Opening a file that is already open has implementation-defined behavior, according to the C Standard, subclause 7.21.3, paragraph 8 [ISO/IEC 9899:2011]:
Functions that open additional (nontemporary) files require a file name, which is a string. The rules for composing valid file names are implementation-defined. Whether the same file can be simultaneously open multiple times is also implementation-defined.
Some platforms may forbid a file simultaneously being opened multiple times, but platforms that allow it may facilitate dangerous race conditions. This is because it is possible for an open file to be moved or deleted on many platforms. If a program re-opens a file using the same pathname, there is no guarantee that the same file is being accessed.
Noncompliant Code Example
This noncompliant code example logs the program's state at runtime:
#include <stdio.h> void do_stuff(void) { FILE *logfile = fopen("log", "a"); if (logfile == NULL) { /* Handle error */ } /* Write logs pertaining to do_stuff() */ fprintf(logfile, "do_stuff\n"); } int main(void) { FILE *logfile = fopen("log", "a"); if (logfile == NULL) { /* Handle error */ } /* Write logs pertaining to main() */ fprintf(logfile, "main\n"); do_stuff(); fclose(logfile); return 0; }
However, the file log
is opened twice simultaneously. The result is implementation-defined and potentially dangerous.
Implementation Details
On a Linux machine running GCC 4.3.2, this program produces
do_stuff main
which does not indicate the order in which data was logged.
Compliant Solution
In this compliant solution, a reference to the file pointer is passed as an argument to functions that need to perform operations on that file. This reference eliminates the need to open the same file multiple times.
#include <stdio.h> void do_stuff(FILE *logfile) { /* Write logs pertaining to do_stuff() */ fprintf(logfile, "do_stuff\n"); } int main(void) { FILE *logfile = fopen("log", "a"); if (logfile == NULL) { /* Handle error */ } /* Write logs pertaining to main() */ fprintf(logfile, "main\n"); do_stuff(logfile); fclose(logfile); return 0; }
Implementation Details
On a Linux machine running GCC 4.3.2, this program produces
main do_stuff
which matches the order in which logging occurred.
Automated Detection
Tool | Version | Checker | Description |
---|---|---|---|
9.7.1 | 75 D | Fully implemented |
Risk Assessment
Simultaneously opening a file multiple times can result in abnormal program termination or data integrity violations.
Rule | Severity | Likelihood | Remediation Cost | Priority | Level |
---|---|---|---|---|---|
FIO31-C | Medium | Probable | High | P4 | L3 |
Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
Related Guidelines
CERT C++ Secure Coding Standard | FIO31-CPP. Do not simultaneously open the same file multiple times |
MITRE CWE | CWE-362, Race conditions CWE-675, Duplicate operations on resource |
Bibliography
[ISO/IEC 9899:2011] | Subclause 7.21.3, "Files" |