Atomic append test.

count.c

#if 0
set -e; [ "$0" -nt "$0.bin" ] &&
gcc -Wall -Wextra -pedantic -std=c99 "$0" -o "$0.bin"
exec "$0.bin" "$@"
#endif

#include <stdio.h>
#include <stdlib.h>

#define LEN(x) (sizeof(x)/sizeof((x)[0]))

int
main(int argc, char *argv[])
{
	(void)argc;

	const int bufsiz = atoi(argv[1]);
	const int loops = atoi(argv[2]);

	int rc = 0;
	int num[24] = {0};

	int curr = 0;
	int ch, prevch = EOF;
	while((ch = getchar()) != EOF) {
		if(prevch != EOF && prevch != ch) {
			if(curr % bufsiz != 0) {
				fprintf(stderr, "%c[%d]\n", prevch, curr);
				rc = -1;
			}
			curr = 0;
		}
		num[ch - 'a']++;
		curr++;
		prevch = ch;
	}
	for(unsigned i = 0; i < LEN(num); i++) {
		if(num[i] != loops * bufsiz) {
			fprintf(stderr, "%c = %d\n", 'a'+i, num[i]);
			rc = -1;
		}
	}
	return rc;
}

test.sh

#!/bin/sh
set -e
rm -f thefile
./write.c $1 $2 >> thefile
< thefile ./count.c $1 $2

write.c

#if 0
set -e; [ "$0" -nt "$0.bin" ] &&
gcc -Wall -Wextra -pedantic -std=c99 -pthread "$0" -o "$0.bin"
exec "$0.bin" "$@"
#endif

#define _XOPEN_SOURCE
#include <pthread.h>
#include <string.h>
#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <unistd.h>

#define LEN(x) (sizeof(x)/sizeof((x)[0]))

static int bufsiz;
static int loops;

void *
writer(void *arg)
{
	char id = (intptr_t)arg;
	char *buf = malloc(bufsiz);
	memset(buf, id, bufsiz);
	for(int i = 0; i < loops; i++) {
		write(STDOUT_FILENO, buf, bufsiz);	
	}
	free(buf);

	return 0;
}

int
main(int argc, char *argv[])
{
	(void)argc;

	bufsiz = atoi(argv[1]);
	loops = atoi(argv[2]);

	pthread_t ths[24];
	for(unsigned i = 0; i < LEN(ths); i++) {
		pthread_create(&ths[i], 0, writer, (void*)(intptr_t)('a'+i));
	}
	for(unsigned i = 0; i < LEN(ths); i++) {
		pthread_join(ths[i], 0);
	}
	return 0;
}