Question

In the following program, file_ptr is NULL, yet it is being initialized properly. Why?

#include <stdio.h>
#include <stdlib.h>


void Fopen(const char * restrict filePath, const char * restrict mode, FILE * restrict filePtr);

int main(int argc, char ** argv)
{
    FILE * file_ptr = NULL;
    Fopen("dummy.dat", "r", file_ptr);
    printf("File pointer is %p\n", file_ptr);

    exit(0);
}

void Fopen(const char * restrict filePath, const char * restrict mode, FILE * restrict filePtr)
{
    if ( (filePtr = fopen(filePath, mode)) != NULL)
            printf("file pointer is %p\n", filePtr);
}

On Command Line:

[jim@cola c++]$ ./readFile
file pointer is 0x740010
File pointer is (nil)

The only explanation is that a copy of FILE * is being passed to Fopen. How can i pass pointer by ref?

Answer 1

Your Fopen function changes the value of its local filePtr and then throws that value away. Nothing in your code can change the value of file_ptr in the caller. You just pass a NULL to the function.

You want this:

FILE* Fopen(const char * restrict filePath, const char * restrict mode)
{
    FILE* filePtr;
    if ( (filePtr = fopen(filePath, mode)) != NULL)
            printf("file pointer is %p\n", filePtr);
    return filePtr;
}