C pthread mutex: Expected expression before `{'

Question

I am using the pthread library to create two threads. I use two queues to transfer data between two streams (producer-consumer) and, therefore, I want to have a mutex for push-pop synchronization in the queue for flows.

The problem I ran into is getting a compilation error like this:

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

Function where I get the error:

 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 } 

Queue structure:

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

If I comment on line 920, the linker starts giving "multiple ad 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 could not find a solution in my search. Is there something fundamentally wrong with my code? Can someone help me determine what I am doing wrong?

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

Many thanks.

Answer 1

• You cannot use PTHREAD_MUTEX_INITIALIZER like this - it should be used as an initializer, and not in a regular assignment. You have two options to fix this: either call pthread_mutex_init() , or add a cast of type PTHREAD_MUTEX_INITIALIZER as a composite literal. Your choice:

   pthread_mutex_init(&q->mutex, NULL); 

  or

   q->mutex = (pthread_mutex_t)PTHREAD_MUTEX_INITIALIZER; 

• Your linker error is related to this command line:

   gcc simple-tun.c simple-tun -lpthread 

  You lack -o , so you are trying to link the program to yourself. This is bad news. You probably want:

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

  Indeed, you should add some warning flags there.

Answer 2

PTHREAD_MUTEX_INITIALIZER is exactly what the initializer for use in declarations:

 pthread_mutex_t foo = PTHREAD_MUTEX_INITIALIZER; 

if you have pthread_mutex_t that was created elsewhere, for example, malloc , initialize it with pthread_mutex_init :

 pthread_mutex_init(&q->mutex, NULL); 

from the SUSV2 documentation for pthread_mutex_init :

In cases where mutex attributes are appropriate by default, the PTHREAD_MUTEX_INITIALIZER macro can be used to initialize obsolete mutexes. The effect is equivalent to dynamic initialization by calling pthread_mutex_init() with the attr parameter specified as NULL , except that error checks are not performed.