FOpen

fopen is a standard C library function declared in stdio.h. It opens a file and returns a pointer to a FILE object that can be used with the standard I/O facilities. The function takes two arguments: the path to the file and a mode string. On success, a stream associated with the file is returned; on error, NULL is returned and errno is set.

Mode strings specify how the file is accessed. Common modes are 'r' for reading, 'w' for writing, and 'a' for appending; a '+' after the letter enables update mode for reading and writing. A 'b' may be appended to request binary mode (for example, 'rb', 'wb', 'ab', 'r+b'). Behavior differs by mode: 'r' requires that the file exists; 'w' creates the file if needed and truncates it; 'a' creates the file if needed and writes go to the end. In update modes, both reading and writing are allowed, with proper file-position handling. Binary mode prevents newline translation on platforms such as Windows.

Return value: On success, a non-null FILE pointer is returned; the caller must call fclose to release resources. On failure, NULL is returned and errno is set to indicate the error, such as ENOENT for a missing file or EACCES for permission issues. It is important to check the return value before use and to detect I/O errors during operations.

Notes: fopen respects the host system's file permissions and path conventions. Some environments provide a secure variant such as fopen_s. For more control, other interfaces like open or fdopen may be used. Related functions include fclose, fread, fwrite, fgetc, fgets, and fprintf.