Problems passing a file pointer to functions

smitchell · 09-28-2008, 06:55 AM

Hi,

I'm having problems passing a file pointer through nested functions.

In the code below, main calls func1 which calls func1a which calls openFile. With some modification to the code (to flush and close the file) openFile can write to the file it has opened.

If func1a is called with the file pointer initialised to NULL, the file pointer is not null before returning from the function. func1a can't write to the file, presumably because the pointer is pointing at something other than the file, but calling fprintf with the file pointer doesn't generate a segmentation fault.

Testing the file pointer in func1 returns NULL. Obviously because of this attempting to write to the file later in the program causes a segmentation fault.

I know the problem has to do with the way I'm passing the file pointer and/or the struct which contains it between functions. I can be fairly certain of this because originally, openFile took a file pointer as a parameter rather than returning it, and testing the file pointer in func1a returned NULL.

I've read several forum posts and tutorials that say that, because a file pointer is already a pointer, it should be passed as-is rather than passing a pointer to it. This doesn't seem to work for me, and I've spent the last few weeks messing around with pointers and pointers to pointers, still with the same results.

When compiling the code using lcc under Windows XP, I get the warning
Warning d:\testpointers.c: 39 Missing prototype for 'openFile'
which refers to the line myFile = openFile();

Thanks in advance for any help you can give me.

Code:

#include <stdio.h>

typedef struct
{
	FILE *myFile;
} myStruct;

FILE* openFile()
{
	FILE *file = fopen("D:\\test1.txt","w");
	if(file == NULL)
		printf("openFile: file is null\n");
	return file;
}

int func1a(FILE *myFile)
{
	myFile = openFile();
	if(myFile == NULL)
		printf("func1a: file is null\n");
	return 0;
}

int func1(myStruct *a)
{
	int status = func1a(a->myFile);
	if(a->myFile == NULL)
		printf("func1: file is null\n");
	return 0;
}

int main()
{
	myStruct a;
	a.myFile = NULL;
	int status = 0;
	status = func1(&a);
}

**Salem** · 09-28-2008, 08:01 AM

To modify a pointer, you need a pointer to a pointer.

One of your functions needs to receive a FILE** parameter.

smitchell · 09-30-2008, 02:05 PM

Thanks for the quick response. After following your advice, I've modified the code to look like this:

Code:

int openFile(FILE ***file)
{
	**file = fopen("D:\\test1.txt","w");
	if(**file == NULL)
		printf("openFile: file is null\n");
	return 0;
}

int func1a(FILE **myFile)
{
	openFile(&myFile);
	if(*myFile == NULL)
		printf("func1a: file is null\n");
	return 0;
}

int func1(myStruct *a)
{
	int status = func1a(&a->myFile);
	if(&a->myFile == NULL)
		printf("func1: file is null\n");
	return 0;
}

It works fine, but is having a pointer to a pointer to a file pointer in openFile the best way to achieve what I want?

tabstop · 09-30-2008, 02:08 PM

I don't know why you went from 1 to 3, and skipped 2:

Code:

int openFile(FILE **file)
{
	*file = fopen("D:\\test1.txt","w");
	if(*file == NULL)
		printf("openFile: file is null\n");
	return 0;
}

smitchell · 09-30-2008, 02:29 PM

No, neither do I

. I think it was because, when I tried two originally, another part of the program stopped working and I put two and two together and came up with 6. Two works fine now though. Thanks.

09-28-2008, 06:55 AM	#1
smitchell Registered User Join Date: Sep 2008 Posts: 3	Problems passing a file pointer to functions Hi, I'm having problems passing a file pointer through nested functions. In the code below, main calls func1 which calls func1a which calls openFile. With some modification to the code (to flush and close the file) openFile can write to the file it has opened. If func1a is called with the file pointer initialised to NULL, the file pointer is not null before returning from the function. func1a can't write to the file, presumably because the pointer is pointing at something other than the file, but calling fprintf with the file pointer doesn't generate a segmentation fault. Testing the file pointer in func1 returns NULL. Obviously because of this attempting to write to the file later in the program causes a segmentation fault. I know the problem has to do with the way I'm passing the file pointer and/or the struct which contains it between functions. I can be fairly certain of this because originally, openFile took a file pointer as a parameter rather than returning it, and testing the file pointer in func1a returned NULL. I've read several forum posts and tutorials that say that, because a file pointer is already a pointer, it should be passed as-is rather than passing a pointer to it. This doesn't seem to work for me, and I've spent the last few weeks messing around with pointers and pointers to pointers, still with the same results. When compiling the code using lcc under Windows XP, I get the warning Warning d:\testpointers.c: 39 Missing prototype for 'openFile' which refers to the line myFile = openFile(); Thanks in advance for any help you can give me. Code: #include <stdio.h> typedef struct { FILE myFile; } myStruct; FILE openFile() { FILE file = fopen("D:\\test1.txt","w"); if(file == NULL) printf("openFile: file is null\n"); return file; } int func1a(FILE myFile) { myFile = openFile(); if(myFile == NULL) printf("func1a: file is null\n"); return 0; } int func1(myStruct *a) { int status = func1a(a->myFile); if(a->myFile == NULL) printf("func1: file is null\n"); return 0; } int main() { myStruct a; a.myFile = NULL; int status = 0; status = func1(&a); }
smitchell is offline

09-28-2008, 08:01 AM	#2
Salem and the hat of Jobseeking Join Date: Aug 2001 Location: The edge of the known universe Posts: 21,710	To modify a pointer, you need a pointer to a pointer. One of your functions needs to receive a FILE** parameter. __________________ If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
Salem is offline

09-30-2008, 02:08 PM	#4
tabstop and the Hat of Guessing Join Date: Nov 2007 Posts: 8,862	I don't know why you went from 1 to 3, and skipped 2: Code: int openFile(FILE *file) { file = fopen("D:\\test1.txt","w"); if(*file == NULL) printf("openFile: file is null\n"); return 0; }
tabstop is offline

09-30-2008, 02:29 PM	#5
smitchell Registered User Join Date: Sep 2008 Posts: 3	No, neither do I . I think it was because, when I tried two originally, another part of the program stopped working and I put two and two together and came up with 6. Two works fine now though. Thanks.
smitchell is offline

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Passing FIle pointer as argt	ganesh bala	C Programming	2	04-08-2009 02:56 AM
Problems with file pointer in functions	willie	C Programming	6	11-23-2008 01:54 PM
Can we have vector of vector?	ketu1	C++ Programming	24	01-03-2008 05:02 AM
airport Log program using 3D linked List : problem reading from file	gemini_shooter	C Programming	3	03-04-2005 02:46 PM
Possible circular definition with singleton objects	techrolla	C++ Programming	3	12-26-2004 10:46 AM