Opening streams

Names specified here
Name	Description			Source	Availability
`FOPEN_MAX`	Maximum number of open files	M		`<stdio.h>`	C89	C90	C95	C99	C11
`fopen()`	Open file as stream		(·)	`<stdio.h>`	C89	C90	C95	C99	C11
`fopen_s()`	Open file as stream		(·)	`<stdio.h>`					C11
`freopen()`	Re-open stream		(·)	`<stdio.h>`	C89	C90	C95	C99	C11
`freopen_s()`	Re-open stream		(·)	`<stdio.h>`					C11

#include <stdio.h>
FILE *fopen(const char *name, const char *mode);
FILE *freopen(const char *name, const char *mode, FILE *stream);

A new stream is opened and associated with a file using fopen, which returns NULL on failure. name is the name of the file, and mode indicates how to access it:

`mode`	Format	Read	Write	Initial position	If file exists	If file does not exist
`r`	Text	Yes		Start		Fail
`rb`	Binary	Yes		Start		Fail
`r+`	Text	Yes	Yes	Start		Fail
`rb+` or `r+b`	Binary	Yes	Yes	Start		Fail
`w`	Text		Yes	Start	Truncate	Create
`wb`	Binary		Yes	Start	Truncate	Create
`w+`	Text	Yes	Yes	Start	Truncate	Create
`wb+` or `w+b`	Binary	Yes	Yes	Start	Truncate	Create
`wx`	Text		Yes	Start	Fail	Create
`wbx`	Binary		Yes	Start	Fail	Create
`w+x`	Text	Yes	Yes	Start	Fail	Create
`wb+x` or `w+bx`	Binary	Yes	Yes	Start	Fail	Create
`a`	Text		Yes	End		Create
`ab`	Binary		Yes	End		Create
`a+`	Text	Yes	Yes	End		Create
`ab+` or `a+b`	Binary	Yes	Yes	End		Create

freopen takes an existing stream, closes it, and re-opens it on another file, returning stream on success, and NULL on failure. It is most useful for redirecting streams such as stdin, stdout and stderr to other files.

#define __STDC_WANT_LIB_EXT1__ 1
#include <stdio.h>
errno_t fopen_s(FILE **pstr, const char *name, const char *mode);
errno_t freopen_s(FILE **pstr, const char *name, const char *mode, FILE *stream);

From C11, fopen_s and freopen_s are provided. These differ from fopen and freopen in that they do not return stream handles, but instead write the new handles to *pstr if pstr is not null. On error, they set *pstr to null and return non-zero.

fopen and freopen also influence permissions on newly created files. Such files will not be accessible to ‘others’, for example, other users, if the environment has such a notion. There are also twelve additional values for mode. A prefix of u on all modes that support file creation indicates that the created file will have default permissions by the time the stream is closed.

Upto FOPEN_MAX streams can be open at once.