在单独的函数中创建动态数组，并从调用函数进行操作

Question

I am working a function that opens a file, a function that reads the contents of that file into a dynamic array, a function that closes the file.

So far I am able to do all of the above except the dynamic array is going out of scope when I get back to the calling location (main). I want to store additional data in the array while in main or even a separate function. Once I am done adding data to the dynamic array I will write the contents of it back to the source file overwriting it with the new data then closing that file. The purpose is to append data to the top of the original file. What am I doing wrong with the function char *LoadFileData(FILE *fp, char* charPtr); that I am not able to access or modify it back in main?

Thanks for help on this.

    FILE *fSource;       // create source file pointer instance
    char mode[] = "a+";  // default file open mode
    char inStr[80];      // string to get input from user
    char *tempFileData;  // dynamic string to hold the existing text file

// Open the source file
    strcpy(mode, "r");   // change the file opnen mode to read
    FileOpen(&fSource, mode);

// Load the source file into a dynamic array
    LoadFileData(fSource, tempFileData);  // this is where I fail that I can tell.

    printf("%s", tempFileData); // print the contents of the (array) source file //(for testing right now)
    FileClose(&fSource);  // close the source file

j

char *LoadFileData(FILE *fp, char* charPtr)
  {
    int i = 0;
    char ch = '\0';
    charPtr = new char; // create dynamic array to hold the file contents
    if(charPtr == NULL)
    {
        printf("Memory can't be allocated\n");
        exit(0);
    }
// loop to read the file contents into the array
   while(ch != EOF)
    {
        ch = fgetc(fp);  // read source file one char at a time
        charPtr[i++] = ch;
    }
    printf("%s", charPtr); // so far so good.
    return charPtr;
  }

Answer 1

Instead of passing in a char * whose value you never use, assign the return value of the function to tempFileData .

So change the function like this:

char *LoadFileData(FILE *fp)
{
    char* charPtr;
    ...

Then call it like this:

tempFileData = LoadFileData(fSource);  

Answer 2

One of the problems is the combination of the following lines:

charPtr = new char; // create dynamic array to hold the file contents

    charPtr[i++] = ch;

You are allocating memory for just one char but proceeding to use it as though it can hold lots characters.

You need to:

1. Find the number of characters present in the file.
2. Allocate memory for all the characters ( +1 if you need to null terminate the array).
3. Read the contents of the file to the allocated memory.

Answer 3

返回一个string怎么样？

string LoadFileData(FILE *fp, char* charPtr)

Answer 4

based on everyone's feedback this is the modification that worked. Thank you!

char* LoadFileData(FILE *fp) 
{
    off_t size; // Type off_t represents file offset value. 
    int i = 0;
    char ch = '\0';
    char *charPtr; // dynamic arrary pointer that will hold the file contents

    fseek(fp, 0L, SEEK_END); // seek to the end of the file
    size = ftell(fp);        // read the file size.
    rewind(fp);              // set the file pointer back to the beginning

    // create a dynamic array to the size of the file
    charPtr = new char[size + 1]; 

    if (charPtr == NULL) {
        printf("Memory can't be allocated\n");
        // exit(0);
    }

    while (ch != EOF) {
        ch = fgetc(fp); // read source file one char at a time
        if (ch < 0) { // do not copy it if it is an invalid char
        }
        else {
            charPtr[i++] = ch;
            // load the char into the next ellement of the array
            // i++;
        }// end else
    } // end while

    return charPtr; 
}