how to detect a file is opened or not in c

13
votes

I'm trying to output some string on a txt file by using c program

however, I need to see if the I have the permission to write on the txt file, if not, I need to print out the error message? However, I don't know how to detect if I successfully open a file or not, could someone help me about this? thanks

The code is like this

File *file = fopen("text.txt", "a");

fprintf(file, "Successfully wrote to the file.");

//TO DO (Which I don't know how to do this)
//If dont have write permission to text.txt, i.e. open was failed
//print an error message and the numeric error number

Thank you for anyone helps, thanks a lot

3
c

3 Answers

18
votes

You need to check the return value of fopen. From the man page:

RETURN VALUE
   Upon successful completion fopen(), fdopen() and freopen() return a FILE pointer.
   Otherwise, NULL is returned and errno is set to indicate the error.

To check whether write is sucessful or not again, check the return value of fprintf or fwrite. To print what is the reason for the failure you can check errno, or use perror to print the error.

f = fopen("text", "rw");
if (f == NULL) {
    perror("Failed: ");
    return 1;
}

perror will print the error like the following (in case of no permission):

Failed: Permission denied
11
votes

You can do some error checking to see if the calls to fopen and fprintf succeeded.

fopen's return value is the pointer to the file object on success and a NULL pointer on failure. You could check for NULL return value.

FILE *file = fopen("text.txt", "a");

if (file == NULL) {
     perror("Error opening file: ");
}

Similarly fprintf return a negative number on error. You could do a if(fprintf() < 1) check.

1
votes
f = fopen( path, mode );
if( f == NULL ) {
  perror( path );
}