std::fopen(3)

NAME
std::fopen - std::fopen

Synopsis
   Defined in header <cstdio>
   std::FILE* fopen( const char* filename, const char* mode );
   Opens a file indicated by filename and returns a file stream associated with that
   file. mode is used to determine the file access mode.

Parameters
   filename - file name to associate the file stream to
   mode - null-terminated character string determining file access mode
  File access flags
    File access Meaning Explanation Action if file Action if file
    mode string already exists does not exist
   "r" read Open a file for read from start return NULL and
                        reading set error
   "w" write Create a file for destroy contents create new
                        writing
   "a" append Append to a file write to end create new
   "r+" read extended Open a file for read from start return NULL and
                        read/write set error
   "w+" write extended Create a file for destroy contents create new
                        read/write
   "a+" append extended Open a file for write to end create new
                        read/write
   File access mode flag "b" can optionally be specified to open a file in binary mode.
   This flag has no effect on POSIX systems, but on Windows, for example, it disables
   special handling of '\n' and '\x1A'.
   On the append file access modes, data is written to the end of the file regardless
   of the current position of the file position indicator.
   File access mode flag "x" can optionally be appended to "w" or "w+" specifiers. This
   flag forces the function to fail if the file exists, instead of overwriting it.
   (C++17)
   The behavior is undefined if the mode is not one of the strings listed above. Some
   implementations define additional supported modes (e.g. Windows).

Return value
   If successful, returns a pointer to the object that controls the opened file stream,
   with both eof and error bits cleared. The stream is fully buffered unless filename
   refers to an interactive device.
   On error, returns a null pointer. POSIX requires that errno is set in this case.

Notes
   The format of filename is implementation-defined, and does not necessarily refer to
   a file (e.g. it may be the console or another device accessible through filesystem
   API). On platforms that support them, filename may include absolute or relative
   filesystem path.
   For portable directory and file naming, see C++ filesystem library or
   boost.filesystem.

Example
// Run this code
 #include <cstdio>
 #include <cstdlib>
 int main()
 {
     int is_ok = EXIT_FAILURE;
     FILE* fp = std::fopen("/tmp/test.txt", "w+");
     if(!fp) {
         std::perror("File opening failed");
         return is_ok;
     }
     int c; // note: int, not char, required to handle EOF
     while ((c = std::fgetc(fp)) != EOF) { // standard C I/O file reading loop
        std::putchar(c);
     }
     if (std::ferror(fp)) {
         std::puts("I/O error when reading");
     } else if (std::feof(fp)) {
         std::puts("End of file reached successfully");
         is_ok = EXIT_SUCCESS;
     }
     std::fclose(fp);
     return is_ok;
 }

Output:
 End of file reached successfully

See also
   fclose closes a file
           (function)
   fflush synchronizes an output stream with the actual file
           (function)
   freopen open an existing stream with a different name
           (function)