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 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 constant1
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) |
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
is 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) { 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); }
References
- 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
See also
closes a file (function) | |
synchronizes an output stream with the actual file (function) | |
(C11) | open an existing stream with a different name (function) |
C++ documentation for fopen |
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