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
The fopen() method in C is a library function that is used to open a file to perform various operations which include reading, writing etc.
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. Similarly fprintf return a negative number on error.
The fopen() function will fail if: [EACCES] Search permission is denied on a component of the path prefix, or the file exists and the permissions specified by mode are denied, or the file does not exist and write permission is denied for the parent directory of the file to be created.
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
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