Help with user file I/O

This is a discussion on Help with user file I/O within the C Programming forums, part of the General Programming Boards category; Hello, I am teaching myself C, and am having difficulty writing to a user-specified file. In the code below, fopen ...

  1. #1
    Registered User
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    May 2009
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    8

    Question Help with user file I/O

    Hello,

    I am teaching myself C, and am having difficulty writing to a user-specified file. In the code below, fopen keeps returning a null file pointer. When the file location is physically written in the code, however, the program works as expected. My question has two parts:

    1) Why does fopen return a null pointer?
    2) What should I change to make the program accept user-specified input files?

    I have not been able to find an answer to these questions in my C book or through online searches.

    Note: The code is written and compiled in Microsoft Visual C++ 2008, if that makes a difference.
    Code:
    #define _CRT_SECURE_NO_WARNINGS
    #define MAX_FILE 100
    #define MAX_TEXT 50
    
    #include <stdio.h>
    
    int main()
    {
    	char file_location[MAX_FILE];
    	char text[MAX_TEXT];
    	FILE *myfile;
    	
    	printf("Enter file location: ");
    	fgets(file_location, MAX_FILE, stdin);
    	myfile = fopen(file_location,"w");
    	//myfile = fopen("c:/c test files/test.txt","w"); //PROGRAM WORKS IN THIS CASE
    
    	if (myfile != NULL)
    	{
    		printf("Enter your text: ");
    		fgets(text, MAX_TEXT, stdin);
    		fprintf(myfile, text);
    	}
    	else
    	{
    		printf("INVALID FILE PATH.\n");
    		return 0;
    	}
    	
    	fclose(myfile);
    
    	return 0;
    }
    Thank you for any assistance.

  2. #2
    ATH0 quzah's Avatar
    Join Date
    Oct 2001
    Posts
    14,826
    Remove the newline from your buffer after you call fgets on it.


    Quzah.
    Hope is the first step on the road to disappointment.

  3. #3
    Registered User
    Join Date
    May 2009
    Posts
    8
    Thank you, quzah.

    For anybody's reference, here is the revised (and working!) code:
    Code:
    #define _CRT_SECURE_NO_WARNINGS
    #define MAX_FILE 100 //maximum length of file_location string, including termination character
    #define MAX_TEXT 50 //maximum length of text string, including termination character
    
    #include <stdio.h>
    
    int main()
    {
    	char file_location[MAX_FILE];
    	char text[MAX_TEXT];
    	int i; //loop counter
    	FILE *myfile;
    	
    	printf("Enter file location: "); //note: fopen doesn't like creating new directories
    	fgets(file_location, MAX_FILE, stdin);
    	
    	for (i=0; i<MAX_FILE; i++) //replace newline character with string termination character
    	{
    		if (file_location[i] == '\n'){
    			file_location[i] = '\0';
    		}
    	}
    
    	printf("File location: %s\n",file_location);
    	myfile=fopen(file_location,"w");
    
    	if (myfile != NULL) //checks if file is valid
    	{
    		printf("Enter your text: ");
    		fgets(text, MAX_TEXT, stdin);
    		fprintf(myfile, text);
    	}
    	else
    	{
    		printf("INVALID FILE PATH.\n");
    		return 0;
    	}
    	
    	fclose(myfile);
    
    	return 0;
    }

  4. #4
    CSharpener vart's Avatar
    Join Date
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    Location
    Rishon LeZion, Israel
    Posts
    6,457
    1. your loop for replacing \n with \0 could be exitted as soon as the \n or \0 char is encountered - no sence to continue
    2. using user provided string as a format for printf is a big security hole. - You should replace fprintf with fputs or use "%s" format in fprintf
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

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