C open() function fails. Are my parameters wrong? - c

Question

This function fails to open the file. Are my parameters wrong or what could be causing this problem?
int CreateFile(const char *filename){
char filepath[strlen(filename) + 3];
sprintf(filepath, "./%s", filename);
int fd = open(filepath, O_CREAT, O_APPEND, S_IWGRP);
if(fd == -1) printf("file read failed\n");
return fd;
}
Xcode prints only "file read failed" to the console. I tried to run this via Terminal aswell but that didn't help either.
I fixed an issue pointed by NetMage:
int CreateFile(const char *filename){
char filepath[strlen(filename) + 3];
sprintf(filepath, "./%s", filename);
int fd = open(filepath, O_CREAT|O_APPEND, S_IWGRP);
if(fd == -1) printf("file read failed\n");
return fd;
}
Unfortunately that didn't fix the issue

Answer 1

Step 1 - Verify that filepath is being set correctly, either by printing it to the terminal or examining it in a debugger.
Step 2 - Verify that the file exists in that path, and that its permissions are set so that you can open it. If filepath is "./foo", then a file named foo had better exist in the current working directory (the directory from which you ran the program), and it needs to have at least read permission.
Step 3 - If the file does not exist, verify that you have permission to create new files in the current working directory.
Step 4 - If after doing all of that you still get an error, check errno. It will give you some additional information beyond "it didn't work."
#include <errno.h>
...
if(fd == -1)
{
switch( errno )
{
case EACCESS: // permission issues
handle_permission_issue();
break;
case EEXIST: // file already exists and you used O_CREAT and O_EXCL
handle_already_exists_issue();
break;
case EFAULT: // bad path
handle_bad_path_issue()
break;
...
}
printf("file read failed\n");
}
NetMage has pointed out one problem - your flags need to be bitwise-OR'd together, rather than listed as separate arguments. Surprised the compiler didn't yell at you over that.

Answer 2

The open function takes only one parameter for oflags, which must be bit-ored together:
#include <errno.h>
#include <string.h>
int fd = open(filepath, O_CREAT|O_APPEND, S_IWGRP);
if (fd == -1) printf("file read failed: %s\n", strerror(errno));

Answer 3

Per the POSIX documentation for open() (somewhat reformatted, and note the bolded text):
SYNOPSIS
#include <sys/stat.h> #include <fcntl.h>
int open(const char *path, int oflag, ...);
...
Values for oflag are constructed by a bitwise-inclusive OR of flags
from the following list, defined in . Applications shall
specify exactly one of the first five values (file access modes)
below in the value of oflag:
O_EXEC
Open for execute only (non-directory files). The result is unspecified if this flag is applied to a directory.
O_RDONLY
Open for reading only.
O_RDWR
Open for reading and writing. The result is undefined if this flag is applied to a FIFO.
O_SEARCH
Open directory for search only. The result is unspecified if this flag is applied to a non-directory file.
O_WRONLY
Open for writing only.
...
You need to include at least one of those five flags, perhaps like:
int fd = open(filepath, O_WRONLY|O_CREAT|O_APPEND, S_IWGRP);
Note that other failures may still occur. As noted in the comments, you're prepending "./" to the file name, which may cause problems if, for example, you get passed "/tmp/filename" and the tmp directory doesn't exist in your current working directory, as open() will not create missing directories in any path.