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Defined in header <stdio.h> | ||
| (1) | ||
FILE*fopen(constchar*filename,constchar*mode); | (until C99) | |
FILE*fopen(constchar*restrict filename,constchar*restrict mode); | (since C99) | |
errno_t fopen_s(FILE*restrict*restrict streamptr, constchar*restrict filename, | (2) | (since C11) |
filename and returns a pointer to the file stream associated with that file.mode is used to determine the file access mode.streamptr and the following errors are detected at runtime and call the currently installedconstraint handler function:streamptr is a null pointerfilename is a null pointermode is a null pointerfopen_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>.Contents |
| filename | - | file name to associate the file stream to |
| mode | - | null-terminated character string determiningfile access mode |
| streamptr | - | pointer to a pointer where the function stores the result (an out-parameter) |
| 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 tofflush,fseek,fsetpos orrewind, and input cannot be followed by output without an intervening call tofseek,fsetpos orrewind, 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 usingfopen_s orfreopen_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 defaultfopen permissions.(C11) | ||||
filename refers to an interactive device. On error, returns a null pointer.POSIX requires thaterrno be set in this case.streamptr is a null pointer itself).The format offilename 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){constchar* fname="/tmp/unique_name.txt";// or tmpnam(NULL);int is_ok=EXIT_FAILURE; 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 EOFwhile((c=fgetc(fp))!=EOF)// standard C I/O file reading loopputchar(c); if(ferror(fp))puts("I/O error when reading");elseif(feof(fp)){puts("End of file is reached successfully"); is_ok=EXIT_SUCCESS;} fclose(fp);remove(fname);return is_ok;}
Possible output:
Hello, world!End of file is reached successfully
| closes a file (function)[edit] | |
| synchronizes an output stream with the actual file (function)[edit] | |
(C11) | open an existing stream with a different name (function)[edit] |
C++ documentation forfopen | |
