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fopen, fopen_s

Defined in header `<stdio.h>`
	(1)
FILE fopen( const char filename, const char *mode );		(until C99)
FILE fopen( const char restrict filename, const char *restrict mode );		(since C99)
errno_t fopen_s( FILE restrict restrict streamptr, const char restrict filename, const char restrict mode );	(2)	(since C11)

1) Opens a file indicated by filename and returns a pointer to the file stream associated with that file. mode is used to determine the file access mode.

2) Same as (1), except that the pointer to the file stream is written to streamptr and the following errors are detected at runtime and call the currently installed constraint handler function:

streamptr is a null pointer
filename is a null pointer
mode is a null pointer

As with all bounds-checked functions, fopen_s is only guaranteed to be available if __STDC_LIB_EXT1__ is defined by the implementation and if the user defines __STDC_WANT_LIB_EXT1__ to the integer constant 1 before including <stdio.h>.

Parameters

filename	-	file name to associate the file stream to
mode	-	null-terminated character string determining file access mode
streamptr	-	pointer to a pointer where the function stores the result (an out-parameter)

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	failure to open
`"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	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 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.
The behavior is undefined if the mode is not one of the strings listed above. Some implementations define additional supported modes (e.g. Windows).
In update mode (`'+'`), both input and output may be performed, but output cannot be followed by input without an intervening call to `fflush`, `fseek`, `fsetpos` or `rewind`, and input cannot be followed by output without an intervening call to `fseek`, `fsetpos` or `rewind`, unless the input operation encountered end of file. In update mode, implementations are permitted to use binary mode even when text mode is specified.
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. (C11)
When using `fopen_s` or `freopen_s`, file access permissions for any file created with `"w"` or `"a"` prevents other users from accessing it. File access mode flag `"u"` can optionally be prepended to any specifier that begins with `"w"` or `"a"`, to enable the default `fopen` permissions. (C11)

Return value

1) If successful, returns a pointer to the new file stream. The stream is fully buffered unless filename refers to an interactive device. On error, returns a null pointer. POSIX requires that errno be set in this case.

2) If successful, returns zero and a pointer to the new file stream is written to *streamptr. On error, returns a non-zero error code and writes the null pointer to *streamptr (unless streamptr is a null pointer itself).

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 though filesystem API). On platforms that support them, filename may include absolute or relative filesystem path.

Example

#include <stdio.h>
#include <stdlib.h>
 
int main(void)
{
    int is_ok = EXIT_FAILURE;
    const char* fname = "/tmp/unique_name.txt"; // or tmpnam(NULL);
    FILE* fp = fopen(fname, "w+");
    if(!fp) {
        perror("File opening failed");
        return is_ok;
    }
    fputs("Hello, world!\n", fp);
    rewind(fp);
 
    int c; // note: int, not char, required to handle EOF
    while ((c = fgetc(fp)) != EOF) { // standard C I/O file reading loop
       putchar(c);
    }
 
    if (ferror(fp)) {
        puts("I/O error when reading");
    } else if (feof(fp)) {
        puts("End of file reached successfully");
        is_ok = EXIT_SUCCESS;
    }
 
    fclose(fp);
    remove(fname);
    return is_ok;
}

Possible output:

Hello, world!
End of file reached successfully

References

C17 standard (ISO/IEC 9899:2018):

7.21.5.3 The fopen function (p: 223-224)
K.3.5.2.1 The fopen_s function (p: 428-429)

C11 standard (ISO/IEC 9899:2011):

7.21.5.3 The fopen function (p: 305-306)
K.3.5.2.1 The fopen_s function (p: 588-590)

C99 standard (ISO/IEC 9899:1999):

7.19.5.3 The fopen function (p: 271-272)

C89/C90 standard (ISO/IEC 9899:1990):

4.9.5.3 The fopen function

fclose	closes a file (function)
fflush	synchronizes an output stream with the actual file (function)
freopenfreopen_s (C11)	open an existing stream with a different name (function)
C++ documentation for `fopen`