Printing variable number of bytes using format strings with printf

Question

Goal: Print variable number of bytes using a single format specifier.

Environment: x86-64 Ubuntu 20.04.3 LTS running in VM on an x86-64 host machine.

Example:

Let %kmagic be the format specifier I am looking for which prints k bytes by popping them from the stack and additing them to the output. Then, for %rsp pointing to a region in memory holding bytes 0xde 0xad 0xbe 0xef, I want printf("Next 4 bytes on the stack: %4magic") to print Next 4 bytes on the stack: deadbeef.

What I tried so far:

1. %khhx, which unfortunately just results in k-1 blank spaces followed by two hex-characters (one byte of data).
2. %kx, which I expected to print k/2 bytes interpreted as one number. This only prints 8 hex-characters (4 bytes) prepended by k - 8 blank spaces.

The number of non-blank characters printed matches the length of the format specifiers, i.e. the expected length of %hhx is 2, which is also the number of non-blank characters printed. The same holds for %x, which one expects to print 8 characters.

Question: Is it possible to get the desired behavior? If so, how?

Answer 1

Is it possible to get the desired behavior? If so, how?

There does not exist printf format specifier to do what you want.

Is it possible

Write your own printf implementation that supports what you want. Use implementation-specific tools to create your own printf format specifier. You can take inspiration from linux kernel printk %*phN format speciifer.

Answer 2

It is not possible to using standard printf. You need to write your own function and customize the printf function.

http://www.gnu.org/software/libc/manual/html_node/Customizing-Printf.html

Example (simple dump):

int printdump (FILE *stream, const struct printf_info *info, const void *const *args)
{
    const unsigned char *ptr = *(const unsigned char **)args[0];
    size_t size = *(size_t*)args[1];
    for(size_t i = 1; i <= size; i++)
    {
        fprintf(stream, "%02X%c", ptr[i-1], i % 8 ? ' ' : '\n');
    }
  return 1;
}

int printdumpargs (const struct printf_info *info, size_t n, int *argtypes)
{
  if (n == 2)
    argtypes[0] = PA_POINTER;
    argtypes[1] = PA_INT;
  return 2;
}

int main(void)
{
    double x[4] = {456543645.6786e45, 456543654, 1e345, -345.56e67};
    register_printf_function ('Y', printdump, printdumpargs);

    printf("%Y\n", &x, sizeof(x));
}

As I see it is depreciated now (probably no one was using it)

https://godbolt.org/z/qKs6e1d9q

Output:

30 18 CB 5A EF 10 13 4B
00 00 00 A6 4D 36 BB 41
00 00 00 00 00 00 F0 7F
C4 5D ED 48 9C 05 60 CE

Answer 3

There is no standard conversion specifier for your purpose, but you can achieve your goal in C99 using an ancillary function and dynamic array:

#include <stdio.h>

char *dump_bytes(char *buf, const void *p, size_t count) {
    const unsigned char *src = p;
    char *dest = buf;
    while (count --> 0) {
        dest += sprintf(dest, "%.2X", *src++);
        if (count)
            *dest++ = ' ';
    }
    *dest = '\0';  // return an empty sting for an empty memory chunk
    return buf;
}

int main() {
    long n = 0x12345;
    printf("n is at address %p with contents: %s\n",
           (void *)&n,
           dump_bytes((char[3 * sizeof(n)]){""}, &n, sizeof(n)));
    return 0;
}

Output: n is at address 0x7fff523f57d8 with contents: 45 23 01 00 00 00 00 00

You can use a macro for simpler invocation:

#define DUMPBYTES(p, n)  dump_bytes((char[3 * (n)]){""}, p, n)

int main() {
    char *p = malloc(5);
    printf("allocated 5 bytes at address %p with contents: %s\n",
           p, DUMPBYTES(p, 5));
    free(p);
    return 0;
}