I am creating random numbers for my code and I am using the seeding method for the purpose of debugging. What I want to do is to store the return value of time(NULL) function which will be used by the srand() function. The value is to be stored in a log text file so that it can be used to analyze problems that occur in the code.
So how should I printf the exact value used as the seed in the srand function?
As already commented, time_t can be virtually anything, the description even changed with versions of the standard, so the only thing you know is that it is a number. I'd say the only portable way to write it to a file is to read the bytes of its representation and store them (*), e.g. in hex format, or directly as binary using fwrite(). For hex format, you could alias it with a char * and read the bytes by indexing from 0 to sizeof(time_t) - 1.
But then, you don't need the exact return value of time() for what you want to achieve, you only need the seed that was actually used! If you look at the srand() function, it takes an unsigned int. So just convert whatever time(NULL) returns to an unsigned int first and store that.
Note that the inner workings of the PRNG aren't specified in the standard either, so if you obtain a logfile from a user, there's no guarantee you can reproduce the same sequence of random numbers from it if he linked against a different C library.
(*) This of course is not portable at runtime between different implementations, as they will have different representations. I guess the question isn't about that, but for completeness, if you really want to store a time value, the way to go would be to store e.g. the components of a struct tm or use something like strftime().
srand initialization is not portable between platforms because the rand algorithm is unspecified and thus could differ. If you need consistent PRN sequences between platforms, you need to use a clearly specified algorithm, including a specified seeding procedure which will use a clearly-defined bitwidth for the seed. As you say, it doesn't matter what the source of the seed is; only the seed itself.The function time() returns time_t type which can be any integer or real-floating type.
But you can cast it to a known type:
printf("%lld\n", (long long) time(NULL)); If you just want to store the binary time_t value in order to read it back later you could do:
Write:
time_t t = time(NULL); /* ... open file handle */ fwrite(&t, sizeof(t), 1, file); Read:
time_t t = 0; /* ... open file handle */ fread(&t, sizeof(t), 1, file); When doing this you have to make sure that wrting and reading is done by the same binary (or different binaries but same compiler).
difftime which is double.
time_tneed not even be an integer in C, but POSIX requires this.uint64_t) print it as such? That's the only way I could see this being in any way close to portable (considering thattime_tusually is an integer type). The problem is that the actual type oftime_tis implementation defined, it could be a floating point type, or even a structure. So the only truly portable way to print it, is as an actual textual date-time stamp.time_tvalue tounsigned long longfirst, and then use that value as the random seedtime().