When a stream that has unwritten buffered data is flushed by exit() there is no way for the calling process to discover whether or not exit() successfully wrote the data to the underlying file descriptor. Therefore, it is strongly recommended that applications always ensure there is no unwritten buffered data in any stream when calling exit(), or returning from the initial call to main(), with a status value that indicates no errors occurred.

For example, the following code demonstrates one way to ensure that stdout has already been successfully flushed before calling exit() with status 0. If the flush fails, the file descriptor underlying stdout is closed so that exit() will not try to repeat the failed write operation. If the flush succeeds, a final check with ferror() is performed to ensure that there were no write errors during earlier flush operations (that were not handled at the time).

int status = 0;
if (fflush(stdout) != 0) {
    perror("appname: standard output");
    close(fileno(stdout));
    status = 1;
}
else if (ferror(stdout)) {
    fputs("appname: write error on standard output\n", stderr);
    status = 1;
}
exit(status);

See APPLICATION USAGE.