this is a hand written lexer..its showing segmentation fault.can any1 help?

[dark lord]

extern void *malloc();
//structure of a token
struct token
{
    int tval;
    char *lexeme;
    struct token *link;
};
 struct token *tfirst=NULL;
char *seentext;
 
 int state=0;int nc=0;
 
//adds token at the beginning in a symbol table
struct token * tokenlist(int val,char *word,struct token *tfirst)
{ 
        struct token *tp;
 
    tp=(struct token *)malloc(sizeof(struct token));
    tp->tval=val;
    tp->lexeme=(char *) malloc(strlen(word) + 1);
    strcpy(tp->lexeme,word);
    tp->link=tfirst;
    return tp->link;
}
 
 
 
int addtoken(int val,char * word)
{       int x;
     if ((x=lookup(word))==0)
    {
     struct token * newtoken;
    newtoken=(struct token *)malloc(sizeof(struct token));
    newtoken->tval=val;
    newtoken->lexeme=(char *)malloc(strlen(word)+1);
    strcpy(newtoken->lexeme,word);
    newtoken->link=tfirst;
    tfirst=newtoken;
    free(seentext);
    return val;
    }
 
    return x;
} 
 
 
int lookup(char *word)
{
    struct token *mark;
    mark=tfirst;
    while(mark!=NULL)
        {
        if(strcmp(mark->lexeme,word)==0)
        return mark->tval;
        mark=mark->link;
        }
    return 0;
}
void delete(struct token *tfirst)
{
  struct token *m;
  if(tfirst==NULL)
  return;
  m=tfirst;
  while(m!=NULL)
  { 
    tfirst=m;
    m=m->link;
    free(tfirst);
  }
}
 
int lex()
{
    char c[2];c[1]='\0';
 
    int BR=0;
 
    while(1)
    {
        switch(state)
        {
            case 0:{
                c[0]=getchar();
                seentext=(char * )malloc(50);
                if(c[0]==EOF) return 0;
                if(c[0]=='-')
                addtoken(MINUS,"-");
 
                else if(c[0]=='+')
                addtoken(PLUS,"+");
 
                else if(c[0]=='*')
                addtoken(MUL,"*");
 
                else if(c[0]=='/')
                addtoken(DIV,"/");
 
                else if(c[0]=='%'){state=0;addtoken(REM,"%");}
                else if(c[0]=='$'){state = 3;break;}
                else if(c[0]=='<'){state=4;break;}
                else if(c[0]=='>'){state=5;break;}
                else if(isalpha(c)){state=1;break;}
                else if(isdigit(c)){state=2;break;}
                if(c[0]==' '||c[0]=='\t'||c[0]=='\n') {state=13;}
                //else error();
                }
            case 1:{
                seentext=(char *)malloc(50);
                strcpy(seentext,c);
                while(1)
                    {
                        c[0]=getchar();
                        if((c[0]==EOF) && !isalpha(c))
                            {
                                ungetc(c[0],stdin);
                                state=0;
                                addtoken(ID,seentext);
 
                            }
                        else
                        strcat(seentext,c);
                    }    
                }
            case 2:{
 
                strcpy(seentext,c);
                while(1)
                    {
                        c[0]=getchar();
                        if ((c[0]==EOF) || !isdigit(c))
                            {
                                ungetc(c[0],stdin);
                                state=0;addtoken(NUM,seentext);
                            }
                        else 
                            strcat(seentext,c);
                    }
                }
 
            case 3:{
                c[0]=getchar();
                if(c[0]=='$')
                {
                BR++;
                state=0;
                addtoken(OPBR,"$$");
                }
                 if(c[0]==' '||c[0]=='\n'||c[0]=='\t')
                {
                BR--;
                state=0;
                addtoken(CLBR,"$");}
                //else error();
                }
            case 4:{
                //seentext=(char *)malloc(50);
                //strcpy(seentext,c);
                c[0]=getchar();
                if(c[0]=='=')
                {
                //strcpy(seentext,c);
                state=0;
                addtoken(ROP,"<=");
                }
                 if(c[0]=='>')
                {
                //strcpy(seentext,c);
                state=0;
                addtoken(ROP,"<>");
                }
                 if(c[0]==' ')
                {
                state=0;ungetc(c[0],stdin);    
                addtoken(ROP,"<");
                }
                //else
                //error();
                            }
            case 5:
                {
                //seentext=(char *)malloc(50);
                //strcpy(seentext,c);
                c[0]=getchar();
                if(c[0]=='=')
                {
                //strcat(seentext,c);
                state=0;            
                addtoken(ROP,">=");
                }
                 if(c[0]==' ')
                {            
                state=0;
                ungetc(c[0],stdin);            
                addtoken(ROP,">");
                }
                //else error();
 
                 }
            case 6:{
                while((c[0]=getchar())!=EOF && (c[0]=='\n'||c[0]=='\t'||c[0]==' '))
                            if(c[0]=='\n') nc++;
 
                            if(c[0]==EOF) return 0;
                            ungetc(c[0],stdin);state=0;
                            break;
                }
        }    
    }   
}
 
int main()
    { 
        int val;
 
        tfirst=tokenlist(MAIN,"main()",tfirst);
        tfirst=tokenlist(PLUS,"+",tfirst);
        tfirst=tokenlist(MINUS,"-",tfirst);
        tfirst=tokenlist(MUL,"*",tfirst);
        tfirst=tokenlist(DIV,"/",tfirst);
        tfirst=tokenlist(REM,"%",tfirst);
        tfirst=tokenlist(iff,"iff",tfirst);
        tfirst=tokenlist(den,"den",tfirst);
        tfirst=tokenlist(orelse,"orelse",tfirst);
        tfirst=tokenlist(till,"till",tfirst);
        tfirst=tokenlist(ASS,"=",tfirst);
        tfirst=tokenlist(ROP,"<=",tfirst);
        tfirst=tokenlist(ROP,"<>",tfirst);
        tfirst=tokenlist(ROP,"<",tfirst);
        tfirst=tokenlist(ROP,">=",tfirst);
        tfirst=tokenlist(ROP,">",tfirst);
 
            while (val=lex()){
                printf("%d \n",val);}
        return 0;
    }
 

[jkmyoung]

Can you give us more information on what it's supposed to do, so we can better understand it?