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fopen, fopen_s

From cppreference.com
< c‎ | io

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:
  • streampr is a null pointer
  • filename is a null pointer
  • mode is a null pointer
As all bounds-checked functions, fopen_s is only guaranteed to be available of __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>.

Contents

[edit] Parameters

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

[edit] Return value

1) 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.

2) Returns zero on error and writes the null pointer to *streamptr (unless streamptr is a null pointer itself)

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

[edit] Example

#include <stdio.h>
#include <stdlib.h>
 
int main(void)
{
    FILE* fp = fopen("test.txt", "r");
    if(!fp) {
        perror("File opening failed");
        return EXIT_FAILURE;
    }
 
    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");
 
    fclose(fp);
}


[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]