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Q&A

Why does fopen return NULL?

+2
−2
#include<stdio.h>
#include<stdlib.h>
int main (void){
  
   FILE *cfPtr; 
   char name[15];
   int ID;
   if ((cfPtr = fopen("students.txt","w")) == NULL){
    printf("File could not be opened...\n"); //program gives me this as an output
   }
   else { //I want the output to be those in this else statement
        do {
            printf("Enter Name:\n"); 
            fflush(stdin); //flushing the input
            scanf("%s", name); 
            printf("Enter Student Number:");
            scanf("%d", &ID);

            fprintf(cfPtr, "%d %s\n", ID, name); 
        } while (ID != -1); 

        fclose(cfPtr); 
   }
    return 0; 
} 

Output:

File could not be opened...

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From one of the documentation pages for fopen... (1 comment)

2 answers

+4
−0
  • The simple explanation would be that you simply don't have write access to the path, which is one possibility.

  • Another weird phenomenon that may happen is that you are running a very old C compiler and it can't find fopen - because you did forget to #include <stdio.h>.

    When this happens on modern compilers, you get a compiler error. On very old compilers (the "ANSI/C90 standard") the compiler would instead cook up some dumb initiative of its own and treat the function as if it returns int. This could create very strange and subtle bugs.

    You could avoid such bugs by updating to the latest version of your C compiler. In case of gcc, clang or icc compilers you should also follow this advise: What compiler options are recommended for beginners learning C?

Unrelated to your question fflush(stdin); is undefined behavior and rarely the correct solution. Instead you should just ensure to consume the line feed character after taking the input. (For example by a getchar() call or by adding a space at the beginning of the scanf format string - but this is a topic for another thread.)

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Regarding the compiler (2 comments)
+3
−0

Besides the possible issues that Lundin told you, there is also the possibility that the current directory of the program is not what you think it is (note that the current directory is not necessarily the directory the executable is in, nor is it necessarily your user directory). This problem is unlikely if you started it directly from the command line, but if you started it through another program (for example from a file manager, or from your development environment), you might want to check that program's documentation on what directory it puts as current when it starts programs.

If the program is started from a directory you don't have a write permissions to, and the file does not yet exist there (which is likely if you are in the “wrong” directory), then the code will try to create it, which fails if you don't have write permission on the directory.

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