sscce

#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include "mpi.h"

#define MAXPROC 16    /* Max number of procsses */
#define TOTAL_FILES 7

int main(int argc, char* argv[]) {
    int i, nprocs, tprocs, me, index;
    const int tag  = 42;    /* Tag value for communication */

    MPI_Request recv_req[MAXPROC];  /* Request objects for non-blocking receive */
    MPI_Request send_req[MAXPROC]; /* Request objects for non-blocking send */
    MPI_Status status;              /* Status object for non-blocing receive */

    char myname[MPI_MAX_PROCESSOR_NAME];             /* Local host name string */
    char hostname[MAXPROC][MPI_MAX_PROCESSOR_NAME];  /* Received host names */
    int namelen;

    MPI_Init(&argc, &argv);                /* Initialize MPI */
    MPI_Comm_size(MPI_COMM_WORLD, &nprocs);    /* Get nr of processes */
    MPI_Comm_rank(MPI_COMM_WORLD, &me);    /* Get own identifier */

    MPI_Get_processor_name(myname, &namelen);  /* Get host name */
    myname[namelen++] = (char)0;              /* Terminating null byte */

    /* First check that we have at least 2 and at most MAXPROC processes */
    if (nprocs<2 || nprocs>MAXPROC) {
        if (me == 0) {
          printf("You have to use at least 2 and at most %d processes\n", MAXPROC);
        }
        MPI_Finalize(); exit(0);
    }

    /* if TOTAL_FILES < nprocs then use only TOTAL_FILES + 1 procs */
    tprocs = (TOTAL_FILES < nprocs) ? TOTAL_FILES + 1 : nprocs;
    int done = -1;

    if (me == 0) {    /* Process 0 does this */

        int send_counter = 0, received_counter;

        for (i=1; i<tprocs; i++) {
            MPI_Isend(&send_counter, 1, MPI_INT, i, tag, MPI_COMM_WORLD, &send_req[i]);
            ++send_counter;
        }

        /* Receive a message from all other processes */
        for (i=1; i<tprocs; i++) {
            MPI_Irecv (hostname[i], namelen, MPI_CHAR, MPI_ANY_SOURCE, tag, MPI_COMM_WORLD, &recv_req[i]);
        }

        for (received_counter = 0; received_counter < TOTAL_FILES; received_counter++){

            /* Wait until at least one message has been received from any process other than 0*/
            MPI_Waitany(tprocs-1, &recv_req[1], &index, &status);

            if (index == MPI_UNDEFINED) perror("Errorrrrrrr");
            printf("Received a message from process %d on %s\n", status.MPI_SOURCE, hostname[index+1]);

            if (send_counter < TOTAL_FILES){ /* si todavia faltan imagenes por procesar */
                MPI_Isend(&send_counter, 1, MPI_INT, status.MPI_SOURCE, tag, MPI_COMM_WORLD, &send_req[status.MPI_SOURCE]);
                ++send_counter;
                MPI_Irecv (hostname[status.MPI_SOURCE], namelen, MPI_CHAR, MPI_ANY_SOURCE, tag, MPI_COMM_WORLD, &recv_req[status.MPI_SOURCE]);
            }
        }

        for (i=1; i<tprocs; i++) {
            MPI_Isend(&done, 1, MPI_INT, i, tag, MPI_COMM_WORLD, &send_req[i]);
        }

    } else if (me < tprocs) { /* all other processes do this */

        int y;
        MPI_Recv(&y, 1, MPI_INT, 0,tag,MPI_COMM_WORLD,&status);

        while (y != -1) {
            printf("Process %d: Received image %d\n", me, y);
            sleep(me%3+1);  /* Let the processes sleep for 1-3 seconds */

            /* Send own identifier back to process 0 */
            MPI_Send (myname, namelen, MPI_CHAR, 0, tag, MPI_COMM_WORLD);
            MPI_Recv(&y, 1, MPI_INT, 0,tag,MPI_COMM_WORLD,&status);
        }
    }

    MPI_Finalize();
    exit(0);
}