fopen - open a stream
fopen_s - open a stream with additional safety checks
#include <stdio.h> FILE *fopen(const char *filename, const char *mode);
#define __STDC_WANT_LIB_EXT1__ 1
#include <stdio.h>
errno_t fopen_s(FILE *restrict *restrict streamptr,
const char *restrict filename, const char *restrict mode);
The fopen() function opens the file whose pathname is the string pointed to by filename, and associates a stream with it.
The argument mode points to a string beginning with one of the following sequences:
Open file for reading
Truncate to zero length or create file for writing
Append; open or create file for writing at end-of-file
Open file for update (reading and writing)
Truncate to zero length or create file for update
Append; open or create file for update, writing at end-of-file
The character b has no effect, but is allowed for ISO C standard conformance (see standards(7)). Opening a file with read mode (r as the first character in the mode argument) fails if the file does not exist or cannot be read.
Opening a file with exclusive mode (x somewhere after the initial w or a in the mode argument) fails if the file already exists or cannot be created.
Opening a file with append mode (a as the first character in the mode argument) causes all subsequent writes to the file to be forced to the then current end-of-file, regardless of intervening calls to fseek(3C). If two separate processes open the same file for append, each process may write freely to the file without fear of destroying output being written by the other. The output from the two processes will be intermixed in the file in the order in which it is written.
When a file is opened with update mode (+ as the second or third character in the mode argument), both input and output may be performed on the associated stream. However, output must not be directly followed by input without an intervening call to fflush(3C) or to a file positioning function (fseek(3C), fsetpos(3C), or rewind(3C)), and input must not be directly followed by output without an intervening call to a file positioning function, unless the input operation encounters end-of-file.
When opened, a stream is fully buffered if and only if it can be determined not to refer to an interactive device. The error and end-of-file indicators for the stream are cleared.
If mode begins with w or a and the file did not previously exist, upon successful completion, fopen() function will mark for update the st_atime, st_ctime and st_mtime fields of the file and the st_ctime and st_mtime fields of the parent directory.
If mode begins with w and the file did previously exist, upon successful completion, fopen() will mark for update the st_ctime and st_mtime fields of the file. The fopen() function will allocate a file descriptor as open(2) does.
The largest value that can be represented correctly in an object of type off_t will be established as the offset maximum in the open file description.
Additional mode characters are supported in this implementation. These additional characters must appear after r, w, or a.
Specifying e in the mode argument indicates the file is being opened with the O_CLOEXEC flag. This sets the FD_CLOEXEC for the underlying file descriptor. For more information, see the open(2) man page.
Specifying f in the mode argument indicates the file is being opened with the O_CLOFORK flag. This sets the FD_CLOFORK for the underlying file descriptor. For more information, see the open(2) man page.
Specifying x in the mode argument indicates the file is being opened in exclusive mode; the file is opened with the O_EXCL flag. For more information, see the open(2) man page.
The fopen_s() function is part of the bounds checking interfaces specified in the C11 standard, Annex K. It is similar to the fopen() function, except for a different return type and an additional parameter, providing extra safety checks in the form of explicit runtime-constraints as defined in the C11 standard. The fopen_s() function provides a new mode character, u which may precede any of the previously defined modes starting with the character w or a. See INCITS/ISO/IEC 9899:2011.
Implementation specific details for fopen_s() include the following:
Files opened for writing are opened with exclusive, (or non-shared), access. The file permission for the file will prevent other system users from accessing the file, that is S_IRUSR | S_IWUSR flags if the file is being created and the first character of the mode string is not u. For more information, see the chmod(2) man page. If the file is being created and first character of the mode string is u, the file shall have the system default file access permissions.
A runtime-constraint violation will be generated if either streamptr, filename, or mode is a null pointer.
Upon successful completion, fopen() returns a pointer to the object controlling the stream. Otherwise, a null pointer is returned and errno is set to indicate the error.
The fopen() function may fail and not set errno if there are no free stdio streams.
If the fopen_s() function succeeds in opening a file, it returns zero. Otherwise, a non-zero value is returned if fopen_s() failed to open the file or if a runtime-constraint violation was encountered.
The fopen() and fopen_s() functions will fail if:
Search permission is denied on a component of the path prefix, or the file exists and the permissions specified by mode are denied, or the file does not exist and write permission is denied for the parent directory of the file to be created.
A signal was caught during the execution of fopen().
The named file is a directory and mode requires write access.
Too many symbolic links were encountered in resolving filename.
There are {OPEN_MAX} file descriptors currently open in the calling process.
The length of the filename exceeds PATH_MAX or a pathname component is longer than NAME_MAX.
The maximum allowable number of files is currently open in the system.
A component of filename does not name an existing file or filename is an empty string.
The directory or file system that would contain the new file cannot be expanded, the file does not exist, and it was to be created.
A component of the path prefix is not a directory.
The named file is a character special or block special file, and the device associated with this special file does not exist.
The current value of the file position cannot be represented correctly in an object of type fpos_t.
The named file resides on a read-only file system and mode requires write access.
The fopen() function may fail if:
The value of the mode argument is not valid.
{FOPEN_MAX} streams are currently open in the calling process.
{STREAM_MAX} streams are currently open in the calling process.
Pathname resolution of a symbolic link produced an intermediate result whose length exceeds {PATH_MAX}.
Insufficient storage space is available.
The file is a pure procedure (shared text) file that is being executed and mode requires write access.
The fopen_s() function will fail if:
Null pointer is passed.
The fopen() function has a transitional interface for 64-bit file offsets. For more information, see the lf64(7) man page.
See attributes(7) for descriptions of the following attributes:
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The fopen() function can be used safely in multithreaded applications.
The fopen_s() function cannot be used safely in a multithreaded application due to the runtime constraint handler. For more information, see the runtime_constraint_handler(3C) man page.
fclose(3C), fdopen(3C), fflush(3C), freopen(3C), freopen_s(3C), fsetpos(3C), rewind(3C), attributes(7), lf64(7), standards(7), runtime_constraint_handler(3C)