#include <stdio.h>
int new_text_file(FILE *text_address)
{
FILE *new_text;
new_text = fopen("weekly.txt", "w");
if (new_text == NULL)
{
printf("Cannot Open Weekly.txt");
return(1);
}
*text_address = &new_text;
return(0);
}
int main(void)
{
char line[10];
FILE *text_address;
if(new_text_file(&text_address) == 1)
{
printf("Error opening file\n\n");
exit(2);
}
fgets(line, sizeof(line), stdin);
}
FILE pointers C
Author
Hi im trying to get a pointer in c to pass right.
I have a function that is stand alone. Im trying to open a file then pass the files address to another file pointer in the main function. So that another function or the main one can write to the file. When I try to compile this I get assignment from incompatible pointer type. Yet both pointers are from the FILE declartion.
Here is the section im working on.
I have not tried it, but I think this will solve your problem. Note that the parameter should be a pointer to a pointer.
Hope that works for you.
BTW, I'm right across the bridge from ya. [smile]
#include <stdio.h>int new_text_file(FILE **text_address) /* DOUBLE POINTER */ { FILE *new_text; new_text = fopen("weekly.txt", "w"); if (new_text == NULL) { printf("Cannot Open Weekly.txt"); return(1); } *text_address = &new_text; return(0); }int main(void) { char line[10]; FILE *text_address; if(new_text_file(&text_address) == 1) { printf("Error opening file\n\n"); exit(2); } fgets(line, sizeof(line), stdin); }Hope that works for you.
BTW, I'm right across the bridge from ya. [smile]
*text_address = new_text;
But where is the fclose()?! Also, I would highly recommend you switch around the 1 and 0 error codes, since 0 automatically goes to false in an if statement, and 1 to true. That way, I can say:
The way you have it, what I wrote would be when a file IS created, which doesn't really make much sense...
#include <stdio.h>int new_text_file(FILE **text_address) { FILE *new_text; new_text = fopen("weekly.txt", "w"); if (new_text == NULL) { printf("Cannot Open Weekly.txt"); return(1); } *text_address = new_text; return(0); }int main(void) { char line[10]; FILE *text_address; if(new_text_file(&text_address) == 1) { printf("Error opening file\n\n"); exit(2); } fgets(line, sizeof(line), stdin); }But where is the fclose()?! Also, I would highly recommend you switch around the 1 and 0 error codes, since 0 automatically goes to false in an if statement, and 1 to true. That way, I can say:
if(!new_text_file(text_address)) { //error }The way you have it, what I wrote would be when a file IS created, which doesn't really make much sense...
I am, by no means, an expert on pointers, but it seems to me that since you're passing the address of(&) text_address to the function, when you use the assignment operator, it should be:
&text_address = &new_text;
Again, I'm no expert by any means, but it's just what my (with my unqualified opinion and all) intuition tells me. Hope it works out for you.
-AJ
EDIT: Seems some people already stopped by with, what I assume is the correct answer. I tried [smile]
&text_address = &new_text;
Again, I'm no expert by any means, but it's just what my (with my unqualified opinion and all) intuition tells me. Hope it works out for you.
-AJ
EDIT: Seems some people already stopped by with, what I assume is the correct answer. I tried [smile]
The address of a local variable is the address of a slot on the stack.
*text_address = &new_text;
There's no guarantee that address will remain valid after the function returns.
You'll need another level of indirection on the new_text_file function. Consider that the code already calls it that way
FILE *text_address;
if(new_text_file(&text_address) == 1)
This is the address of a pointer - or a pointer pointer. However, the function is declared to simply accept a pointer
int new_text_file(FILE *text_address)
I hope you're starting to see the problem here.
One solution is to change the function to accept a pointer pointer
int new_text_file(FILE **text_address)
and then assign the contents of the local variable to the dereferenced argument
*text_address = new_text;
*text_address = &new_text;
There's no guarantee that address will remain valid after the function returns.
You'll need another level of indirection on the new_text_file function. Consider that the code already calls it that way
FILE *text_address;
if(new_text_file(&text_address) == 1)
This is the address of a pointer - or a pointer pointer. However, the function is declared to simply accept a pointer
int new_text_file(FILE *text_address)
I hope you're starting to see the problem here.
One solution is to change the function to accept a pointer pointer
int new_text_file(FILE **text_address)
and then assign the contents of the local variable to the dereferenced argument
*text_address = new_text;
Author
Ok when I fixed that in and compiled it I got another error to go away. That one was about assigning pointers between the functions. But the assignment from incompatible pointer type error is still coming up. As if new_text and text_address where not from the same varible type FILE.
Btw thanks for the last bit of info. Much apperciated.
Btw thanks for the last bit of info. Much apperciated.
Author
Nvm. I got it thanks to you guys.
That was a nice pickle for my first day back to C in some time.
Again thanks for all the help.
That was a nice pickle for my first day back to C in some time.
Again thanks for all the help.
The most obvious (and easier) solution is for the function to just return the file pointer, or NULL or error. It's a common convention (return a pointer, or NULL on error), easy to work with (FILE* some_file = gimme_a_file()), and removes a redundant way of returning data (the data returned is "a file" or "not a file"). Also, error reporting is best handled by higher-level code (unless you have a more generic way of reporting an error message).
Isn't that nice and clean?
FILE* new_text_file() { FILE *new_text = fopen( "weekly.txt", "w" ); if( new_text == NULL ) fprintf( stderr, "Could not open file\n" ); // Also handles returning NULL, since that's also what fopen returns return new_text;}Isn't that nice and clean?
Quote:Original post by jonahrowley
The most obvious (and easier) solution is for the function to just return the file pointer, or NULL or error. [...]
*** Source Snippet Removed ***
Isn't that nice and clean?
Quoted for truth. [smile]
This topic is closed to new replies.
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