Permission denied issue in mmap function in shared memory in C - c

Question

In the child process the mapping is giving me issue. the parent is creating a shared memory and child is using it for writing a message to parent but when the mmap executes, it gives an error of permission denied. kindly help me to get rid of this issue. everything else is running file. when I try to create shared memory in child process the issue is resolved but the requirement is creating shared memory in parent process, that is why it is taking time to solve it.
/**
* Simple program demonstrating shared memory in POSIX systems.
*
* This is the consumer process
*
* Figure 3.18
*
* To compile, enter
* gcc shm-posix-consumer.c -lrt
*
* #author Gagne, Galvin, Silberschatz
* Operating System Concepts - Tenth Edition
* Copyright John Wiley & Sons - 2018
*/
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <string.h>
#include <errno.h>
#include <sys/shm.h>
#include <sys/stat.h>
#include <sys/wait.h>
#include <sys/mman.h>
int main()
{
const int SIZE = 4096;
const char *name = "OS";
//shm_unlink(name);
int shm_fd;
void *ptr;
pid_t pid;
pid=fork();
if(pid==-1) {
printf("Error in creating a child process\n");
return -1;
}
else if(pid==0) {
sleep(2);
printf("child process is executing\n");
/* open the shared memory segment */
shm_fd = shm_open(name, O_RDWR, S_IRWXU);
if (shm_fd == -1) {
printf("opening of shared memory failed\n");
exit(-1);
}
if(ftruncate(shm_fd,SIZE)==-1){
printf("Error in configuring the size of shared memory");
exit(1);
}
/* now map the shared memory segment in the address space of the process */
ptr = mmap(0,SIZE,PROT_WRITE, MAP_SHARED, shm_fd, 0);
if (ptr == MAP_FAILED) {
perror("Map failed in child process\n");
printf("error:%s %d",strerror(errno),errno);
return -1;
}
char *message0;
strcpy(message0,"Greeting to parent");
printf("Message to parent from child:%s\n",message0);
sprintf(ptr,"%s",message0);
ptr += strlen(message0);
if(munmap(ptr,SIZE)==-1){
printf("munmap error:\n");
exit(1);
}
close(shm_fd);
}
else {
printf("parent process is executing\n");
/* create the shared memory segment */
shm_fd = shm_open(name, O_CREAT | O_RDWR, S_IRWXU);
/* configure the size of the shared memory segment */
if(ftruncate(shm_fd,SIZE)==-1){
printf("Error in configuring the size of shared memory");
exit(1);
}
printf("parent process wait after creating shared memory\n");
wait(NULL);
printf("back to parent process after child termination\n");
/* now map the shared memory segment in the address space of the process */
ptr = mmap(0,SIZE, PROT_READ, MAP_SHARED, shm_fd, 0);
if (ptr == MAP_FAILED) {
printf("Map failed in parent\n");
return -1;
}
printf("%s\n",(char *)ptr);
if (shm_unlink(name) == -1) {
printf("Error removing %s\n",name);
exit(-1);
}
}
return 0;
}

Answer 1

At least three problems:
char *message0;
strcpy(message0,"Greeting to parent");
You are using message0 uninitialized, switch to something like
char message0[64];
strcpy(message0,"Greeting to parent");
or simply:
char *message0 = "Greeting to parent";
Here:
void *ptr;
...
ptr += strlen(message0);
You can't use pointer arithmetic with void *, switch to char *ptr;
And here:
ptr += strlen(message0);
if(munmap(ptr,SIZE)==-1){
you need to rewind ptr to the original position returned by mmap before calling munmap, it seems that you can remove this line:
ptr += strlen(message0);
Now it works for me adding #define _XOPEN_SOURCE 500 at the very beginning (for ftruncate).