fopen

 

C

Language

Standard Library

Type support

Dynamic memory management

Error handling

Program utilities

Date and time utilities

Strings library

Algorithms

Numerics

Input/output support

Localization support

Thread support (C11)

Atomic operations (C11)

 

File input/output

Functions
File access
fopen freopen fclose fflush	fwide setbuf setvbuf
Direct input/output
fread
fwrite
Unformatted input/output
fgetc fgets fputc fputs getchar gets putchar puts ungetc	fgetwc fgetws fputwc fputws getwchar putwchar ungetwc
Formatted input/output
scanffscanfsscanf vscanfvfscanfvsscanf (C99)(C99)(C99) printffprintfsprintfsnprintf (C99) vprintfvfprintfvsprintfvsnprintf (C99)	wscanffwscanfswscanf vwscanfvfwscanfvswscanf (C99)(C99)(C99) wprintffwprintfswprintf vwprintfvfwprintfvswprintf
File positioning
ftell
fgetpos
fseek
fsetpos
rewind
Error handling
clearerr
feof
ferror
perror
Operations on files
remove
rename
tmpfile
tmpnam

 

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)

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 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)

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

Example

fopen with error checking. Code opens a file for writing data.

Run this code

#include <stdio.h>
#include <stdlib.h>
 
int main(void)
{
    FILE *fp = fopen("data.txt","w");
    if (fp == NULL)
    {
       perror("fopen()");
       fprintf(stderr,"fopen() failed in file %s at line # %d\n", __FILE__,__LINE__-4);
       return EXIT_FAILURE;
    }
 
    /* Normal processing continues here. */
 
    fclose(fp);
    return EXIT_SUCCESS;
}

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