            /* UNIX standard function definitions */
#include <fcntl.h>            /* File control definitions */
#include <ctype.h>

int main(int argc, char *argv[])
{
      if(execlp("htpasswd","htpasswd","/home/www/.htpasswd","user2","pass2",0)<0)
      {
            perror("execl:");
      }

      printf("Location: /ssi/access.shtml\n");      //redirect back to access.shtml
      exit(EXIT_SUCCESS);
}
/**********************************************************************************/

htpasswd is the bin executable that change username and password setting in .htpasswd file
.htpasswd is a text file that store the user name and password
(Both files located under /home/www)

The web server I am using is thttpd (http://www.acme.com/software/thttpd/) and it is run as root.
I can run the CGI script correctly on a console using any login including "root" and "nobody".

I am struggle with this problem for over a week now and I would like to know if anyone can give me some suggestion  on how to solve the problem.
sctccommAsked:
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jlevieCommented:
Open a file in /tmp and have the error results written to that file. You might also look at the logs for your web server and see if there's anything interesting.
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jlevieCommented:
Have you tried specifying the absolute path to htpasswd? It may be in your PATH in an interactive session and can't be found from the PATH of the web server.
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sctccommAuthor Commented:
I try the following before and it behave the same

//      sprintf(command,"/home/www/htpasswd /home/www/.htpasswd user2 pass2");
//      system(command);
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jlevieCommented:
Well that wouldn't work as written, but:

command = sprintf("/home/www/htpasswd /home/www/.htpasswd %s %s", user2 pass2);
system(command);

might, assuming that the variables user2 & pass2 have been set.
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sctccommAuthor Commented:
I am not sure what you mean there jlevie

SYNOPSIS
       #include <stdio.h>
 
       int printf(const char *format, ...);
       int fprintf(FILE *stream, const char *format, ...);
       int sprintf(char *str, const char *format, ...);
       int snprintf(char *str, size_t size, const char *format, ...);
......
   Return value
       Upon successful return, these functions return the number of characters
       printed  (not  including  the  trailing  ’\0’  used  to  end  output to
       strings).  
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jlevieCommented:
My mistake... I had Perl on my mind.

What you had is correct for C code.  When you try using the "system(command" what error is returned?
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sctccommAuthor Commented:
When I run the program in console, there is no error and the program runs correctly.

When it is run as a CGI program, I am not sure how to look at the error message since any printf statment will not show on the terminal.
Do you have any suggestion?
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