C pthread mutex: Expected expression before `{'

Question

I am using the pthread library to create two threads. I am using two queues to communicate the data between the two threads (producer-consumer) and hence want to have a mutex to sync the push-pops in the queue by the threads.

But I get a compile error as follows:

$ gcc simple-tun.c simple-tun -lpthread
simple-tun.c: In function ‘new_queue’:
simple-tun.c:920:13: error: expected expression before ‘{’ token

The the function where I get the error is:

908 struct queue * new_queue () {
909 
910     struct queue * q;
911     q = (struct queue *) malloc (sizeof(struct queue));
912 
913     if (q == NULL)
914         return NULL;
915 
916 
917     q->head = NULL;
918     q->tail = NULL;
919     q->is_empty = 1;
920     q->mutex = PTHREAD_MUTEX_INITIALIZER;
921 
922     return q;
923 }

structure queue is:

    struct queue {
 80     struct node * head;
 81     struct node * tail;
 82     int is_empty;
 83     pthread_mutex_t mutex;
 84 };

If I comment out line 920, the linker starts giving 'multiple declaration errors'

$ gcc simple-tun.c simple-tun -lpthread
simple-tun: In function `settun':
(.text+0x2b7): multiple definition of `settun'
/tmp/cc5Ms4xP.o:simple-tun.c:(.text+0x1cb): first defined here
simple-tun: In function `_fini':
(.fini+0x0): multiple definition of `_fini'
/usr/lib/gcc/x86_64-linux-gnu/4.7/../../../x86_64-linux-gnu/crti.o:(.fini+0x0): first defined here
simple-tun: In function `mktun':
(.text+0x1e2): multiple definition of `mktun'
/tmp/cc5Ms4xP.o:simple-tun.c:(.text+0xf6): first defined here
simple-tun: In function `net_connect':
(.text+0xe27): multiple definition of `net_connect'
/tmp/cc5Ms4xP.o:simple-tun.c:(.text+0x1115): first defined here
simple-tun: In function `data_start':
(.data+0x0): multiple definition of `__data_start'
/usr/lib/gcc/x86_64-linux-gnu/4.7/../../../x86_64-linux-gnu/crt1.o:(.data+0x0): first defined here
simple-tun: In function `client_connect':
(.text+0xe95): multiple definition of `client_connect'
/tmp/cc5Ms4xP.o:simple-tun.c:(.text+0x1183): first defined here
simple-tun: In function `data_start':
(.data+0x8): multiple definition of `__dso_handle'
/usr/lib/gcc/x86_64-linux-gnu/4.7/crtbegin.o:(.data+0x0): first defined here
simple-tun:(.rodata+0x0): multiple definition of `_IO_stdin_used'
/usr/lib/gcc/x86_64-linux-gnu/4.7/../../../x86_64-linux-gnu/crt1.o:(.rodata.cst4+0x0): first defined here
simple-tun: In function `server_connect':
(.text+0xfa2): multiple definition of `server_connect'
/tmp/cc5Ms4xP.o:simple-tun.c:(.text+0x1290): first defined here
simple-tun: In function `print_usage':
(.text+0xe05): multiple definition of `print_usage'
/tmp/cc5Ms4xP.o:simple-tun.c:(.text+0x10f3): first defined here
simple-tun: In function `_init':
(.init+0x0): multiple definition of `_init'
/usr/lib/gcc/x86_64-linux-gnu/4.7/../../../x86_64-linux-gnu/crti.o:(.init+0x0): first defined here
/usr/lib/gcc/x86_64-linux-gnu/4.7/crtend.o:(.tm_clone_table+0x0): multiple definition of `__TMC_END__'
simple-tun:(.data+0x10): first defined here
/usr/bin/ld: error in simple-tun(.eh_frame); no .eh_frame_hdr table will be created.
collect2: error: ld returned 1 exit status

I was not able to find a solution in my searches. Is there something fundamentally wrong with my code? Can someone help me spot what I am doing wrong?

Please let me know if I need to post more snippets or more outputs.

Answer 1

1. You can't use PTHREAD_MUTEX_INITIALIZER like that - it has to be used as an initializer, not in a regular assignment expression. You have two choices to fix it - either call pthread_mutex_init(), or add a typecast to use PTHREAD_MUTEX_INITIALIZER as a compound literal. Your choice of:

   pthread_mutex_init(&q->mutex, NULL);
   

   or:

   q->mutex = (pthread_mutex_t)PTHREAD_MUTEX_INITIALIZER;
   

2. Your linker error problem is due to this command line:

   gcc simple-tun.c simple-tun -lpthread
   

   You're missing a -o, so you're trying to link the program with itself. That's bad news. What you probably want is:

   gcc simple-tun.c -o simple-tun -lpthread
   

   And really, you should add some warning flags in there, too.

Answer 2

PTHREAD_MUTEX_INITIALIZER is exactly that, an initializer to use in declarations:

pthread_mutex_t foo = PTHREAD_MUTEX_INITIALIZER;

if you have a pthread_mutex_t that was created elsewhere - such as by malloc - initialize it with pthread_mutex_init:

pthread_mutex_init(&q->mutex, NULL);

from the SUSV2 documentation for pthread_mutex_init:

In cases where default mutex attributes are appropriate, the macro PTHREAD_MUTEX_INITIALIZER can be used to initialise mutexes that are statically allocated. The effect is equivalent to dynamic initialisation by a call to pthread_mutex_init() with parameter attr specified as NULL, except that no error checks are performed.