segmentation fault occur on thread program - c

Question

I am new in thread program. I wrote a C program for executing threads which reverse the command line string and print the both original and reversed string. My program is here:
#include<stdio.h>
#include<stdlib.h>
#include<unistd.h>
#include<sys/types.h>
#include<pthread.h>
#include<string.h>
typedef struct
{
char *input_string;
char *rev_string;
}data;
void* rev_string(void* arg)
{
int len = 0,index1 = 0,
index2 = 0;
data* names = NULL; /*bject creation*/
names = (data*)malloc(sizeof(data));
if( NULL == names)
{
printf("malloc failure\n");
exit(1);
}
printf("thread recvd = %s\n",(char*)arg);
len = strlen((char*)arg);
names->input_string = (char*)malloc(sizeof(char)*(len+1));
if( NULL == names->input_string)
{
printf("malloc failure\n");
exit(1);
}
strncpy(names->input_string,(char*)arg,len); /*copying input_string to the struct*/
names->rev_string = (char*)malloc(sizeof(char)*(len+1));
if( NULL == names->rev_string)
{
printf("malloc failure\n");
exit(1);
}
for(index1 = len-1 ; index1 >= 0 ; index1--)
{
names->rev_string[index2] = names->input_string[index1];
index2++;
}
pthread_exit((void*)names);
}
Main program:
int main(int argc,char* argv[])
{
int no_of_strings = 0,
len = 0,
status = 0,
ret = 0 ,
index = 0;
pthread_t id[index]; /*thread identifier*/
void* retval = NULL; /*retval to store the value returned by thread job */
data *strings = NULL;/*object creation*/
if(1 >= argc)
{
printf(" please do enter the commands as of below shown format\n");
printf("<exe.c> <string1> <string2> ...<string(N)>\n");
exit(1);
}
no_of_strings = argc - 1 ; /* no of strings entered */
/*creation of threads*/
for(index = 0; index < no_of_strings ;index++)
{
status = pthread_create(&id[index],NULL,rev_string,(void*)argv[index + 1]);
if(status != 0)
{
printf("ERROR in creating thread\n");
exit(1);
}
else
{
printf("thread %d created\n",index+1); }
}
for(index = 0 ;index < no_of_strings;index++)
{
ret = pthread_join(id[index],&retval);
if(ret)
{
printf("Error in joining %d\n", ret);
exit(1);
}
printf("the input_string = %s and its reverse = %s\n",((data*)retval)->input_string,((data*)retval)->rev_string);
}
// free(retval->input_string);
// free(retval->rev_string);
// free(retval);
pthread_exit(NULL);
exit(0);
}
It works on 2 string from command line argument. But got segmentation fault when more than strings. Why? Any errors? Help me.

Answer 1

pthread_t id[index];
id has zero length because index == 0. So any assignment to its elements is undefined behavior. Initialize the array with some positive length, e.g:
const int MAX_THREADS = 10;
pthread_t id[MAX_THREADS]; /*thread identifier*/