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)
C documentation for
fopen