fopen, fopen_s

Defined in header <stdio.h>
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>.


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).


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.


#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);
    int c; // note: int, not char, required to handle EOF
    while ((c = fgetc(fp)) != EOF) { // standard C I/O file reading loop
    if (ferror(fp)) {
        puts("I/O error when reading");
    } else if (feof(fp)) {
        puts("End of file reached successfully");
        is_ok = EXIT_SUCCESS;
    return is_ok;

Possible output:

Hello, world!
End of file reached successfully


  • C17 standard (ISO/IEC 9899:2018):
    • The fopen function (p: 223-224)
    • K. The fopen_s function (p: 428-429)
  • C11 standard (ISO/IEC 9899:2011):
    • The fopen function (p: 305-306)
    • K. The fopen_s function (p: 588-590)
  • C99 standard (ISO/IEC 9899:1999):
    • The fopen function (p: 271-272)
  • C89/C90 standard (ISO/IEC 9899:1990):
    • The fopen function

See also

closes a file
synchronizes an output stream with the actual file
open an existing stream with a different name
C++ documentation for fopen

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