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POSIX API
6-3-4 DIAGNOSING A FILE ERROR
The file maintains flags that reflect errors encountered in the previous file access, and
subsequent accesses will fail (under certain conditions outlined here) unless these flags are
explicitly cleared (using fs_clearerr() ). There are actually two sets of flags. One reflects
whether the file encountered the end-of-file (EOF) during the previous access, and if this is
set, writes will not fail, but reads will fail. The other reflects device errors, and no
subsequent file access will succeed (except file close) unless this is first cleared. The
functions fs_ferror() and fs_feof() can be used to get the state of device error and
EOF conditions, respectively.
6-3-5 ATOMIC FILE OPERATIONS USING FILE LOCK
If a file is shared between several tasks in an application, the file lock can be employed to
guarantee that a series of file operations are executed atomically. fs_flockfile() (or its
non-blocking equivalent fs_ftrylockfile() ) acquires the lock for a task (if it does not
already own it). Accesses from other tasks will be blocked until fs_funlockfile() is called.
Each file actually has a lock count associated with it. This allows nested calls by a task to
acquire a file lock; each of those calls must be matched with a call to fs_funlockfile( ).
void App_Fnct (void)
{
unsigned char data1[50];
unsigned char data2[10];
.
.
.
if (App_FilePtr != (FS_FILE *)0) {
fs_flockfile(App_FilePtr);
/* Lock file.
/* See Note #1.
/* Wr data atomically.
*/
*/
*/
fs_fwrite(data1, 1, sizeof(data1), App_FilePtr);
fs_fwrite(data2, 1, sizeof(data1), App_FilePtr);
fs_funlockfile(App_FilePtr); /* Unlock file.
*/
}
.
.
.
}
Listing 6-4 Example file lock usage.
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