How can I print to stderr in C without using printf or puts or any function from their families?

Is there a C function to print to stderr without using printf or puts or any function from their families?

Using plain standard C there's only the functions you mention that can write to stdout, stderr, and read from stdin.

However, those are modeled on the UNIX standard output, error and input file descriptors. So if you're on a system like Linux or macOS then you can use the write system call to write to STDERR_FILENO and STDOUT_FILENO (defined in the <unistd.h> header file).

Windows have something similar of course, with GetStdHandle and WriteFile.

Use the write system call:

#include <unistd.h>
#include <string.h>

int main(int argc, char *argv[]) {
   char buf[] = "Hello, world!\n";
   write(2, buf, strnlen(buf,sizeof(buf)));
}

Use fwrite from stdio.h, it writes data from array to a file:

size_t fwrite(const void *ptr, size_t size, size_t nmemb, FILE *stream)

The function allow you to specify the file. This can be stderr in your case:

char str[] = "This is a string";
fwrite(str, sizeof(char), strlen(str), stderr);

You can use the standard fwrite() function and specify stderr as the last argument:

#include <stdio.h>

size_t fwrite(const void *restrict ptr, size_t size, size_t nitems,
              FILE *restrict stream);

Alternatively, you can use the write syscall on POSIX-compliant systems and specify STDERR_FILENO as the first argument:

#include <unistd.h>

ssize_t write(int fd, const void *buf, size_t count);