I have a program that reads from a Random Access File and is to return the smallest and largest number in the file. One requirement is that this is done with 4 processes using fork() and piping the results. I divide the file up into 4 chunks and have each process evaluate a chunk of the file. I find the max and min of each chunk and write them to a pipe. At the end I will compare the piped values and find the largest and smallest of the values.
I am having trouble reading from the pipes as they are returning -1. Any insight on what I am doing wrong? Thanks!
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
int findMin(int start, int end, const char * filename);
int findMax(int start, int end, const char * filename);
//Calculates minimum and maximum of a number
int main(int argc, char * argv[])
{
const char * filename; // name of file to read
FILE * ft; // file handle for the file
int pid, // process id of this process
num, // the number of integer values in the file
i, // loop control variable for reading values
temp=0; // used to store each value read from the file
long size; // size in bytes of the input file
/*********************************************************************/
filename = argv[1]; // read the file named on the command line
ft= fopen(filename, "rb");
if (ft)
{
pid = getpid();
fseek (ft,0,SEEK_END); //go to end of file
size = ftell(ft); //what byte in file am I at?
fseek (ft,0,SEEK_SET); //go to beginning of file
num = (int)size / (int)sizeof(int); // number of integer values
printf("file size: %li bytes\n", size);
printf("sizeof(int) = %i bytes\n",(int) sizeof(int));
printf("how many integers = %i\n\n", num);
fclose(ft);
}
//Split file size into quarters to make 4 processes
int increment = num/4;
int num1 = increment;
int num2 = num1 + increment;
int num3 = num2 + increment;
int num4 = num;
int status;
int pid1 = -1;
int pid2 = -1;
//Pipes
int fdmin1[2];
int fdmax1[2];
int fdmin2[2];
int fdmax2[2];
int fdmin3[2];
int fdmax3[2];
int fdmin4[2];
int fdmax4[2];
//initializing pipes
if(pipe(fdmin1) == -1)
{
perror("Piping fd1 failed");
return 0;
}
if(pipe(fdmax1) == -1)
{
perror("Piping fd2 failed");
return 0;
}
if(pipe(fdmin2) == -1)
{
perror("Piping fd3 failed");
return 0;
}
if(pipe(fdmax2) == -1)
{
perror("Piping fd4 failed");
return 0;
}
if(pipe(fdmin3) == -1)
{
perror("Piping fd3 failed");
return 0;
}
if(pipe(fdmax3) == -1)
{
perror("Piping fd4 failed");
return 0;
}
if(pipe(fdmin4) == -1)
{
perror("Piping fd3 failed");
return 0;
}
if(pipe(fdmax4) == -1)
{
perror("Piping fd4 failed");
return 0;
}
//temp variables for pipes
int temp1;
int temp2;
int temp3;
int temp4;
int temp5;
int temp6;
int temp7;
int temp8;
pid1 = fork();
printf("pid1: %d \n", pid1);
if(pid1 > 0)
{
//Process 1
temp1 = findMin(0, num1, filename);
temp2 = findMax(0, num1, filename);
close(fdmin1[0]);
if(write(fdmin1[1], &temp1, sizeof(int)) == -1)
{
printf("Error writting to pipe");
}
close(fdmin1[1]);
close(fdmax1[0]);
if(write(fdmax1[1], &temp2, sizeof(int)) == -1)
{
printf("Error writting to pipe");
}
close(fdmax1[1]);
}
else if(pid1 == 0)
{
//Process 2
temp3 = findMin(num1, num2, filename);
temp4 = findMax(num1, num2, filename);
close(fdmin2[0]);
if(write(fdmin2[1], &temp3, sizeof(int)) == -1)
{
printf("Error writting to pipe");
}
close(fdmin2[1]);
close(fdmax2[0]);
if(write(fdmax2[1], &temp4, sizeof(int)) == -1)
{
printf("Error writting to pipe");
}
close(fdmax2[1]);
pid2 = fork();
printf("pid2: %d \n", pid2);
if(pid2 > 0)
{
//Process 3
temp5 = findMin(num2, num3, filename);
temp6 = findMax(num2, num3, filename);
close(fdmin3[0]);
if(write(fdmin3[1], &temp5, sizeof(int)) == -1)
{
printf("Error writting to pipe");
}
close(fdmin3[1]);
close(fdmax3[0]);
if(write(fdmax3[1], &temp6, sizeof(int)) == -1)
{
printf("Error writting to pipe");
}
close(fdmax3[1]);
}
else if(pid2 == 0)
{
//Process 4
temp7 = findMin(num3, num4, filename);
temp8 = findMax(num3, num4, filename);
close(fdmin4[0]);
if(write(fdmin4[1], &temp7, sizeof(int)) == -1)
{
printf("Error writting to pipe");
}
close(fdmin4[1]);
close(fdmax4[0]);
if(write(fdmax4[1], &temp8, sizeof(int)) == -1)
{
printf("Error writting to pipe");
}
close(fdmax4[1]);
}
}
//Close all pipe ends in all processes
close(fdmin1[0]);
close(fdmin1[1]);
close(fdmin2[0]);
close(fdmin2[1]);
close(fdmin3[0]);
close(fdmin3[1]);
close(fdmin4[0]);
close(fdmin4[1]);
close(fdmax1[0]);
close(fdmax1[1]);
close(fdmax2[0]);
close(fdmax2[1]);
close(fdmax3[0]);
close(fdmax3[1]);
close(fdmax4[0]);
close(fdmax4[1]);
//Wait for all processes to finish
int returnStatus;
waitpid(pid1, &returnStatus, 0);
int returnStatus2;
waitpid(pid2, &returnStatus2, 0);
//Make sure we are in parant process
if(pid1 > 0)
{
//Variables to compare min and max returned from processses
int min1;
int max1;
int min2;
int max2;
int min3;
int max3;
int min4;
int max4;
//read from pipe (error is occuring here)
close(fdmin1[1]);
if(read(fdmin1[0], &min1, sizeof(int)) == -1)
{
printf("Error reading");
}
close(fdmin1[0]);
printf("min1: %d \n", min1);
}
return 0;
}
//function to find the minimum in the file
int findMin(int start, int end, const char * filename)
{
int temp;
int smallestNum;
int i;
int length = end - start;
FILE * ft2;
ft2= fopen(filename, "rb");
fseek (ft2,start,SEEK_SET);
fread(&smallestNum,sizeof(int),1,ft2);
for(i = 0; i < length; i++)
{
fread(&temp,sizeof(int),1,ft2);
//printf("%d \n", temp);
if(temp < smallestNum)
{
smallestNum = temp;
}
/*
printf("%5i: %7i ",pid,temp);
if ((i+1)%5 == 0)
printf("\n");
*/
}
fclose(ft2);
printf("SmallestNum: %d \n", smallestNum);
return smallestNum;
}
//function to find maximum in file
int findMax(int start, int end, const char * filename)
{
int temp;
int largestNum;
int i;
int length = end - start;
FILE * ft3;
ft3= fopen(filename, "rb");
fseek (ft3,start,SEEK_SET);
fread(&largestNum,sizeof(int),1,ft3);
for(i = 0; i < length; i++)
{
fread(&temp,sizeof(int),1,ft3);
//printf("%d \n", temp);
if(temp > largestNum)
{
largestNum = temp;
}
/*
printf("%5i: %7i ",pid,temp);
if ((i+1)%5 == 0)
printf("\n");
*/
}
fclose(ft3);
printf("Largest Num: %d \n", largestNum);
return largestNum;
}
Here is the code for generating the Random Access File
/*
* This file generates a binary output file containing integers. It
* requires the output filename as a parameter and will take an
* argument indicating the number of values to generate as input.
*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#define BIAS 0 // a bias value added to the numbers to "bias" the file
// contents to provide an offset to the min and max
int main(int argc, char * argv[]) {
const char * filename; // name of the output file
FILE * ft; // file handle for output file
int numtogen = 1000000; // default is to generate 1,000,000 numbers
int randomnum, i; // variables used in the loop generating numbers
if (argc<2) { // not enough arguments, need output file name
printf("Usage: gendata <filename> [number of numbers]\n");
return 1;
}
if (argc == 3) // optional third argument for number of numbers
numtogen = atoi(argv[2]);
filename=argv[1]; // use the filename entered to store numbers
srand(time(NULL)); // seed the random number generator
ft= fopen(filename, "wb") ;
if (ft) {
for (i = 0; i < numtogen; i++){
randomnum = rand() % numtogen + BIAS;
fwrite(&randomnum,sizeof(int),1,ft);
}
fclose(ft);
}
return 0;
}
I am having trouble reading from the pipes as they are returning -1. Any insight on what I am doing wrong? Thanks!
this is because in the main process you close two times the pipe, doing
printf("pid1: %d \n", pid1);
if(pid1 > 0)
{
...
close(fdmin1[0]); <<< HERE
and
//Close all pipe ends in all processes
close(fdmin1[0]); <<< HERE
so it is closed when you do :
if(read(fdmin1[0], &min1, sizeof(int)) == -1)
do not close fdmin1[0] before to read in but the reverse.
Note you also close two times fdmin1[1] and fdmax1[0] and fdmax1[1].
The usage of the pipes is very strange and probably no what you want :
fdmin1 is a pipe between the main process and itself, the main process does if(write(fdmin1[1], &temp1, sizeof(int)) == -1)and later if(read(fdmin1[0], &min1, sizeof(int)) == -1) so that pipe is useless and min1 is temp1
the main process does if(write(fdmax1[1], &temp2, sizeof(int)) == -1) but nobody read that value, that pipe is useless and temp2 = findMax(0, num1, filename); is done for nothing.
the main process child does if(write(fdmin2[1], &temp3, sizeof(int)) == -1) and if(write(fdmax2[1], &temp4, sizeof(int)) == -1) and if(write(fdmin3[1], &temp5, sizeof(int)) == -1) and if(write(fdmax3[1], &temp6, sizeof(int)) == -1) but nobody read, these four pipes are useless and all the min/max computing are done for nothing.
it is the same for the third created process doing if(write(fdmin4[1], &temp7, sizeof(int)) == -1) and if(write(fdmax4[1], &temp8, sizeof(int)) == -1) but nobody read, these two pipes are useless and the min/max computing are done for nothing.
That means at the end you do not get the right min/max value in the main process, but only the min value of the first quarter computing by the main process and all other computing are lost.
The code
//Wait for all processes to finish
int returnStatus;
waitpid(pid1, &returnStatus, 0);
int returnStatus2;
waitpid(pid2, &returnStatus2, 0);
is executed by all the child processes, because you do not exit or return when you have to do.
You also have an undefined behavior because you have a race condition between your processes, the execution is not the same depending on where I had usleep in your code. A parent process must wait for the end of its child process when needed, you do not at the right moment. Note your process numbering is wrong, there are only the main process and two children, so 3 processes rather than 4, //process4 does not exist and that comment is in process 2.
Except in the main process you do not read from the right position in the file because for findMin and findMax the parameter start correspond to a rank of int rather than a position in the file, you must replace
fseek (ft2,start,SEEK_SET);
fseek (ft3,start,SEEK_SET);
by
fseek (ft2,start*sizeof(int),SEEK_SET);
fseek (ft3,start*sizeof(int),SEEK_SET);
You also (try to) read one int too many doing
int length = end - start;
...
fread(&smallestNum,sizeof(int),1,ft2);
for(i = 0; i < length; i++)
{
fread(&temp,sizeof(int),1,ft2);
for instance replace the loop to have
for(i = 1; i < length; i++)
There also are a lot of useless variables in your program, if I compile with option -Wall :
bruno#bruno-XPS-8300:/tmp$ gcc -Wall -g p.c -o p
p.c: In function ‘main’:
p.c:250:16: warning: unused variable ‘max4’ [-Wunused-variable]
int max4;
^
p.c:249:16: warning: unused variable ‘min4’ [-Wunused-variable]
int min4;
^
p.c:248:16: warning: unused variable ‘max3’ [-Wunused-variable]
int max3;
^
p.c:247:16: warning: unused variable ‘min3’ [-Wunused-variable]
int min3;
^
p.c:246:16: warning: unused variable ‘max2’ [-Wunused-variable]
int max2;
^
p.c:245:16: warning: unused variable ‘min2’ [-Wunused-variable]
int min2;
^
p.c:244:16: warning: unused variable ‘max1’ [-Wunused-variable]
int max1;
^
p.c:48:12: warning: unused variable ‘status’ [-Wunused-variable]
int status;
^
p.c:20:8: warning: unused variable ‘temp’ [-Wunused-variable]
temp=0; // used to store each value read from the file
^
p.c:19:8: warning: unused variable ‘i’ [-Wunused-variable]
i, // loop control variable for reading values
^
p.c:17:8: warning: variable ‘pid’ set but not used [-Wunused-but-set-variable]
int pid, // process id of this process
^
bruno#bruno-XPS-8300:/tmp$
Out of that
You must check the value of argc before to do filename = argv[1];.
If fopen(filename, "rb"); fails you must stop the execution, currently you continue with an undefined behavior.
Note also your program can be simplified using array of pipe rather than separated variables for them, allowing you to use a loop rather than the sequence of if(pipe(fdmin1) == -1) ... if(pipe(fdmax4) == -1) .... It is the same to start the child processes, rather than to duplicate the code use a function to write it only one time. Doing that you can have a definition allowing any number of child process rather than dedicated to 4 only.
Going back to the statement
I divide the file up into 4 chunks and have each process evaluate a chunk of the file
This is an extreme case but you have to manage the case the file is too small to be divided by 4, this is not the case in your proposal.
this is done with 4 processes
Considering the main process is count among the 4, 3 children must be created. But rather than to have each child creating an other one if needed, it is more simple to have the 3 children created by the main process and the parallelism is a little better.
A program must be simple, I already said you have a lot of variables for nothing and lot of code is duplicated, also :
It is useless to have so many pipes, only one is enough to allow each child to send the min/max it computed because the pipe reads and writes are guaranteed to be atomic up to PIPE_BUF (larger than the size of 2 int)
It is useless to read the file so many times, you can search for the min and the max at the same time.
And finally a proposal :
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#define N 4 /* including the main process */
/* to send/receive result atomicaly through the pipe */
typedef struct {
int min, max;
} MinMax;
void findMinMax(long offset, long n, FILE * fp, MinMax * minmax);
//Calculates minimum and maximum of a number
int main(int argc, char * argv[])
{
const char * filename; // name of file to read
FILE * fp; // file handle for the file
long num; // the number of integer values in the file
long size; // size in bytes of the input file
long offset; // offset in file
int pp[2]; // the unique pipe
int pids[N-1];
MinMax minmax;
int i;
if (argc != 2) {
fprintf(stderr, "Usage: %s <filename>\n", *argv);
exit(-1);
}
filename = argv[1];
fp = fopen(filename, "rb");
if (fp == NULL) {
perror("cannot open file");
exit(-1);
}
/* get file size */
if (fseek(fp, 0, SEEK_END) == -1) { //go to end of file
perror("cannot fseek");
fclose(fp); /* also done automaticaly when exiting program */
exit(-1);
}
size = ftell(fp); //what byte in file am I at?
num = size / sizeof(int); // number of integer values
printf("file size: %li bytes\n", size);
printf("how many integers = %li\n\n", num);
if (num < N) {
fprintf(stderr, "the input file is too small, it must contains at least %i int\n", N);
fclose(fp); /* also done automaticaly when exiting program */
exit(-1);
}
//initializing pipe
if(pipe(pp) == -1) {
perror("Piping failed");
exit(-1);
}
offset = 0;
for (i = 0; i != N-1; ++i) {
pids[i] = fork();
switch (pids[i]) {
case 0:
/* child */
{
FILE * fp2 = fopen(filename, "rb");
if (fp2 == NULL) {
perror("child cannot open file");
exit(-1);
}
findMinMax(offset, num/N, fp2, &minmax);
printf("min max child %d : %d %d\n", i, minmax.min, minmax.max);
if (write(pp[1], &minmax, sizeof(minmax)) != sizeof(minmax)) {
perror("Error writting to pipe");
exit(-1);
}
}
exit(0);
case -1:
/* parent */
perror("Cannot fork");
exit(-1);
default:
/* parent, no error */
offset += (num/N)*sizeof(int);
}
}
findMinMax(offset, (size - offset)/sizeof(int), fp, &minmax);
printf("min max main : %d %d\n", minmax.min, minmax.max);
for (i = 0; i != N-1; ++i) {
int status;
MinMax mm;
if ((waitpid(pids[i], &status, 0) != -1) &&
(status == 0) &&
(read(pp[0], &mm, sizeof(mm)) == sizeof(mm))) {
if (mm.min < minmax.min)
minmax.min = mm.min;
if (mm.max > minmax.max)
minmax.max = mm.max;
}
else
fprintf(stderr, "cannot get result for child %d\n", i);
}
printf("global min max : %d %d\n", minmax.min, minmax.max);
return 0;
}
// function to find the minimum and maximum in the file
// n > 1
void findMinMax(long offset, long n, FILE * fp, MinMax * minmax)
{
int v;
if (fseek(fp, offset, SEEK_SET) == -1) {
perror("cannot fseek");
exit(-1);
}
if (fread(&minmax->min, sizeof(minmax->min), 1, fp) != 1) {
fclose(fp); /* also done automaticaly when exiting program */
perror("cannot read int");
exit(-1);
}
minmax->max = minmax->min;
while (--n) {
if (fread(&v, sizeof(v), 1, fp) != 1) {
fclose(fp); /* also done automaticaly when exiting program */
perror("cannot read int");
exit(-1);
}
if (v < minmax->min)
minmax->min = v;
if (v > minmax->max)
minmax->max = v;
}
fclose(fp); /* also done automaticaly when exiting program */
}
As you can see the code is much simple and I just have to modify #define N 4 to an other value to change the number of processes working in parallel.
Using your second program to generate 1000000 int in aze, compilation and execution of my proposal :
bruno#bruno-XPS-8300:/tmp$ gcc -g -Wall p.c
bruno#bruno-XPS-8300:/tmp$ ./a.out aze
file size: 4000000 bytes
how many integers = 1000000
min max main : 2 999995
min max child 0 : 10 999994
min max child 2 : 0 999998
min max child 1 : 3 999999
global min max : 0 999999
bruno#bruno-XPS-8300:/tmp$
I am working on pipeline system with string in C. For example I write this line to console ./pipeline LOWERCASE REVWORD SQUASHWS < stringFile.txt. It should be any like this P0 -> P1 -> P2 -> ...-> Pn. P0 load string from file stringFile.txt and send it to P1.. I can work with pipe(send and read), but I dont know how to work with N processes. It should be any like this. Can you give me any advice or example? Thank you
while(is_something_to_read) {
load_from_previous_process();
do_process(); // for example LOWERCASE
send_to_next_process();
}
Patrik,
I wrote a program that simulate a shell that will spawn a given amount of children and each process will then communicate with another process. Since the number of processes can vary I decided to use a 2D array for the pipes. In the below code NUM_PROCS refers to the amount of processes that will be running (including the parent).
I declare it
int pipes[NUM_PROCS][2];
After this, I create the pipes
for(i = 0; i < NUM_PROCS; i++)
{
if((pipe(pipes[i])) < 0)
{
perror("Failed to open pipe");
}
}
This is a shell program that i wrote for practise.
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <fcntl.h>
#define MAXARGS 256
char ** getargs(char * cmd) {
char** argsarray;
int nargs = 0;
int nlen = strlen(cmd);
int i = 0;
argsarray = (char**) malloc(sizeof(char*) * MAXARGS);
argsarray[0] = strtok(cmd," ");
i = 0;
while (argsarray[i] != NULL){
i++;
argsarray[i] = strtok(NULL," ");
}
return argsarray;
}
int main(void){
pid_t childpid;
int fd[256][2];
char cmd[256];
char * sepCmd[256];
char * pch;
printf("Please enter a command sequence: \n");
gets(cmd);
printf("You have entered: %s ....%d\n", cmd,strlen(cmd));
printf("Attempting to split up command: \n");
pch = strtok (cmd, "|");
int count = 0;
while (pch != NULL && count < 256) {
printf("%s\n", pch);
sepCmd[count] = pch;
printf("The value in this array value is: %s\n", sepCmd[count]);
pch = strtok (NULL, "|");
count++;
}
char ** argue;
int k;
/* Block that deals with the first command given by the user */
k = 0;
pipe(fd[k]);
if(!fork()) {
dup2(fd[k][1], STDOUT_FILENO);
close(fd[k][0]);
argue = getargs(sepCmd[k]);
execvp(argue[0], argue);
perror(argue[0]);
exit(0);
}
/*Loop that will control all other comands except the last*/
for(k = 1; k <= count - 2; k++) {
close(fd[k-1][1]);
pipe(fd[k]);
if(!fork()) {
close(fd[k][0]);
dup2(fd[k-1][0], STDIN_FILENO);
dup2(fd[k][1], STDOUT_FILENO);
argue = getargs(sepCmd[k]);
execvp(argue[0], argue);
perror(argue[0]);
exit(0);
}
}
/*Block that will take care of the last command in the sequence*/
k = count - 1;
close(fd[k-1][1]);
if(!fork()) {
dup2(fd[k-1][0], STDIN_FILENO);
argue = getargs(sepCmd[k]);
execvp(argue[0], argue);
perror(argue[0]);
exit(0);
}
while(waitpid(-1, NULL, 0) != -1);
}
I cant even get to print "Main" on the screen. It seems like none of my code runs. When i dont specify any command line arguments in prints out the warning. my input file contains integers on each line.
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <unistd.h>
#include <semaphore.h>
#include <sys/types.h>
int patientCount,treatedPatientCount,maxPatient,allRegistered;
int *list;
FILE *input,*output;
sem_t semOutputFile,semGlobalVars;
void* Nurse();
void* Doctor(void *);
int main( int argc, char *argv[] ){
printf("Main");
if(argc != 3){
printf("Command line argument count is different then expected. Aborting!");
return -1;
}
input = fopen(argv[1],"r");
output = fopen(argv[2],"w");
allRegistered = 0;
maxPatient = 5;
treatedPatientCount = 0;
patientCount = 0;
list = malloc(sizeof(int)*maxPatient);
pthread_t nurse,doc1,doc2;
sem_init(&semGlobalVars, 0, 1);
sem_init(&semOutputFile, 0, 1);
pthread_create(&nurse, NULL, &Nurse, NULL);
pthread_create(&doc1, NULL, &Doctor, (void*) 1);
pthread_create(&doc2, NULL, &Doctor, (void*) 2);
pthread_exit(NULL);
}
void* Nurse(){
char buff[255],*eof;
while(1){
eof = fgets(buff, 255, input);
if (eof == NULL) {
allRegistered = 1;
pthread_exit(NULL);
}
int k = atoi(buff);
sem_wait(&semGlobalVars);//Critical region 1 starts
if(patientCount == maxPatient){
maxPatient *=2;
list = realloc(list,sizeof(int)*maxPatient);
}
list[patientCount++] = k;
sem_post(&semGlobalVars);//Critical region 1 ends
sem_wait(&semOutputFile);//Critical region 2 starts
fputs("Nurse registered a patient!\n",output);
sem_post(&semOutputFile);//Critical region 2 ends
sleep(2);
}
}
void* Doctor(void * id){
printf("Doctor");
char buff[255];
int waitTime = 0;
while(1){
printf("%d %d %d",allRegistered,treatedPatientCount,patientCount);
if(allRegistered == 1 && treatedPatientCount==patientCount) pthread_exit(NULL);
sem_wait(&semGlobalVars);//Critical region 1 starts
waitTime = list[treatedPatientCount++];
sem_post(&semGlobalVars);//Critical region 1 ends
sprintf (buff, "Doctor %d treated a patient\n", (int) id);
sem_wait(&semOutputFile);//Critical region 2 starts
fputs(buff,output);
sem_post(&semOutputFile);//Critical region 2 ends
sleep(waitTime);
}
}
You can add exit(0) to the end of main
What exit(0) does is flush all buffers, (and other good things,) making sure anything that was buffered (like that printf of "Main") gets written out.
All C streams (open with functions in <cstdio>) are closed (and flushed, if buffered), and all files created with tmpfile are removed.
http://www.cplusplus.com/reference/cstdlib/exit/
I know what every command does within my code, I just don't know the reasons they're there in the first place. This is insanely difficult to search answers for as my questions relate mostly to my own program. Apologies if it's still un-answerable, I will endeavor to improve my future questions :).
I need to write a program that can communicate across shared memory, taking turns to create and delete processes. I'm trying to understand the piece of code I was given, in particular the bit below. At the very bottom I've included the whole producer code in case it helps anyone answer my question.
THE QUESTION: Why is *randNum incremented past 101 when later, the condition for it to print the output IS it being equal to 101?
Does this hint at the Consumer having to change the value contained in the location *randNum in order for the condition to be met?
for(A = 0; A < size; A++) // for loop to reset all priority values so that they are clear to be used in the next set
{
*randNum = 101;
*randNum++;
}
The if command later on:
if(*randNum == 101)
{
*randNum = rand() % (100 - 1) + 1;
*pidNum = getpid();
printf("priority: %d Process ID: %d \n", *randNum, *pidNum);
x = 1;
}
As promised, full program below for completion purposes (trying to make it easier on you and prevent questions; also to provide context)
#include <stdio.h>
#include <stdlib.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/types.h>
#include <unistd.h>
void shm2sz(int size);
int main(int argc, char *argv[])
{
int shmid, A, B, count, *shm, *randNum, *pidNum, pid, *memSig;
key_t key;
int size = atoi(argv[1]);
int shmsz = (size * 2) + 1; // declaring size of shared mem to be twice the size of user input, + 1 for owner ID
int x = 0;
int noToCreate = atoi(argv[2]);
shm2sz(shmsz);
key = 2060; // Identifier key for SharedMem
shmid = shmget(key, shmsz, IPC_CREAT | 0666); //creating Sharedmem
if(shmid < 0) // variable if sharedmem is less than 0, print error.
{
perror("shmget"); // eror mesage print
exit(1);
}
shm = shmat(shmid, NULL, 0); //Attach to shared mem, if fails.. proceed with error message
if(shm == (int *) -1) // eror message
{
perror("shmat");
exit(1);
}
randNum = shm; // declare randNum equal to shm
pidNum = shm + size; // set pid to the first bit of the second part of the shared mem
memSig = shm + shmsz; // set memsig as final value in shared mem
*memSig = 0;
for(A = 0; A < size; A++) // for loop to reset all priority values so that they are clear to be used in the next set
{
*randNum = 101;
*randNum++;
}
count = 0; // set count back to 0
randNum = shm; //check randNum equal to shm
pidNum = shm + size;
while(*memSig != 2)
{
while(*memSig == 1) // set memsignature to sleep while..
{
sleep(1);
}
for(B = 0; B < noToCreate; B++)
{
pid = fork();
if(pid == -1)
{
perror("Error forking");
exit(1);
}
else if(pid > 0)
{
wait(0);
}
else
{
srand(getpid());
while(x == 0)
{
if(*randNum == 101)
{
*randNum = rand() % (100 - 1) + 1;
*pidNum = getpid();
printf("priority: %d Process ID: %d \n", *randNum, *pidNum);
x = 1;
}
else
{
*randNum++;
*pidNum++;
}
}
exit(0);
}
} /* Closes main for loop */
if(*memSig == 0)
{
*memSig = 1;
}
} /* Closes main while loop */
}
void shm2sz(int size)
{
int shmid, *shm2;
key_t key;
key = 9876;
shmid = shmget(key, 2, IPC_CREAT | 0666);
if(shmid < 0)
{
perror("shmget2");
exit(1);
}
shm2 = shmat(shmid, NULL, 0);
if(shm2 == (int *) -1)
{
perror("shmat2");
exit(1);
}
*shm2 = size;
}
The operator precedence for the postfix increment operator is higher than the pointer dereference operator. This means that *randNum++ actually increases the pointer randNum.
If you want to increment the value pointed to by randNum you have to use parentheses:
(*randNum)++;
This question already has an answer here:
Closed 10 years ago.
Possible Duplicate:
C programming print a certain amount of bytes to screen
I would like to read partSize amount of bytes from one file, which can be of any type, and print that same exact amount that was read to a new file which already exists. The program I wrote seems to write less than it is suppose to and gives a segmentation fault.
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/stat.h>
#include <sys/types.h>
#define PERMS 0777
#include <errno.h>
int main()
{
int createDescriptor;
int openDescriptorOriginal;
int closeCreateDescriptor;
char fileNameOriginal[15]="picture.jpg";
//char fileNameOriginal[15]="myFile.txt";
//char fileNameNew[15]="NEWFILE.txt";
char fileName[15]="NEWFILE.jpg";
int parts;
int partSize;
parts=2;
int bytesRemaining;
int partNumber;
char BUFFER[512];
int readDescriptor;
int openDescriptor;
if ((openDescriptorOriginal = open(fileNameOriginal, O_RDONLY )) == -1)
{
printf("Error opening %s", fileNameOriginal);
exit(EXIT_FAILURE);
}
struct stat buf;
int r = fstat(openDescriptorOriginal, &buf);
if(r)
{
fprintf(stderr, "error: fstat: %s\n",(char *)strerror(errno));
exit(1);
}
int originalFileSize=buf.st_size;
printf("The file is %d Bytes large.\n",originalFileSize);
partSize=((originalFileSize+parts)-1)/parts;
printf("Each part is %.9f Kilobytes large.\n",(double)partSize/1024 );
partNumber=1;
printf("Part number: %d\n", partNumber);
if ((openDescriptor = open(fileName, O_WRONLY )) == -1)
{
printf("Error creating %s\n", fileName);
exit(EXIT_FAILURE);
}
ssize_t count, total;
total = 0;
char *bufff = BUFFER;
while (partSize) {
count = read(openDescriptorOriginal, bufff, partSize);
if (count < 0) {
// handle error
break;
}
if (count == 0)
break;
bufff += count;
total += count;
partSize -= count;
}
write (openDescriptor, BUFFER, total);
printf("\n");
return 0;
}
Some initial problems:
add the CREAT flag to your open() in case the file isn't there.
partSize should not be adjusted
Take out the line where you adjust partSize and it should work.
int bytesReceived;
.... open files ....
while ((bytesReceived = read(openDescriptorOriginal, BUFFER, sizeof(BUFFER)) > 0) {
if (bytesReceived != write(openDescriptor, BUFFER, bytesReceived) {
printError(...);
}
}