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Question : C code linking
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hi people
I was trying to compile a code using some
user defined shared libraries.
the name of the library he has kept is as
---------------------------------------------------
g++ -lsemphore -L./fifoimplementation semcreate.c
->undefined reference to sem_open(const char *, int. ...)
->undefined referenc to sem_close(sem_t *)
---------------------------------------------------
directory : ./fifoimplementation
***** libsemaphore.so.1,
libsemaphore.so -> libsemaphore.so.1
---------------------------------------------------
I used nm command on libsemaphore.so.1
(which is placed in ./fifoimplementation directory)
and it says "sem_close" and "sem_open" declared
with a T (normal) status.
I used gcc -c sem_open.c [to produce the sem_open.o file]
gcc -c sem_close.c [to produce the sem_close.o file]
gcc -shared -o libsemaphore.so.1 *.o [to produce
the .so file]
below are the .c files .. u can cut these and try to
compile the library and use it
---------------------------------------------------
********************
/*semcreate.c - uses sem_open and sem_close :*/
#include
#include
#include
#include
#include
#include
#include "./fifoimplementation/semaphore.h"
int
main(int argc, char **argv)
{
int c, flags;
sem_t *sem;
unsigned int value;
flags = O_RDWR | O_CREAT;
value = 1;
while ( (c = getopt(argc, argv, "ei:")) != -1) {
switch (c) {
case 'e':
flags |= O_EXCL;
break;
case 'i':
value = atoi(optarg);
break;
}
}
if (optind != argc - 1)
{printf("usage: semcreate [ -e ] [ -i initalvalue ] ");
exit(0);}
sem = sem_open(argv[optind], flags, S_IRWXU | S_IROTH | S_IRGRP, value);
sem_close(sem);
exit(0);
}
******************************
------------------------------------
when I try to compile it with
g++ -lsemaphore -L./fifoimplementation semcreate.c
is says undefined reference to sem_open and sem_close.
but they are there in the library and also in the header
file included.
-----------------------------------------
****************************
/*semaphore.h*/
typedef struct {
int sem_fd[2]; /*2 fd's for reading and writing from the FIFO*/
int sem_magic; /*magic no*/
} sem_t;
#define SEM_MAGIC 0x92843
#ifdef SEM_FAILED
#undef SEM_FAILED
#define SEM_FAILED ((sem_t *) (-1)) /*this aviods compiler warnings ... I dont know*/
#endif
sem_t * sem_open(const char *, int, ...);
int sem_close(sem_t *);
int sem_wait(sem_t *);
int sem_post(sem_t *);
int sem_unlink(const char *);
***********************
*********************
/*sem_open.c
opening a semaphore in FIFO implementation
sem_open(const char *pathname, int oflags, ...)
if the O_CREAT flag is there in oflags we need
the rest of the two arguments namely int mode, int value*/
#include
#include
#include
#include
#include
#include
#include
#include
sem_t *sem_open(const char *pathname, int oflags, ...)
{
int save_errno;
va_list ap;
mode_t mode; /*mode of the file : O_RDWR, O_RDONLY etc*/
unsigned int value;
sem_t *sem;
int flags;
char c;
int i;
if (oflags & O_CREAT ==0) /*we are creating the semaphore */
{
va_start(ap, oflags);
mode = va_arg(ap, mode_t);
value = va_arg(ap, unsigned int);
va_end(ap);
if (mkfifo(pathname, mode)<0) /*the FIFO is created*/
if ( (errno ==EEXIST) && (oflags & O_EXCL)==0) /*file already exists and we have not set the O_EXEL flag*/
oflags = ~O_CREAT; /* we dont want to write the value of the semaphore */
/*in the fifo as it is already there*/
else
return ((sem_t *) -1);
}
/*allocate memory to the sem_t structure*/
sem = (sem_t *) malloc(sizeof(sem_t));
/*fifo created*/
( sem->sem_fd[0] = open(pathname, O_RDONLY | O_NONBLOCK) <0) /*an opening of the FIFO will block if */
goto error; /*i.e unlink the fifo created the other end hasen't opened it */
/*we specify O_NONBLOCK to avoid the deadlock */
/*and then turn off the O_NONBLOCK option */
/*to wait for the semaphore values see fifo(4)*/
if (sem->sem_fd[1] = open(pathname, O_WRONLY | O_NONBLOCK) <0)
goto error;
flags = fcntl(sem->sem_fd[0], F_GETFL); /*get the current flags fot this file*/
flags &= ~O_NONBLOCK; /*reset O_NONBLOCK*/
fcntl(sem->sem_fd[0], F_SETFL, flags); /*reset the flags*/
if (oflags & O_CREAT) /*first time ... initialize the semaphore*/
{
for (i=0;i if (write(sem->sem_fd[1], &c, 1)<0)
goto error;
}
sem->sem_magic = SEM_MAGIC;
return (sem);
error:
save_errno = errno; /*save errno lest it should get modified */
if (oflags & O_CREAT ) unlink(pathname);
close(sem->sem_fd[0]); /*in this call*/
close(sem->sem_fd[1]);
free((void *) sem);
errno = save_errno; /*restore it*/
return ((sem_t *) (-1));
}
********************
***********************
/*sem_close.c*/
#include
#include
#include
#include
int sem_close(sem_t *sem)
{
if (sem->sem_magic != SEM_MAGIC)
{
errno = EINVAL;
return (-1);
}
sem->sem_magic = 0; /*reset the magic number ... semaphore is now closed*/
if ( close(sem->sem_fd[0] ==-1 ) || close(sem->sem_fd[1] == -1) )
{
free(sem);
return (-1);
}
free(sem);
return (0);
}
*********************
-----------------------------------------------------------
if anybody can tell me the commands of how to
make libraries and also how to use the built in libraries,
I will be highly obliged !!!!!!!
Thankzzz
Gaurav.
-------------------------------------------------------------
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Answer : C code linking
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semcreate.o links with the routines in libpthread.so just fine for me.
To compile semcreate.c for the libpthread versions, I would need the interface header files for sem_open() and sem_close() and I either don't have them or don't know what they are. In fact, I can't find any evidence that those are real interfaces. The process synchronization calls in Posix threads are the "mutex" routines. Are you sure these aren't internal library subroutines?
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