全局写入时出现分段错误

我有一个链接到main.c的c++库。在这两个文件之间使用了一个全局变量。变量在sharedata.h中声明

#ifndef __SHARE_DATA_H__
#define __SHARE_DATA_H__
#include <stdio.h>
#include <pthread.h>
#ifdef __cplusplus
extern "C" {
#endif
// declare your two vars in the header file as extern. 
extern pthread_mutex_t mutex;
extern int *ptr;

#ifdef __cplusplus
}
#endif
#endif /* __SHARE_DATA_H__ */

在main.c内部，我有ptr的声明和内存分配。我可以read and print of data in *ptr。

main.c
#include "sharedata.h"
pthread_mutex_t mutex;
int *ptr;
int main(){
pthread_mutex_lock( &mutex );
ptr = (int *)malloc((4 * 5) * sizeof(int)); 
pthread_mutex_unlock( &mutex );
/*do some other things
%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%
*/
pthread_mutex_lock( &mutex ); 
for (int x = 0; x < 5; x++)
{     
printf("%d ", *(ptr+x));         
}
printf("n");
pthread_mutex_unlock( &mutex ); 
}

在gstdsexample.cpp中，我有全局变量的声明和初始化。但是，当我尝试将数据写入*ptr时，我在这行有segmentation fault*(ptr+x) = 1;可能出了什么问题？

#include "sharedata.h"
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
int *ptr;
{
printf("before lockn");
pthread_mutex_lock( &mutex ); 
for (int x = 0; x < 5; x++)
{     
printf("before pointern");
*(ptr+x) = 1;      
}
pthread_mutex_unlock( &mutex ); 
}