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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.

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.

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, {{open()}} 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.

Care should be taken when using {{O_EXCL}} with remote file systems because it does not work with NFS version 2. NFS version 3 added support for {{O_EXCL}} mode in {{open()}}. IETF RFC 1813 defines the {{EXCLUSIVE}} value to the {{mode}} argument of {{CREATE}} \[[Callaghan 95|AA. C References#Callaghan 95]\].

Section 12.3 of the GNU C Library says: \[[Loosemore 07|AA. C References#Loosemore 07]\]

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 \[[Open Group 04|AA. C References#Open Group 04]\].

\[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\] Section 7.19.3, "Files," and Section 7.19.4, "Operations on Files"
\[[ISO/IEC TR 24731-1:2007|AA. C References#SO/IEC TR 24731-1-2007]\] Section 6.5.2.1, "The {{fopen_s}} function"
\[[Loosemore 07|AA. C References#Loosemore 07]\] [Section 12.3, "Opening Streams"|http://www.gnu.org/software/libc/manual/html_node/Opening-Streams.html]
\[[Open Group 04|AA. C References#Open Group 04]\]
\[[Seacord 05a|AA. C References#Seacord 05]\] Chapter 7, "File I/O"