error with running multiple thread with C example - c

Question

I have found an example of multi thread programming which help to determine the prime numbers for a given integar n.it will also take number of thread as a input from the user.But the problem is when i try to execute it give me some errors which very hard to solve.Can anyone help?I am newbies at coding so any type of help and advice will be greatly appreciated.
#include <stdio.h>
#include <math.h>
#include <pthread.h> // required for threads usage
#define MAX_N 100000000
#define MAX_THREADS 25
int nthreads, n, prime[MAX_N+1], nextbase; // next sieve multiplier to be used
// lock for the shared variable nextbase
pthread_mutex_t nextbaselock = PTHREAD_MUTEX_INITIALIZER;
// ID structs for the threads
pthread_t id[MAX_THREADS];
// "crosses out" all odd multiples of k
void crossout(int k)
{ int i;
for (i = 3; i*k <= n; i += 2) {
prime[i*k] = 0;
}
}
// each thread runs this routine
void *worker(int tn) // tn is the thread number (0,1,...)
{ int lim,base,
work = 0; // amount of work done by this thread
// no need to check multipliers bigger than sqrt(n)
lim = sqrt(n);
do {
pthread_mutex_lock(&nextbaselock);
base = nextbase;
nextbase += 2;
// unlock the lock
pthread_mutex_unlock(&nextbaselock);
if (base <= lim) {
// don't bother crossing out if base known composite
if (prime[base]) {
crossout(base);
work++; // log work done by this thread
}
}
else return work;
} while (1);
}
main(int argc, char **argv)
{ int nprimes, // number of primes found
i,work;
n = atoi(argv[1]);
nthreads = atoi(argv[2]);
for (i = 3; i <= n; i++) {
if (i%2 == 0) prime[i] = 0;
else prime[i] = 1;
}
nextbase = 3;
// get threads started
for (i = 0; i < nthreads; i++) {
pthread_create(&id[i],NULL,worker,i);
}
for (i = 0; i < nthreads; i++) {
pthread_join(id[i],&work);
printf("%d values of base done\n",work);
}
nprimes = 1;
for (i = 3; i <= n; i++)
if (prime[i]) {
nprimes++;
}
printf("the number of primes found was %d\n",nprimes);
}
I have the following error while compiling:
In function ‘worker’:
Primes.c:67:12: warning: return makes pointer from integer without a cast [enabled by default]
else return work;
^
Primes.c: In function ‘main’:
Primes.c:88:7: warning: passing argument 3 of ‘pthread_create’ from incompatible pointer type [enabled by default]
pthread_create(&id[i],NULL,worker,i);
^
In file included from Primes.c:15:0:
/usr/include/pthread.h:244:12: note: expected ‘void * (*)(void *)’ but argument is of type ‘void * (*)(int)’
extern int pthread_create (pthread_t *__restrict __newthread,
^
Primes.c:88:7: warning: passing argument 4 of ‘pthread_create’ makes pointer from integer without a cast [enabled by default]
pthread_create(&id[i],NULL,worker,i);
^
In file included from Primes.c:15:0:
/usr/include/pthread.h:244:12: note: expected ‘void * __restrict__’ but argument is of type ‘int’

Answer 1

using: gcc -c -Wall -Wextra -Wconversion -std=gnu99 %f
where %f is the name of the file being compiled
The compiler output the following messages.
I added some commentary for each message that should point you to how to fix the problem.
compiler warning message:
:28:14: warning: conversion to 'int' from 'double' may alter its' value [-Wconversion]
lim = sqrt(n);
The function: sqrt() returns a 'double' but 'lim' is declared a 'int'
Suggest: cast the returned value to 'int'
lim = (int)sqrt(n);
compiler warning message:
:43:12: warning: return makes pointer from integer without a cast [enbled by default]
else return work;
the return type from the worker() function is void*
always exit the worker() function by:
pthread_exit( &work );
compiler warning message:
:24:18: warning: unused parameter 'tn' [-Wunused-parameter]
means the parameter 'tn' is not used. fix this by inserting in that function the line:
(void)tn;
compiler warning message:
47:1: warning: return type defaults to 'int' [enabled by default]
The signature of the main() function is not correct. without using the environment parameter, (almost never used), there are only 2 valid main() signatures and one optional signature.
use the correct signature for what your program needs.
int main( void )
int main( int argc, char *argv[] )
int main() // optional signature
compiler warning message:
50:4: warning: implicit declaration of function 'atoi' [-Wimplicit-function-declaration]
means the header file: stdlib.h has not been #include'd suggest inserting at top of file:
#include <stdlib.h>
compiler warning message:
61:7: warning: passing argument 3 of 'pthread_create' from incompatible pointer type [enabled by default]
means the third parameter to the function: pthread_create() was not a void pointer. suggest:
pthread_create(&id[i],NULL,worker,(void*)&i);
compiler warning message:
67:7: warning: passing argument 2 of 'pthread_join' from incompatible pointer type [enabled by default]
means the variable 'work' should be declared, in file global space, not in the thread function, as:
void * work;
there are other compiler warning messages output, but the above will eliminate them.
I.E. always start with the first compiler message, fix that, then re-compile. then fix the new first message.
Most of the problems with the syntax of the code could have been avoided by paying attention to the man pages for the system functions that were called in the posted code.
Note: the crossout() functions' logic is not correct. the code needs to start at k+k, continue until k<=n and step by k+=k I.E.
for( int k=i+i; k<(n+1); k+=k )
the code seems to be trying to implement a eratosthenes sieve for prime numbers google for the details.
unless your required to use threads, don't, they will just slow things down due to all the context swapping, etc.
this code logic:
for (i = 3; i <= n; i++)
{
if (i%2 == 0) prime[i] = 0;
else prime[i] = 1;
}
seems to be wrong, for one thing, the array prime[] will be initially all 0 because it is in the file global space. As far as I can tell, that code block is not needed at all.
because the work variable will be in the file global space, suggest an array of
void* work[ nthreads ];
then have the thread function: worker() actually use the passed parameter to select which of the entries in the array to be updating.