fprintf() is not working in ubuntu - c

Question

I'm trying to learn File I/O concepts in C programming language. I'm using GNU / Linux ( Ubuntu 16.04 LTS ) and my IDE is eclipse 3.8. when I try to write in a file through fprintf() method, it doesn't create any files or if the file is even created, it doesn't write in it. I tried to fix the problem by using fflush() or setbuf(file_pointer, NULL) methods as is suggested here but still no change. I guess I'm writing the address of the file in a wrong way.
Here is the code:
#include <stdio.h>
int main(void){
FILE *file_pointer;
file_pointer=fopen("~/.textsfiless/test.txt","w+");
setbuf(file_pointer,NULL);
fprintf(file_pointer,"Testing...\n");
fclose(file_pointer);
return EXIT_SUCCESS;
}
Can someone explain what's wrong here?

Answer 1

On Linux, the ~ in ~/.textsfiless/test.txt is not expanded by the C library fopen... When you use ~ on the command line, it is expanded by your shell (but not by the program using it, started by the shell doing some execve(2)...) into your home directory; the expansion is called globbing. Read glob(7). You are very unlikely to have a directory named ~.
You should read Advanced Linux Programming
So you should check if fopen failed (it is very likely that it did fail). If you want to get a file in the home directory, you'll better use getenv(3) with "HOME" (or perhaps getpwuid(3) & getuid(2)...). See environ(7)
Perhaps a better code might be:
char*homedir = getenv("HOME");
if (!homedir) { perror("getenv HOME"); exit(EXIT_FAILURE); };
char pathbuf[512]; /// or perhaps PATH_MAX instead of 512
snprintf(pathbuf, sizeof(pathbuf),
"%s/.textsfiless/test.txt", homedir);
FILE *file_pointer = fopen(pathbuf, "r");
if (!file_pointer) { perror(pathbuf); exit(EXIT_FAILURE); };
and so on.
Notice that you should check against failures most C standard library (& POSIX) functions. The perror(3) function is useful to report errors to the user on stderr.
(pedantically, we should even test that snprintf(3) returns a length below sizeof(pathbuf) or use and test against failure asprintf(3) instead; I leave that test as an exercise to the reader)
More generally, read the documentation of every external function that you are using.
Beware of undefined behavior (your code is probably having some, e.g. fprintf to a NULL stream). Compile your code with all warnings & debug info (so gcc -Wall -g) and use the gdb debugger. Read What every C programmer should know about undefined behavior.
BTW, look into strace(1) and try it on your original (faulty) program. You'll learn a lot about the system calls used in it.

Answer 2

Most likely your call to fopen() fails. You don't have any checking in your program to ensure fopen even worked. It may not have, and this could be due to a variety of things, like you spelling the path wrong, wrong file or process permissions, etc.
To see what really happened, you should check fopen's return value:
#include <stdio.h>
int main(void){
FILE *file_pointer;
file_pointer=fopen("~/.textsfiless/test.txt","w+");
if (file_pointer == NULL) {
printf("Opening the file failed.");
return EXIT_FAILURE;
}
setbuf(file_pointer,NULL);
fprintf(file_pointer,"Testing...\n");
fclose(file_pointer);
return EXIT_SUCCESS;
}
Edit: Since your comment, you getting the path wrong is most certainly what happened. If you're executing your program from the current directory, and your file is in a folder called "textfiless" in your current directory and your file is called "test.txt", then you'd call fopen like this:
file_pointer=fopen("/textsfiless/test.txt","w+");