C memcpy()分割故障

Question

我一次又一次地分割故障。我的目的是将文件复制到共享内存中，以便其他读者进程可以读取它。要复制文件，我已将M大小数据数组分配在结构中。我已经使用了numberOfbuffers(n(和缓冲(b(，以便m = n*b。m是文件的一半。因此，文件大小约为2m。我认为在此代码中，memcpy((正在生成分段故障。最初，我的目标是通过此作者在共享内存中的全M大小。我的操作系统是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, '', 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 %sn", 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

由于文件大小大于numberOfbuffers，因此此代码要求mod difiss不访问m-> data Array之外的数据。

#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, '', 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 %sn", 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;
}