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The C99 {{fopen()}} function is used to open an existing file or create a new one \[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\]. However, {{fopen()}} does not indicate if an existing file has been opened for writing or a new file has been created. This may lead to a program overwriting or accessing an unintended file. |
Noncompliant Code Example
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(fopen())
In this noncompliant code example, the file referenced by file_name is opened for writing. This example is noncompliant if the programmer's intent was to create a new file, but the referenced file already exists.
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|---|---|---|
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char *file_name;
FILE *fp;
/* initialize file_name */
fp = fopen(file_name, "w");
if (!fp) {
/* Handle error */
}
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Noncompliant Code Example
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(fopen_s()
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, ISO/IEC TR 24731-1)
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The ISO/IEC TR 24731-1 {{fopen_s()}} function is designed to improve the security of the {{fopen()}} function \[[ISO/IEC TR 24731-1:2007|AA. C References#SO/IEC TR 24731-1-2007]\]. However, like {{fopen()}}, {{fopen_s()}} provides no mechanism to determine if an existing file has been opened for writing or a new file has been created. |
| Code Block | ||
|---|---|---|
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char *file_name;
FILE *fp;
/* initialize file_name */
errno_t res = fopen_s(&fp, file_name, "w");
if (res != 0) {
/* Handle error */
}
|
Compliant Solution
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(open()
...
, POSIX)
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The {{open()}} function, as defined in the Open Group Base Specifications Issue 6 \[[Open Group 04|AA. C References#Open Group 04]\], is available on many platforms and provides finer control than {{fopen()}}. In particular, {{fopen()}} accepts the {{O_CREAT}} and {{O_EXCL}} flags. When used together, these flags instruct the {{open()}} function to fail if the file specified by {{file_name}} already exists. |
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For examples on how to check for the existence of a file without opening it, see FIO10-C. Take care when using the rename() function.
Compliant Solution
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(fopen()
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, GNU)
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Section 12.3 of the GNU C Library says: \[[Loosemore 07|AA. C References#Loosemore 07]\] |
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Use of this (nonportable) extension allows for the easy remediation of legacy code.
Compliant Solution
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(fdopen()
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, POSIX)
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For code that operates on {{FILE}} pointers and not file descriptors, the POSIX {{fdopen()}} function can be used to associate an open stream with the file descriptor returned by {{open()}}, as shown in this compliant solution \[[Open Group 04|AA. C References#Open Group 04]\]. |
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