Pass by reference 2D dynamically allocated double array in C

Question

I'm trying to convert my string into a dynamic array of doubles. Each space of my string represents a column, each ";" represents a new row. When this code runs, it only works for when *F[0][col]. When it gets to *F[1][col] it gives me the error "Unhandled exception at 0x00e4483c in CCode.exe: 0xC0000005: Access violation reading location 0xcccccccc." Anyone know why?

void main(void) {  
    double **F = NULL;  
    F = malloc(row * sizeof (double *));  
    for (m=0; m < row;m++) {  
        F[m] = malloc(col * sizeof(double ));
    }  
    FParser(string, &F);
for (m=0;m<rowF;m++)
    free(F[m]);
free(F);
}

void FParser(char string[256], double ***F) {  
  while (dummyChar_ptr != NULL) {
    dummyChar_ptr = strtok(dummyChar_ptr," ");
    while ((dummyChar_ptr) != NULL) {
      *F[row][col] = atof(dummyChar_ptr);
      dummyChar_ptr = strtok(NULL," ");
      col++;
    }
    col=0;
    row++;
    strcpy(dummyChar,string);
    dummyChar_ptr = strtok(dummyChar,";");
    for (x=0;x<row;x++) 
      dummyChar_ptr = strtok(NULL,";");  

  }

//example String: 1 0.1 0 0; 0 1 0 0; 0 0 1 0.1; 0 0 0 0.1

why don't you post complete code? we can't find definition of your row and col. Are they global variables? Maybe the problem is you shared row/col between main and FParser. — Francis
– Francis, Commented Jan 24, 2010 at 6:08
can you fix your code listing , I have think it is missing declarations and closing brace, it is hard to read. — Anycorn
– Anycorn, Commented Jan 24, 2010 at 6:09

jamesdlin · Accepted Answer · 2010-01-24 06:26:55Z

8

[] has a higher precedence than unary * in C, so *F[row][col] is actually *(F[row][col]), and you're indexing into the wrong memory location.

Try (*F)[row][col]).

Incidentally, there's no reason for FParser to take a double*** anyway. F is already a pointer; just pass that. The only reason you'd need to use an extra level of indirection is if FParser needed to modify what main()'s F points to.

Other miscellaneous bits of advice:

Check whether malloc succeeded.
Avoid global variables.
Don't use strcpy unless you've checked that the source string won't overflow the destination buffer.
The function parameter char string[256] doesn't actually guarantee that the input argument is an array of 256 (or more) elements, so IMO it's kind of pointless and might as well be char* string.

edited Jan 24, 2010 at 6:26

answered Jan 24, 2010 at 6:20

jamesdlin

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2 Comments

anonymous Over a year ago

It works! Thank you, Jamesdlin, for your response and advice.

caf Over a year ago

In fact, the char string[256] parameter is of type char * (sizeof string will not evaluate to 256, for example).

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