The "a" mode doesn't remove the EOF marker before appending to the file. The file pointer can be repositioned by using fseek or rewind, but it's always moved back to the end of the file before any write operation is carried out so that existing data can't be overwritten. When a file is opened by using the "a" or "a+" access type, all write operations occur at the end of the file. The EOF marker isn't restored after writing is completed. The appending operation includes the removal of the EOF marker before new data is written to the file. If the file exists, its contents are destroyed. Opens an empty file for both reading and writing. Opens for writing at the end of the file (appending) without removing the end-of-file (EOF) marker before new data is written to the file. If the given file exists, its contents are destroyed. If the file doesn't exist or can't be found, the fopen_s call fails. The character string mode specifies the kind of access that's requested for the file, as follows. (This behavior applies to a mode only, not a+.) In either case, fopen_s then reopens the file with write-only access. If successful, the function reads the BOM to determine the encoding for the file if unsuccessful, the function uses the default encoding for the file. If mode is "a, ccs=UNICODE", "a, ccs=UTF-8", or "a, ccs=UTF-16LE", fopen_s first tries to open the file with both read access and write access. Encodings used based on ccs flag and BOM ccs flagįiles that are opened for writing in Unicode mode have a BOM written to them automatically. The following table summarizes the modes for various ccs flag values that are given to fopen_s and for BOMs in the file. The ccs encoding is only used when no BOM is present or if the file is a new file.īOM-detection only applies to files that are opened in Unicode mode that is, by passing the ccs flag. The BOM encoding takes precedence over the encoding that's specified by the ccs flag. If the file already exists and is opened for reading or appending, the byte order mark (BOM), if present in the file, determines the encoding. If no value is specified for ccs, fopen_s uses ANSI encoding. To open a new or existing Unicode file, pass a ccs flag that specifies the desired encoding to fopen_s, for example:įopen_s(&fp, "newfile.txt", "w+, ccs=UNICODE") Īllowed values of the ccs flag are UNICODE, UTF-8, and UTF-16LE. Unicode supportįopen_s supports Unicode file streams. To change it, see Global state in the CRT. For more information, see errno, _doserrno, _sys_errlist, and _sys_nerr.īy default, this function's global state is scoped to the application. If an error occurs, the error code is returned, and the global variable errno is set. If pFile, filename, or mode is a null pointer, these functions generate an invalid parameter exception, as described in Parameter validation.Īlways check the return value to see if the function succeeded before you do any further operations on the file. These functions validate their parameters. You can use either forward slashes (/) or backslashes (\) as the directory separators in a path. When you construct paths for fopen_s, don't make assumptions about the availability of drives, paths, or network shares in the execution environment. _wfopen_s and fopen_s behave identically, otherwise.įopen_s accepts paths that are valid on the file system at the point of execution UNC paths and paths that involve mapped network drives are accepted by fopen_s as long as the system that's executing the code has access to the share or mapped network drive at the time of execution. _wfopen_s is a wide-character version of fopen_s and the arguments to _wfopen_s are wide-character strings. The fopen_s function opens the file specified by filename. If you need to share the file, use _fsopen or _wfsopen with the appropriate sharing mode constant-for example, use _SH_DENYNO for read/write sharing. The fopen_s and _wfopen_s functions can't open a file for sharing. For more information about these error codes, see errno, _doserrno, _sys_errlist, and _sys_nerr. Zero if successful an error code on failure. Syntax errno_t fopen_s(Ī pointer to the file pointer that receives the pointer to the opened file. These versions of fopen, _wfopen have security enhancements, as described in Security features in the CRT.
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