< cpp‎ | io‎ | c
C-style I/O
Types and objects
File access
Direct input/output
Unformatted input/output
Formatted input
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.


[edit] Parameters

filename - file name to associate the file stream to
mode - null-terminated character string determining file access mode

[edit] File access flags

File access
mode string
Meaning Explanation Action if file
already exists
Action if file
does not exist
"r" read Open a file for reading read from start return NULL and set error
"w" write Create a file for writing destroy contents create new
"a" append Append to a file write to end create new
"r+" read extended Open a file for read/write read from start return NULL and set error
"w+" write extended Create a file for read/write destroy contents create new
"a+" append extended Open a file for read/write write to end create new
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).

[edit] 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.

[edit] 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.

[edit] Example

#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
    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;
    return is_ok;


End of file reached successfully

[edit] See also

closes a file
(function) [edit]
synchronizes an output stream with the actual file
(function) [edit]
open an existing stream with a different name
(function) [edit]
C documentation for fopen