File IO unit test in C [closed] - c

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I need to write unit tests for a simple logger implemented in C (unit tests are in c++).
I'm searching for a way to mock an unsuccessful 'write' to the log file ,in order to see that the logger to responds appropriately and returns an error.
I tried renaming the log-file before writing on to it:
static bool fileWriteMiss() {
const char* testFile = "missLogFile.log";
const char* movedFile = "missedLogFile.log";
ASSERT_TRUE(
spLoggerCreate(testFile, SP_LOGGER_DEBUG_INFO_WARNING_ERROR_LEVEL) ==
SP_LOGGER_SUCCESS);
rename(testFile, movedFile);
ASSERT_TRUE(spLoggerPrintError("MSGA", "sp_logger_unit_test.c", __func__,
__LINE__) == SP_LOGGER_WRITE_FAIL);
spLoggerDestroy();
return true;
}
and I tried deleting the log-file before writing on to it:
static bool fileWriteMiss() {
const char* testFile = "missLogFile.log";
ASSERT_TRUE(
spLoggerCreate(testFile, SP_LOGGER_DEBUG_INFO_WARNING_ERROR_LEVEL) ==
SP_LOGGER_SUCCESS);
remove(testFile);
ASSERT_TRUE(spLoggerPrintError("MSGA", "sp_logger_unit_test.c", __func__,
__LINE__) == SP_LOGGER_WRITE_FAIL);
spLoggerDestroy();
return true;
}
ASSERT_TRUE function is implemented in an assertion library that is assumed to work properly.
spLoggerCreate is the initialization function of the logger structure (I'm being careful not to call it construction and object since the implementation is not oop, but pure C code).

Answer 1

A working hack is to write to file that you cannot write to (please note that you can write to non-existent file in an existing directory - it would be created). There are few ways, some os-dependent:
write to a file in non-existing directory. While it is complicated in a generic case, you can use some tricks like using chars unlikely to meet in a filename, like '\x01'.
const char* testFile = "non-existent\x01dir/missLogFile.log";
write to a file you have no rights to write to. In UNIX you can issue
chmod a-w file.log
command on an existing file to revoke write permission.
write to a system read-only pseudo-file.
const char* testFile = "/proc/uptime"; // linux
const char* testFile = "CLOCK$"; // windows / dos
I believe unit-tests should never access OS, and your test becomes non-unit this way, but this is a terminology question and a holy war subject.
Update:
Your comment actually makes it a poorly asked but pretty interesting question. I think you can edit it and I will try to answer. As things go hacky here, I'll focus on Linux only, answering the question
"How to make file open-for-write succeed, but subsequent write fail?"
Probably the most common error real application face is "No space left on device". This is easy to reproduce in hermetic way: let us create a fat16 filesystem in local file, precisely:
$ dd if=/dev/urandom of=test_disk bs=1048576 count=1
$ mkdosfs test_disk
$ mkdir /tmp/mnt
$ sudo mount -o loop,rw,umask=000 test_disk /tmp/mnt/
Now fill the filesystem. Alternatively we could use smaller size.
$ perl -e 'print "a"x(1024*1002)' > /tmp/mnt/a.txt
$ echo test > /tmp/mnt/test || echo "can't write test file"
And finally hit the error:
$ echo `perl -e 'print "a"x4096'` > /tmp/mnt/test || echo "test succeeded"
bash: echo: write error: No space left on device
test succeeded
To be precise the error is encountered writing byte 1009:
$ ls -l /tmp/mnt/
total 1004
-rwxrwxrwx 1 root root 1026048 Mar 27 17:56 a.txt
-rwxrwxrwx 1 root root 1009 Mar 27 17:57 test
A nice side effect of FAT filesystem is that now you are free to open more files there and write will fail from byte 0, as you asked.