Passing FILE pointer to a function

Question

I'm little bit confused over here, not quite sure about this. What I'm trying to do is to pass the name of a file through terminal/cmd that will be opened and read from.

myfunction(char* fileName, FILE* readFile)
{
    if((readFile = fopen(fileName,"r")) == NULL)
    {
        return FILE_ERROR;
    }
    return FILE_NO_ERROR;
}

int main(int argc, char **argv)
{
FILE* openReadFile;
    if(myfunction(argv[1], openReadFile) != FILE_NO_ERROR)
    {
        printf("\n %s : ERROR opening file. \n", __FUNCTION__);
    }
}

My question is if i pass a pointer openReadFile to myfunction() will a readFile pointer to opened file be saved into openReadFile pointer or do i need to put *readFile when opening.

You need to pass &openReadFile into the function if you want the updated pointer passed back to the caller. The parameter declaration of the function would be FILE** readFile — Jerry Jeremiah
– Jerry Jeremiah, Commented Oct 27, 2015 at 20:38
... or pass the successful pointer back as function value, or NULL. — Weather Vane
– Weather Vane, Commented Oct 27, 2015 at 20:41
If you want to read from the file in main() you can't, because it's pointer value is lost after the function returns. — Weather Vane
– Weather Vane, Commented Oct 27, 2015 at 20:42

Weather Vane · Accepted Answer · 2015-10-27 20:44:22Z

16

FILE * needs to be a pointer, so in main openReadFile stays as a pointer. myfunction takes **, so we can update the FILE * with the result from fopen *readFile = fopen... updates the pointer.

int myfunction(char* fileName, FILE** readFile) /* pointer pointer to allow pointer to be changed */
{
    if(( *readFile = fopen(fileName,"r")) == NULL)
    {
        return FILE_ERROR;
    }
    return FILE_NO_ERROR;
}

int main(int argc, char **argv)
{
    FILE* openReadFile; /* This needs to be a pointer. */
    if(myfunction(argv[1], &openReadFile) != FILE_NO_ERROR) /* allow address to be updated */
    {
        printf("\n %s : ERROR opening file. \n", __FUNCTION__);
    }
}

edited Oct 27, 2015 at 20:44

Weather Vane

34.8k7 gold badges43 silver badges63 bronze badges

answered Oct 27, 2015 at 20:41

mksteve

13.4k3 gold badges31 silver badges53 bronze badges

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1 Comment

Sir DrinksCoffeeALot Over a year ago

Think i got it, if i would have only one * in int myfunction(char* fileName, FILE** readFile) i would be basicly changing on what readFile is pointing, and with double * im changing on which pointer from main is pointing too. Yay.

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