inttypes.h

inttypes.h solves a portability trap that bites you the first time you try to printf a uint64_t. On one platform uint64_t is unsigned long, so %lu works. On another it is unsigned long long, so you need %llu. Get it wrong and you get a compiler warning on one platform and silently wrong output on another. The format macros in this header expand to whichever specifier is correct for the current platform.

#include <inttypes.h>
#include <stdio.h>
 
uint64_t bytes = 1099511627776ULL;   // 1 TiB
 
printf("%" PRIu64 " bytes\n", bytes);    // always correct
printf("%" PRIx64 " hex\n",   bytes);    // lowercase hex
printf("%" PRIX64 " HEX\n",   bytes);    // uppercase hex

The naming pattern: PRI for printf, SCN for scanf; then the conversion letter (d, i, u, x, X, o); then the bit width. The full set covers 8, 16, 32, 64, and the FAST/LEAST variants from <stdint.h>:

uint32_t id;
scanf("%" SCNu32, &id);

The header also declares strtoimax and strtoumax for converting strings to intmax_t/uintmax_t, and imaxabs/imaxdiv for arithmetic on those types.

The portable workaround without these macros — casting everything to unsigned long long and using %llu — works in practice today but generates warnings and will break if you move the code to a platform with a different long long size. The macros are the correct solution.

// compile: gcc -o intdemo intdemo.c
// run: ./intdemo
// description: print fixed-width integers portably using PRI macros
 
#include <inttypes.h>
#include <stdio.h>
 
int main(void) {
    uint8_t  a = 255;
    uint32_t b = 0xDEADBEEF;
    uint64_t c = UINT64_MAX;
    int64_t  d = INT64_MIN;
 
    printf("uint8:  %" PRIu8  "\n",              a);
    printf("uint32: %" PRIu32 " (0x%" PRIx32 ")\n", b, b);
    printf("uint64: %" PRIu64 "\n",              c);
    printf("int64:  %" PRId64 "\n",              d);
    return 0;
}

Compile this on a 32-bit cross-compilation target and the output will be identical to a 64-bit build because the macros automatically expand to the right specifier. Try replacing PRIu64 with %lu and compile for both targets — you will get a warning on at least one. That warning is the bug the macros fix.