...
Additionally, there are well-known recommendations for dealing with common file operations securely that use non-standard functions. This recommendation opens those options up to implementers of this standard.
Non-Compliant Example 1
The C99 standard function fopen() is typically used to open existing, and create new files. However, fopen() does not provide a way to test file existence potentially allowing a program to overwrite or access and unintended file.
In this example, a file name is supplied to fopen() to create and open for writing. Howerver, there is no gauruntee that the file referenced by file_name does not exist prior to calling fopen(). This may cause an unintended file to be overwritten.
| Code Block |
|---|
...
FILE * fptr = fopen(file_name, "w");
if (!fptr) {
/* Handle Error */
}
...
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Compliant Solution 1.
The open() function (defined in the POSIX standard) provides a a way to test for file existence . If the O_CREAT and O_EXCL flags are used together, the open() function will fail if the file file specified by file_name already exists.
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|---|
...
int fd = open(file_name, O_CREAT | O_EXCL | O_WR_ONLY, 0600);
if (fd == -1) {
/* Handle Error */
}
...
|
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 04b