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A somewhat better solution involves using the POSIX {{access()}} function which can check for the existence of a file explicitly \[[Open Group 04|AA. C References#Open Group 04]\]. As with the {{fopen()}} solution, this code contains an unavoidable race condition and consequently can only be safely executed within a secure directory. In fact the existence check is not really needed on POSIX systems, because POSIX specifies the behavior of {{rename()}} when the file referenced by {{new}} exists. |
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char const *old = /* ... */;
char const *new = /* ... */;
if (access(new,F_OK) != 0) {
if (rename(old, new) != 0) {
/* Handle Error */
}
} else {
/* Handle Error */
}
|
While the likelihood of access() returning a false negative is lower than that of fopen(), on file systems where the program does not have sufficient permissions in the directory to view the file, access() may return -1 even when the file exists. In such cases, rename() will also likely fail since the program does not have adequate permissions inside the directory.
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On Windows, [{{_access_s()}}|http://msdn.microsoft.com/en-us/library/a2xs1dts(VS.80).aspx] allows one to check for the existence of a file \[[MSDN|AA. C References#MSDN]\]. As with the {{fopen()}} solution, this code contains an unavoidable race condition and consequently can only be safely executed within a secure directory. |
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char const *old = /* ... */;
char const *new = /* ... */;
if (_access_s(new,0) != 0) {
if (rename(old, new) != 0) {
/* Handle Error */
}
} else {
/* Handle Error */
}
|
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