linguini1 opened a new issue, #15840:
URL: https://github.com/apache/nuttx/issues/15840
### Description / Steps to reproduce the issue
This may not be considered a bug, but the behaviour of `fflush` on files in
a littlefs filesystem does not match what I would expect to see.
My setup is an RP2040 based MCU connected to an SD card over SPI1. The SD
card has two partitions, the first one is a FAT filesystem and the second is a
littlefs file system. The littlefs file system is mounted at `/pwrfs`
When I run the following code with a littlefs file system and interrupt its
execution by removing power to my board, none of logs that were written in the
main loop are visible in the log file. The file was created, but that's it:
```c
/* Open power safe file system */
pwrfs = fopen("/pwrfs/somefile.bin", "w");
if (pwrfs < 0)
{
err = errno;
fprintf(stderr, "Couldn't open power safe log file: %d\n", err);
pthread_exit((void *)(long)err);
}
pthread_cleanup_push(close_fd, pwrfs);
/* Log sensor data continuously */
char data[] = "Some data\n";
for (;;)
{
printf("Logging...\n");
b_written = fwrite(data, 1, sizeof(data), pwrfs);
if (b_written <= 0)
{
err = errno;
fprintf(stderr, "Couldn't write data to logfile: %d\n", err);
continue;
}
err = fflush(pwrfs);
if (err == EOF)
{
err = errno;
fprintf(stderr, "Couldn't flush logfile: %d\n", err);
}
printf("Logged\n");
usleep(10000);
}
pthread_cleanup_pop(1); /* Close pwrfs */
```
I have also tried limiting the loop to 10 iterations, then closing the file
with `fclose` after the loop and then cutting power. The file still remains
empty.
After some searching through file system debug logs, I saw the `echo`
command makes calls to `lfs_file_sync`. This appears only to be accessible
through `littlefs_sync` and `littlefs_open` from my grepping in the source
code. I decided to try operating on my log file using file descriptors instead
of the `FILE` API. The following code performs exactly how I would expect:
```c
/* Open power safe file system */
pwrfs = open("/pwrfs/somefile.bin", O_WRONLY | O_CREAT);
if (pwrfs < 0)
{
err = errno;
fprintf(stderr, "Couldn't open power safe log file: %d\n", err);
pthread_exit((void *)(long)err);
}
pthread_cleanup_push(close_fd, pwrfs);
/* Log sensor data continuously */
char data[] = "Some data\n";
for (;;)
{
printf("Logging...\n");
b_written = write(pwrfs, data, sizeof(data));
if (b_written <= 0)
{
err = errno;
fprintf(stderr, "Couldn't write data to logfile: %d\n", err);
continue;
}
err = fsync(pwrfs);
if (err < 0)
{
err = errno;
fprintf(stderr, "Couldn't sync logfile: %d\n", err);
}
printf("Logged\n");
usleep(10000);
}
pthread_cleanup_pop(1); /* Close pwrfs */
```
No matter when I pull the power on my board, logs are preserved in the log
file and I can access the contents on the next boot, instead of being greeted
by an empty file.
I don't know if there would be any other similar/equivalent `FILE` API
function to `fsync` besides `fflush`, so I would intuitively expect that this
function would sync the file system to "disk" underneath the hood. Maybe that's
an incorrect/naive assumption. If so, could anyone tell me an alternative
option so I can use the `FILE` API but still have my log file be synced? I've
let the program run for quite a few iterations and I still observe an empty
file. Does the file ever get synced when modified through the `FILE` API? And
when?
### On which OS does this issue occur?
[OS: Linux]
### What is the version of your OS?
Linux 6.13.2-arch1-1 #1 SMP PREEMPT_DYNAMIC Sat, 08 Feb 2025 18:54:55 +0000
x86_64 GNU/Linux
### NuttX Version
e5388558a6deb779d0cd880e9e1a2042d2f1bc57
### Issue Architecture
[Arch: arm]
### Issue Area
[Area: File System]
### Host information
_No response_
### Verification
- [x] I have verified before submitting the report.
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