C Shared Memory Reader-Writer Segmentation Fault - c

Question

Here is a Synchronized Reader and Writer. The target is passing data between these two Processes via a Shared Memory.
The Writer opens a Shared Memory through a Structure and writes Some Data. I am getting Segmentation Fault(Core Dumped) error message.
The code is compiled through the following command in Ubuntu.
g++ Writer.c -o Writer -lrt
g++ Reader.c -o Reader -lrt
And these two Processes are run by-
./Writer
./Reader
The Writer.c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/stat.h>
#include <sys/mman.h>
int main(void){
struct MemData{
char* FileName;
int LastByteLength;
int ReadPointer;
int WritePointer;
char Data[512000];//MEMORY BLOCK SIZE: 500 KB
};
int SD;
struct MemData *M;
int NumberOfBuffers=10;
int BufferSize=51200;//FILE BUFFER SIZE 50 KB
SD= shm_open("/program.shared", O_RDWR|O_CREAT, S_IREAD|S_IWRITE);
if(SD< 0){
printf("\nshm_open() error \n");
return EXIT_FAILURE;
}
fchmod(SD, S_IRWXU|S_IRWXG|S_IRWXO);
if(ftruncate(SD, sizeof(MemData))< 0){
printf ("ftruncate() error \n");
return EXIT_FAILURE;
}
//THE FOLLOWING TYPECASTING AVOIDS THE NEED TO ATTACH THROUGH shmat() in shm.h HEADER I GUESS.
M=(struct MemData*)mmap(NULL, sizeof(MemData), PROT_READ|PROT_WRITE, MAP_SHARED, SD, 0);
if(M== MAP_FAILED){
printf("mmap() error");
return EXIT_FAILURE;
}else{
M->FileName=(char*)"xaa";
M->LastByteLength=0;
M->ReadPointer=-1;
M->WritePointer=-1;
memset(M->Data, '\0', strlen(M->Data));
}
/*
FILE *FP= fopen(FileName, "rb");
if(FP!= NULL){
unsigned long int FilePosition;
fseek(FP, 0, SEEK_SET);
FilePosition=ftell(FP);
fclose(FP);
}
*/
close(SD);
return 0;
}
The Reader.c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/stat.h>
#include <sys/mman.h>
int main(void){
struct MemData{
char* FileName;
int LastByteLength;
int ReadPointer;
int WritePointer;
char Data[512000];//MEMORY BLOCK SIZE: 500 KB
};
int SD;
struct MemData *M;
int NumberOfBuffers=10;
int BufferSize=51200;//FILE BUFFER SIZE 50 KB
SD= shm_open("/program.shared", O_RDWR|O_CREAT, S_IREAD|S_IWRITE);
if(SD< 0){
printf("\nshm_open() error \n");
return EXIT_FAILURE;
}
fchmod(SD, S_IRWXU|S_IRWXG|S_IRWXO);
if(ftruncate(SD, sizeof(MemData))< 0){
printf ("ftruncate() error \n");
return EXIT_FAILURE;
}
//THE FOLLOWING TYPECASTING AVOIDS THE NEED TO ATTACH THROUGH shmat() in shm.h HEADER I GUESS.
M=(struct MemData*)mmap(NULL, sizeof(MemData), PROT_READ|PROT_WRITE, MAP_SHARED, SD, 0);
if(M== MAP_FAILED){
printf("mmap() error");
return EXIT_FAILURE;
}else{
printf("\n%s", M->FileName);
printf("\n%d", M->LastByteLength);
printf("\n%d", M->ReadPointer);
printf("\n%d", M->WritePointer);
}
/*
FILE *FP= fopen(FileName, "rb");
if(FP!= NULL){
unsigned long int FilePosition;
fseek(FP, 0, SEEK_SET);
FilePosition=ftell(FP);
fclose(FP);
}
*/
munmap(M,sizeof(MemData));
close(SD);
return 0;
}

Answer 1

Based on your comments, the issue is because of the way you're assigning and passing the FileName value.
M->FileName=(char*)"xaa";
This results in M->FileName holding a pointer to a string in the writer process' memory. Dereferencing this pointer in the reader process results in a segmentation fault due to the filename being stored in the writer process memory, which is not shared with the reader. You need to store the characters themselves in the shared memory, not a pointer to writer process memory.
If you can safely assume the maximum length of the filename string, you can change your struct to store the entire string rather than a pointer: change char* FileName; to char FileName[256]; or some other fixed length value. You will need to use strcpy rather than direct assignment after making this change: change M->FileName=(char*)"xaa"; to strcpy(M->FileName, "xaa");.
If you want a dynamic length string, you can call mmap again to allocate shared memory for just the string, and then store the pointer to this shared memory string in FileName.