C printf 说明符 %lld 打印字符“ld”

Question

I am developing using gcc (-std=gnu99) for an embedded toolchain (Myriota), and I have problems with printf.

when I try the following code:

long long int time = TimeGet();
printf("\nSeconds since epoch: %lld\r\n", time);

it prints:

Seconds since epoch: ld

Using "%" PRId64 prints the same "ld".

Any ideas? I'd appreciate if you could point me to the right place.

Edit variable type corrected long long int time

Answer 1

Most likely, your C library, specifically its implementation of printf doesn't support C99.

The type long long int and the %lld format were introduced by the 1999 ISO C standard (C99). Using gcc -std=c99 makes the compiler attempt to conform to C99, but it can't make the runtime library do things that it doesn't implement. You have a mismatch between what the compiler supports and what the runtime library supports.

In C90, calling printf with %lld in the format string had undefined behavior.

Does %ld work for an argument of type long int ? If the argument doesn't exceed LONG_MAX , converting and using %ld might be a good workaround. If you need to print values that exceed LONG_MAX , implementing a long long int to string conversion isn't horribly difficult.

Answer 2

The Myriota SDK uses newlib-nano which does not support int64_t. I'll have to implement my own function to convert to char string, or cast to uint32_t.