FIO03-A. Do not make assumptions about fopen() and file creation

The ISO/IEC 9899-1999 C standard function fopen() is typically used to open an existing file or create a new one. However, fopen() does not provide a mechanism to determine if an existing file has been opened or a new file has been created. This may lead to a program overwriting or accessing an unintended file.

Non-Compliant Example

In this example, a file name is supplied to fopen() to create and open for writing. However, there is no guarantee that the file referenced by file_name does not exist prior to calling fopen(). This may cause an unintended file to be accessed or overwritten.

...
FILE * fptr = fopen(file_name, "w");
if (!fptr) {
  /* Handle Error */
}
...

Compliant Solution 1

The ISO/IEC 9899-1999 C standard does not provide a mechanism to determine if an existing file has been opened or a new file has been created. However, the open() function as defined in the Open Group Base Specifications Issue 6 [[Open Group 04]] is available on many platforms and provides such a mechanism. If the O_CREAT and O_EXCL flags are used together, the open() function fails when the file specified by file_name already exists.

...
int fd = open(file_name, O_CREAT | O_EXCL | O_WRONLY, new_file_mode);
if (fd == -1) {
  /* Handle Error */
}
...

Compliant Solution 2

The function fdopen() [[Open Group 04]] can be used in conjunction with open() to determine if a file is opened or created, and then associate a stream with the file descriptor.

...
FILE *fp;
int fd;

fd = open(file_name, O_CREAT | O_EXCL | O_WRONLY, new_file_mode);
if (fd == -1) {
  /* Handle Error */
}

fp = fdopen(fd,"w");
if (fp == NULL) {
  /* Handle Error */
}
...

Priority: Level:

The ability to determine if an existing file has been opened, or a new file has been created provides greater assurance that the file accessed is the one that was intended.

References

• Seacord 05 Chapter 7, File I/O
• ISO/IEC 9899-1999 Sections 7.19.3, Files
• ISO/IEC 9899-1999 Sections 7.19.4, Operations on Files
• Open Group 04 The open function