Thread: Help with error

I'm trying to run this sample code and I keep getting this error:

"process1.cpp:92: implicit declaration of function `int wait(...)' "

I thought the error was that I didn't include a specific library but I think I have every thing I need. Help please.

Code:

#include <iostream.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <unistd.h>
#include <stdlib.h>

#define SHMSZ    1024

using namespace std;

int write(int *b)
{
    char c;
    int shmid;
    key_t key;
    int *shm;
    char *s;

    /*
    * We'll name our shared memory segment
    * "5678".
    */
    key = 5678;

    /*
    * Create the segment.
    */
    if ((shmid = shmget(key, SHMSZ, IPC_CREAT | 0666)) < 0) {
        cerr<<"shmget";
        exit(1);
    }

    /*
    * Now we attach the segment to our data space.
    */
    if ((shm = (int *)/*(char *)*/shmat(shmid, 0, 0)) == /*(char *)*/(int *) -1) {
        cerr<<"shmat";
        exit(1);
    }

    /*
    * Now put some things into the memory for the
    * other process to read.
    */
    // s = shm;
    int i;
    for (i=0; i<=1000; i++,shm++)
        *shm = i;

    *b=0;
    cout<<"write is complited"<<endl;
    cout<<*b<<endl;
    return(*b);
}
main()
{

    char c;
    int shmid;
    key_t key;
    static int *b;

    key = 5679;

    /*
    * Create the one bit  segment.
    */
    if ((shmid = shmget(key, 1, IPC_CREAT | 0666)) < 0) {
        cerr<<"shmget";
        exit(1);
    }

    /*
    * Now we attach the segment to our data space.
    */
    if ((b = ( int *)shmat(shmid, 0, 0)) == (int *) -1) 
    {
        cerr<<"shmat";
        exit(1);

    }
    cout<<*b<<endl;
    while(1)
    {
        if(*b==1)
        {
            write(b);
        }
        else
            wait(5);
    }

Where is wait() declared? There is one in sys/wait.h, but that one takes a pointer, not an integer.

Also, main() should be declared as returning an int.

Ok, I declared wait and now it gives me this error. No idea what this means.

/tmp/ccjgTrxM.o: In function `main':
/tmp/ccjgTrxM.o(.text+0x212): undefined reference to `wait(void)'
collect2: ld returned 1 exit status

Well, I've tried both "int wait()" and "void wait()" and I get the same outcome.

Code:

#include <iostream.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <unistd.h>
#include <stdlib.h>

#define SHMSZ    1024

using namespace std;

int wait();

int write(int *b)
{
    char c;
    int shmid;
    key_t key;
    int *shm;
    char *s;

    /*
    * We'll name our shared memory segment
    * "5678".
    */
    key = 5678;

    /*
    * Create the segment.
    */
    if ((shmid = shmget(key, SHMSZ, IPC_CREAT | 0666)) < 0) {
        cerr<<"shmget";
        exit(1);
    }

    /*
    * Now we attach the segment to our data space.
    */
    if ((shm = (int *)/*(char *)*/shmat(shmid, 0, 0)) == /*(char *)*/(int *) -1) {
        cerr<<"shmat";
        exit(1);
    }

    /*
    * Now put some things into the memory for the
    * other process to read.
    */
    // s = shm;
    int i;
    for (i=0; i<=1000; i++,shm++)
        *shm = i;

    *b=0;
    cout<<"write is complited"<<endl;
    cout<<*b<<endl;
    return(*b);
}
main()
{

    char c;
    int shmid;
    key_t key;
    static int *b;

    key = 5679;

    /*
    * Create the one bit  segment.
    */
    if ((shmid = shmget(key, 1, IPC_CREAT | 0666)) < 0) {
        cerr<<"shmget";
        exit(1);
    }

    /*
    * Now we attach the segment to our data space.
    */
    if ((b = ( int *)shmat(shmid, 0, 0)) == (int *) -1) 
    {
        cerr<<"shmat";
        exit(1);

    }
    cout<<*b<<endl;
    while(1)
    {
        if(*b==1)
        {
            write(b);
        }
        else
            wait();
    }
}

I think you are misunderstanding how functions work. To call a function (wait() in this case), it must be both declared, and implemented. Just writing:

Code:

int wait();

declares the function wait(), but you still cannot call it without an implementation like so:

Code:

int wait() { return something; }

Now sometimes the function you want to call is implemented in an external library. In this case you need to #include that library's header file, and then link to that library. If the library is part of the platform's C library, then it will be linked to automatically. For instance, this wait() function is implemented in the GNU C library, so you just need to include sys/wait.h in order to use it.

Well, how exactly will it be implemented? I'm still sort of a novice at this stuff. Like I said, this is just a sample code that I want to run to understand shared memory. I included the wait header but it still didn't help.

Originally Posted by matsp

And where do you expect the compiler to find the function wait()? There is no wait function DEFINED, you have only declared it. That tells the compiler "you can expect to find this function somewhere when you link, I'll define it later". But the linker says "Uhoh! No wait function defined", because you haven't actually defined the function.

What is wait supposed to do?

--
Mats

Its supposed to wait on the other process to finish writing. The author says "One wants to write the data and another wants to read the data to the shared memory."

So I'm guessing it reads while it wait for the other process to finish writing.