sin el codigo completo es algo dificil adivinar de donde viene el error...

#include <sys/socket.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <netdb.h>
#include <unistd.h>
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <limits.h>
#include <sys/types.h>
#include <signal.h>
 
#define MAX_dani 0xF0000 
 
struct params{
  char web[20];
  int port; 
  int thread;
  int nrequests;
  int sleeptime;
};
 
int total_requests;
pthread_mutex_t lock;
int nthreads;
 
void* t(void *p)
{
  int n = ((struct params*)p)->thread + 1;
  printf("Proceso %i: presente\n", n);
  char request[128];
  char buff[10000] = {'\0'};
  struct hostent *host = gethostbyname(((struct params*)p)->web);
  if(!host)
  {
    printf("Proceso %i: No se ha podido resolver la direccion del servidor\n", n);
    return NULL;
  }
  printf("Proceso %i: host->h_addr: %s\n", n, inet_ntoa(*((struct in_addr *)host->h_addr)));
  struct sockaddr sockbuf;
  int stsize = sizeof(sockbuf);
  struct sockaddr_in sock;
  sock.sin_family = AF_INET;
  sock.sin_port = htons(((struct params*)p)->port);
  sock.sin_addr.s_addr = inet_addr(inet_ntoa(*((struct in_addr *)host->h_addr)));
  printf("Proceso %i: Direccion: %d\n", n, sock.sin_addr.s_addr);
 
 int sockfd = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
 if(sockfd==-1)
 {
  printf("Proceso %i: No se pudo crear el socket\n", n);
  return NULL;
 }
 printf("Proceso %i: Socket creado\n", n);
 printf("Proceso %i: Conectando...\n", n);
 int aux;
 connect(sockfd, (struct sockaddr*) &sock, sizeof(struct sockaddr));
 printf("Proceso %i: Conectado\n", n);
 sprintf(request, "GET / HTTP/1.1\nHost: %s\nUser-Agent: Mozilla/4.0\n\n ", host->h_name);
 printf("Proceso %i: Peticion en request\n", n);
 for(aux=0; aux<(((struct params*)p)->nrequests); aux++)
 {
   switch(getpeername(sockfd, &sockbuf, &stsize))
   {
    case 0: break; 
    case ENOTCONN:
     printf("Proceso %i: reconectando socket...", ((struct params*)p)->thread);
     close(sockfd);
     sockfd = socket(AF_INET, SOCK_STREAM, 0);
     if(connect(sockfd, (struct sockaddr*) &sock, sizeof(struct sockaddr))==-1) continue;
     else break;
   }
   write(sockfd, request, strlen(request));
   printf("Proceso %i: %i peticion/es\n", n, aux+1);
   pthread_mutex_lock(&lock);
   total_requests++;
   sleep((((struct params*)p)->sleeptime)/nthreads);
   pthread_mutex_unlock(&lock);
 }
 close(sockfd);
 pthread_exit(NULL);
}
 
int main(int argc, char *argv[])
{
 pthread_mutex_init(&lock, NULL);
 pthread_t *mythreads = (pthread_t*)malloc(atoi(argv[1])*sizeof(pthread_t));
 pthread_attr_t a;
 struct params *p = (struct params*)malloc(atoi(argv[1])*sizeof(struct params));
 int i, ret, j;
 nthreads = atoi(argv[1]);
 for(j=0; j<nthreads; j++)
 {
  strcpy(p[j].web, argv[2]);
  p[j].thread = j;
  p[j].port = atoi(argv[3]);
  p[j].nrequests = atoi(argv[4]);
  p[j].sleeptime = atoi(argv[5]);
 }
 pthread_attr_init(&a);
 pthread_attr_setstacksize(&a, MAX_dani);
 for(i=0; i<nthreads; i++)
 {
  ret = pthread_create(&mythreads[i], &a, t, &p[i]);
  switch(ret)
  {
   case 0:
    printf("Thread %i creado\n", i+1);
    break;
   case EAGAIN:
    printf("EAGAIN\n");
    _exit(1);
   case EINVAL:
    printf("EINVAL\n");
    _exit(2);
   case EPERM:
    printf("EPERM\n");
    _exit(3);
  }
 }
 pthread_attr_destroy(&a);
 for(j=0; j<nthreads; j++)
 pthread_join(mythreads[j], NULL);
 pthread_mutex_destroy(&lock);
 printf("Retornaron los hilos\n");
 printf("Total de peticiones: %i\n", total_requests);
 return 0;
}