Posix Semaphore compilation error using the -lrt [duplicate] - c

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Closed 11 years ago.
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sem_open() error: “undefined reference to sem_open()” on linux (Ubuntu 10.10)
Having issues with compilation of posix semaphores. My goal is to create a shared memory segment and protect it by semaphores. shared memory works fine but semaphores code gives me compilation errors even though I included semaphore.h and added -lrt to compilation flags
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <semaphore.h>
#include <time.h>
int main (int argc, char** argv) {
FILE *configFile;
int i,j;
int CashDesksNo=0;
int maxCashDesksNo;
int n,m;
int TmaxServe;
int custPerc; //customer ratio policy
int maxCapacity;
char * nlptr=NULL;
char * pch=NULL;
char line[125];
char termInput[30];
char tmpString[40];
int flg1,flg2;
int flag;
int execResult=0;
int fd;
int rc;
int randNum;
int status;
pid_t ch_pid;
int a,b,c;
int shmid=0;
int *shm_ptr;
int * err;
int retval;
sem_t *sp;
char semName[10];
strcpy(semName,"mutex");
//---------- Davasma kai elegxos in line parameters-----------------
// read inline params and config file
.
.
.
//------------ Print Configuration Data ----------------------------
if(CashDesksNo>maxCashDesksNo || CashDesksNo<1)
{
printf("\n# of Cash Desks should be between 1 and %d",maxCashDesksNo);
printf("\nsupermarket will now exit...\n");
exit(1);
}
printf("\n//-----------------------------------------------");
printf("\nSupermarket initialization");
printf("\n# of Cash Desks: %d",CashDesksNo);
printf("\n# of max products: %d",n);
printf("\nMax price: %d",m);
printf("\nMaximum serving time(secs): %d",TmaxServe);
printf("\n%% Customer/Cashier Percentage: %d/%d",custPerc,100-custPerc);
printf("\nMax Supermarket capacity: %d",maxCapacity);
printf("\n//-----------------------------------------------\n");
printf("\nAbout to create customer and cashier processes");
// ----------- Shared Memory Attachment --------------------------------
shmid=shmget(IPC_PRIVATE,sizeof(int*),0666);
if (shmid < 0)
{
perror("shmget");
exit(1);
}
shm_ptr=(int*)shmat(shmid,(void *)0,0);
if (shm_ptr == (int *)(-1))
{
perror("shmat");
exit(1);
}
a=0; //shm
shm_ptr=(int*)a;
printf("shmPtr:%d",(int)shm_ptr);
// ----- create & initialize semaphore ---------------------------------
sp = sem_open(semName,O_CREAT,0644,1);
if(sp == SEM_FAILED)
{
perror("unable to create semaphore");
exit(-1);
}
while(1)
{
printf("\nPress enter to start a new day at the supermarket(type exit to quit)\n");
fgets(termInput,sizeof(termInput),stdin);
nlptr = strchr(termInput, '\n');// termatismos string
if (nlptr) *nlptr = '\0';
if(strcmp(termInput,"exit")==0)//exit apo tin efarmogi
{
printf("\nExiting Supermarket..\n");
exit(0);
}
i=0;
while(i<maxCapacity)
{
//-Fork new process for the simulation --------------------
ch_pid = fork();
if (ch_pid == -1) {
perror("\nFailed to fork initial spliter/merger process \n");
exit(1);
}
if ( ch_pid == 0 ) //this is the child process
{
srand (getpid());//pid based seed
// "itoa" - Metatropi ari8mou se string
sprintf( tmpString, "%d", shmid );
randNum=rand() % 100 + 1;
if(randNum<custPerc)// customer : cashier ratio
{
execResult=execl("customer","customer","0",tmpString,NULL);
//printf("\nCreated a customer,randNum %d",randNum);
}else
{
execResult=execl("cashier","cashier","0",tmpString,NULL);
//printf("\nCreated a cashier,randNum %d",randNum);
}
if(execResult==-1)
{
perror("Could not perform exec to create cashier/customer process");
}
}
i++;
}
//Root process
//wait for childs to terminate
for (i = 0; i < maxCapacity; ++i)
{
waitpid(-1,&status,0);
}
printf("supermarket shared memory content: %d",(int)shm_ptr);
sem_close(sp);
sem_unlink(semName);
err = (int*)shmctl(shmid, IPC_RMID, 0);
if (err == -1)
perror ("Shared Memory Removal.");
else
printf("Shared Memory Removed. %d\n", (int)(err));
}
}
this is the makefile:
OBJS = supermarket.o cashier.o customer.o
SOURCE = supermarket.c cashier.c customer.c
HEADER = struct.h
OUT = supermarket cashier customer
CC = gcc
FLAGS = -lrt -g -c
LIBS = -lm
# -g option enables debugging mode
# -c flag generates object code for separate files
# -lm math library
all: supermarket cashier customer
supermarket: supermarket.c
$(CC) supermarket.c -o supermarket
cashier: cashier.c
$(CC) cashier.c -o cashier
customer: customer.c
$(CC) customer.c -o customer
# clean house
clean:
rm -f $(OBJS) $(OUT)
# do a bit of accounting
count:
wc $(SOURCE) $(HEADER)
I keep getting this error:
george#george-System-Product-Name:~/Desktop/prj3$ make
gcc supermarket.c -o supermarket
supermarket.c: In function ‘main’:
supermarket.c:310:11: warning: comparison between pointer and integer [enabled by default]
/tmp/ccYxA2Wi.o: In function `main':
supermarket.c:(.text+0x75c): undefined reference to `sem_open'
supermarket.c:(.text+0x9b0): undefined reference to `sem_close'
supermarket.c:(.text+0x9bf): undefined reference to `sem_unlink'
collect2: ld returned 1 exit status
make: *** [supermarket] Error 1
what can i do?
this is a project for operating systems class, my system is linux Ubuntu

Answer 1

I think you should link against pthread as well:
-lpthread
Example makefile from an old project of mine (see comments):
CC=gcc
CFLAGS=-c -Wall -O3 -g
LDFLAGS=-pthread
SOURCES=chatzor.c clientlist.c messagequeue.c
OBJECTS=$(SOURCES:.cpp=.o)
EXECUTABLE=chatzor_server
all: $(SOURCES) $(EXECUTABLE)
$(EXECUTABLE): $(OBJECTS)
$(CC) $(LDFLAGS) $(OBJECTS) -o $#
.cpp.o:
$(CC) $(CFLAGS) $< -o $#
clean:
rm -rf *.o ${EXECUTABLE}

Answer 2

You're going to have sore shins once you're done with this.
The compilation trace says:
$ make
gcc supermarket.c -o supermarket
supermarket.c: In function ‘main’:
...
Your makefile says:
supermarket: supermarket.c
$(CC) supermarket.c -o supermarket
It should say (at minimum):
supermarket: supermarket.c
$(CC) supermarket.c -o supermarket $(LIBS)
Indeed, it should probably be saying:
supermarket: supermarket.c
$(CC) $(CFLAGS) supermarket.c -o supermarket $(LDFLAGS) $(LIBS)