How to print a int64

C99 standard has integer types with bytes size like int64_t. I am using the following code:

#include <stdio.h>
#include <stdint.h>
int64_t my_int = 999999999999999999;
printf("This is my_int: %I64dn", my_int);

and I get this compiler warning:

warning: format ‘%I64d’ expects type ‘int’, but argument 2 has type ‘int64_t’

I tried with:

printf("This is my_int: %lldn", my_int); // long long decimal

But I get the same warning. I am using this compiler:

~/dev/c$ cc -v
Using built-in specs.
Target: i686-apple-darwin10
Configured with: /var/tmp/gcc/gcc-5664~89/src/configure --disable-checking --enable-werror --prefix=/usr --mandir=/share/man --enable-languages=c,objc,c++,obj-c++ --program-transform-name=/^[cg][^.-]*$/s/$/-4.2/ --with-slibdir=/usr/lib --build=i686-apple-darwin10 --program-prefix=i686-apple-darwin10- --host=x86_64-apple-darwin10 --target=i686-apple-darwin10 --with-gxx-include-dir=/include/c++/4.2.1
Thread model: posix
gcc version 4.2.1 (Apple Inc. build 5664)

Which format should I use to print my_int variable without having a warning?

---

For int64_t type:

#include <inttypes.h>
int64_t t;
printf("%" PRId64 "n", t);

for uint64_t type:

#include <inttypes.h>
uint64_t t;
printf("%" PRIu64 "n", t);

you can also use PRIx64 to print in hexadecimal.

cppreference.com has a full listing of available macros for all types including intptr_t ( PRIxPTR ). There are separate macros for scanf, like SCNd64 .

---

A typical definition of PRIu16 would be "hu" , so implicit string-constant concatenation happens at compile time.

For your code to be fully portable, you must use PRId32 and so on for printing int32_t , and "%d" or similar for printing int .

---

The C99 way is

#include <inttypes.h>
int64_t my_int = 999999999999999999;
printf("%" PRId64 "n", my_int);

Or you could cast!

printf("%ld", (long)my_int);
printf("%lld", (long long)my_int); /* C89 didn't define `long long` */
printf("%f", (double)my_int);

If you're stuck with a C89 implementation (notably Visual Studio) you can perhaps use an open source <inttypes.h> (and <stdint.h> ): http://code.google.com/p/msinttypes/

---

With C99 the %j length modifier can also be used with the printf family of functions to print values of type int64_t and uint64_t :

#include <stdio.h>
#include <stdint.h>

int main(int argc, char *argv[])
{
    int64_t  a = 1LL << 63;
    uint64_t b = 1ULL << 63;

    printf("a=%jd (0x%jx)n", a, a);
    printf("b=%ju (0x%jx)n", b, b);

    return 0;
}

Compiling this code with gcc -Wall -pedantic -std=c99 produces no warnings, and the program prints the expected output:

a=-9223372036854775808 (0x8000000000000000)
b=9223372036854775808 (0x8000000000000000)

This is according to printf(3) on my Linux system (the man page specifically says that j is used to indicate a conversion to an intmax_t or uintmax_t ; in my stdint.h, both int64_t and intmax_t are typedef'd in exactly the same way, and similarly for uint64_t ). I'm not sure if this is perfectly portable to other systems.

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