CGI guestbook

/**** Beginning of file guestbook.c ****/
 
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <conio.h>
#include <ctype.h>
#include <time.h>
 
 
/* here are the names of data files needed */
char*header="header.txt";
char*guestbook="guestbook.txt";
char*footer="footer.txt";
 
/* this will hold 0 or a message for the user */
char*msg=0;
 
 
/* data structure for one cgi parameter */
typedef struct{
 char*name;
 char*value;
} Arg;
 
 
 
/* prints error message to stdout and commits seppuku */
void err(char*s){
 puts("Guestbook has encountered an error!");
 puts(s);
 exit(1);
}
 
 
/* returns value of one hex digit */
int getHex(char c){
 if(isdigit(c))return c-'0';
 if(isalpha(c))return tolower(c)-'a'+10;
 return 0;
}
 
 
/* decodes 1 character from hex*/
char decode(char*s){
 return getHex(s[0])*16+getHex(s[1]);
}
 
 
/* to prevent making tags in html */
int safeChar(char c){
 switch(c){
  case '<':
  case '>':
  return 0;
 }
 return 1;
}
 
 
/* returns 1 if s points to 2 valid consecutive hex digits */
int isSafe(char*s){
 if(!isxdigit(s[0]) || !isxdigit(s[1])) return 0;
 return 1;
}
 
 
/* decodes hex values, converts + to space,
 and converts newline to <br> */
char*fixData(char*i){
 int l=strlen(i);
 char*m=malloc(l*4+8);
 char*o=m;
 if(!m)err("Memory allocation failure for fixData");
 while(*i){
  if(*i=='+')*o=' ';
  else if(*i=='%'&&isSafe(i+1)){
   char d=decode(i+1);
   if(safeChar(d)){
    if(d=='\n')o+=sprintf(o,"<br>")-1;
    else if(d=='\r')--o;
    else *o=d;
   }else o+=sprintf(o,"&#%d;",d)-1;
   i+=2;
  }else *o=*i;
  ++i;
  ++o;
 }
 *o=0;
 m=realloc(m,o-m+1);
 if(!m)err("Memory reallocation failure for fixData");
 return m;
}
 
 
/* prints a basic http header to stdout */
void init(){
 printf("Content-Type: text/html\n\n");
}
 
 
/* gets input string from stdin */
char*getInput(){
 char*v=getenv("CONTENT_LENGTH");
 char*s=0;
 if(v){
  int l=atoi(v);
  if(l){
   s=malloc(l+1);
   FILE*f=stdin;
   if(!s)err("Memory allocation failure for input");
   fread(s,1,l,f);
   s[l]=0;
  }
 }
 return s;
}
 
 
/* returns count of variables encoded in string */
int countVars(char*s){
 int n=1;
 while(*s){
  if(*s=='&')++n;
  ++s;
 }
 return n;
}
 
 
/* Moves pointer-pointer s just past next occurrence of character c,
 which is then set to 0, or returns 1 if end of string is reached */
int skipTo(char**s,char c){
 while(**s != c){
  if(!**s)return 1;
  ++*s;
 }
 **s=0;
 ++*s;
 return 0;
}
 
 
/* Parses input string for args */
Arg*parseInput(char*s){
 int l=countVars(s);
 Arg*arg=malloc((l+1)*sizeof(Arg));
 Arg*a=arg;
 if(!a)err("Memory allocation failure for args list");
 while(*s){
  a->name=s;
  if(skipTo(&s,'='))goto Done;
  a->value=s;
  if(skipTo(&s,'&'))goto Done;
  ++a;
 }
 Done:
 arg[l].name=0;/* marks the end of arg list */
 return arg;
}
 
 
/* returns cgi parameter in array a  that has name==s */
char*getArg(Arg*a,char*s){
 while(a->name){
  if(!strcmp(a->name,s))return fixData(a->value);
  ++a;
 }
 return "";
}
 
 
/* returns 1 if parameter list has required inputs */
int isValid(Arg*a){
 char*s1=getArg(a,"name");
 char*s2=getArg(a,"text");
 int b=*s1 && *s2;
 free(s1);
 free(s2);
 return b;
}
 
 
/* saves posted message to guestbook data file */
void saveOutput(Arg*a){
 FILE*f=fopen(guestbook,"at");
 char*s;
 if(!f)err("Can't save file!");
 s=getArg(a,"name");
 fprintf(f,"<p><div class=txt><b>%s</b>\n",s);
 free(s);
 {
  time_t rawtime;
  struct tm*timeinfo;
  time(&rawtime);
  timeinfo=localtime(&rawtime);
  fprintf(f,"&nbsp;%s<br><hr>",asctime(timeinfo));
 }
 s=getArg(a,"text");
 fprintf(f,"%s",s);
 free(s);
 fputs("</div>\n\n",f);
 fclose(f);
}
 
 
/* prints a file to stdout */
void showFile(char*s){
 FILE*f=fopen(s,"rt");
 if(f){
  while(1){
   char c=getc(f);
   if(c==EOF)break;
   putchar(c);
  }
  fclose(f);
 }
}
 
 
/* prints whole page to stdout */
void showOutput(){
 showFile(header);
 if(msg)puts(msg);
 showFile(guestbook);
 showFile(footer);
}
 
 
int main(){
 char*i;
 Arg*a;
 init();
 i=getInput();
 if(i){
  a=parseInput(i);
  if(isValid(a)){
   saveOutput(a);
   msg="Your message has been added!";
  }else msg="Invalid input!";
  free(i);
  free(a);
 }
 showOutput(a);
 return 0;
}
 
 
/**** End of file guestbook.c ****/
 
/**** Beginning of file header.txt ****/
 
<html><head><title>Guestbook</title><style>
body,.txt,input,textarea,button{color:#fff;background-color:#000;font:16px verdana}
textarea{width:90%;height:256px}
.txt{background-color:#008}
.txt{border:1px solid #0f0;padding:4px}
</style></head><body>
 
/**** End of file header.txt ****/
 
/**** Beginning of file footer.txt ****/
 
<p><form action="guestbook.exe" method=post>
<center>Name:<br><input name=name maxlength=32><p>
Message:<br><textarea name=text></textarea>
<p><input type=submit value="Post">
</form></center></body></html>
 
/**** End of file footer.txt ****/