C memcpy() Segmentation Fault

Question

I have been getting Segmentation Fault again and again. My purpose is to copy a File into a Shared Memory so that another Reader Process can read it. To copy File, I have allocated M size Data Array in a Struct. I have used NumberOfBuffers(N) and BufferSize(B) so that M=N*B. The M is half of the file. So File Size is approximately 2M. I think in this code memcpy() is generating the Segmentation Fault. Initially my target is to full M size in Shared Memory through this Writer.c to see Copy at least is working. My Operating System is Ubuntu.

#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[128];//POINTER PUTS DATA INTO NON-SHARED MEMORY
        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
    unsigned char Buf[BufferSize];

    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{
        strcpy(M->FileName, "xaa");
        M->LastByteLength=0;
        M->ReadPointer=-1;
        M->WritePointer=-1;
        memset(M->Data, '\0', strlen(M->Data));
    }
    char FileName[128]="xaa";
    FILE *FP= fopen(FileName, "rb");
    if(FP!= NULL){
        struct stat StatBuf;
        if(stat(FileName, &StatBuf)==-1){
            printf("failed to fstat %s\n", FileName);
            exit(EXIT_FAILURE);
        }
        long long FileSize=StatBuf.st_size;
        printf("\n File Size: %lld", FileSize);
        long long FilePosition=ftell(FP);
        FilePosition=ftell(FP);
        long long CopyableMemorySize=FileSize-FilePosition;
        printf("\n Copyable File Size: %lld", CopyableMemorySize);
        int NumberOfFileBuffers=CopyableMemorySize/BufferSize;
        printf("\n Number Of File Buffers: %d", NumberOfFileBuffers);
        for(int i=0; i<NumberOfFileBuffers; i++){
            if(abs(M->WritePointer-M->ReadPointer)==NumberOfBuffers){
                //WAIT
            }else{
                fseek(FP, i*BufferSize, SEEK_SET);
                fread(Buf, sizeof(unsigned char), BufferSize, FP);
                memcpy(&M->Data[i*BufferSize], Buf, sizeof(Buf)*sizeof(unsigned char));
           }
        }
        fclose(FP);
    }
    close(SD);
    return 0;
}

Answer 1

As the file size is larger than the NumberOfBuffers, then this code requires a Mod Division not to access data outside M->Data array.

#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[128];//POINTER PUTS DATA INTO NON-SHARED MEMORY
        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
    unsigned char Buf[BufferSize];

    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{
        strcpy(M->FileName, "xaa");
        M->LastByteLength=0;
        M->ReadPointer=-1;
        M->WritePointer=-1;
        memset(M->Data, '\0', strlen(M->Data));
    }
    char FileName[128]="xaa";
    FILE *FP= fopen(FileName, "rb");
    if(FP!= NULL){
        struct stat StatBuf;
        if(stat(FileName, &StatBuf)==-1){
            printf("failed to fstat %s\n", FileName);
            exit(EXIT_FAILURE);
        }
        long long FileSize=StatBuf.st_size;
        printf("\n File Size: %lld", FileSize);
        long long FilePosition=ftell(FP);
        FilePosition=ftell(FP);
        long long CopyableMemorySize=FileSize-FilePosition;
        printf("\n Copyable File Size: %lld", CopyableMemorySize);
        int NumberOfFileBuffers=CopyableMemorySize/BufferSize;
        printf("\n Number Of File Buffers: %d", NumberOfFileBuffers);
        for(int i=0; i<NumberOfFileBuffers; i++){
            int BufferModCount=i%NumberOfBuffers;
            if(abs(M->WritePointer-M->ReadPointer)==NumberOfBuffers){
                //WAIT
            }else{
                fseek(FP, i*BufferSize, SEEK_SET);
                fread(Buf, sizeof(unsigned char), BufferSize, FP);
                printf("\n Checking: %d", i*BufferSize);

                memcpy(&M->Data[BufferModCount*BufferSize], &Buf, sizeof(Buf)*sizeof(unsigned char));
            }
        }
        fclose(FP);
    }
    close(SD);
    return 0;
}