shell.sh

#!/bin/bash

str="aaaaaaaaaaaaaaaaaa";

while true; do
  if [ "$str" = "aaaaaaaaaaaaaaaaaa" ]; then
    str="bbbbbbbbbbbbbbbbbbbbb"
  else
    str="aaaaaaaaaaaaaaaaaa"
  fi
  echo "$str
$str
$str" > /dev/shm/test.tmp
  mv /dev/shm/test.tmp /dev/shm/test
done

test-shm.c

#include <apr_file_io.h>
#include <apr_pools.h>
#include <apr_strings.h>
#include <stdio.h>
#include <stdlib.h>
#include <apr_thread_proc.h>
#include <apr_thread_mutex.h>

static void _remove_lineends(char *str) {
  if(str) {
      size_t len = strlen(str);
      while(len>0) {
          if(str[len-1] == '\n' || str[len-1] == '\r') {
              str[len-1] = '\0';
              len--;
            } else {
              break;
            }
        }
    }
}

static void* APR_THREAD_FUNC work_thread(apr_thread_t *thread, void *data) {
  apr_pool_t *pool;
  apr_file_t *f;
  apr_pool_create(&pool,NULL);
  while(1) {
      int rv;
      char *line1=NULL,*line2=NULL,*line3=NULL;
      char line[1024];
      rv = apr_file_open(&f, "/dev/shm/test",
                         APR_FOPEN_READ|APR_FOPEN_BUFFERED|APR_FOPEN_BINARY,APR_OS_DEFAULT,
                         pool);

      if(rv != APR_SUCCESS) {
          char errmsg[120];
          printf("failed to open file %s\n",apr_strerror(rv,errmsg,120));
          exit(1);
        }
      if( (rv = apr_file_gets(line,1024,f))== APR_SUCCESS) {
          _remove_lineends(line);
          line1 = apr_pstrdup(pool,line);
        }
      if( (rv = apr_file_gets(line,1024,f))== APR_SUCCESS) {
          _remove_lineends(line);
          line2 = apr_pstrdup(pool,line);
        }
      if( (rv = apr_file_gets(line,1024,f))== APR_SUCCESS) {
          _remove_lineends(line);
          line3 = apr_pstrdup(pool,line);
        }
      apr_file_close(f);
      if(!line1||!line2||!line3) {
          printf("one line was NULL\n");
          exit(1);
        }
      if(strcmp(line1,line2)||strcmp(line1,line3)) {
          printf("one line does not match\n");
          exit(1);
        }
      apr_pool_clear(pool);

    }
}

int main(int argc, char **argv) {
  apr_thread_t **threads;
  int nthreads=128,n;
  apr_status_t rv;
  apr_threadattr_t *thread_attrs;
  apr_pool_t *pool;
  apr_initialize();
  apr_pool_create(&pool,NULL);
    apr_threadattr_create(&thread_attrs, pool);
    threads = (apr_thread_t**)apr_pcalloc(pool, nthreads*sizeof(apr_thread_t*));
    for(n=0; n<nthreads; n++) {
      apr_thread_create(&threads[n], thread_attrs, work_thread, NULL, pool);
    }
    for(n=0; n<nthreads; n++) {
      apr_thread_join(&rv, threads[n]);
    }

 }