获取结构中 pthread_mutex 初始化的缺少大括号警告

Question

I have a struct fifo that contains types pthread_mutex_t and pthread_cond_t . Is there something special about these types that I shouldn't initialize them with {0} ?

#include <stdio.h>
#include <pthread.h>

struct fifo {
        pthread_mutex_t head_mutex;
        pthread_cond_t cond_add;
};
typedef struct fifo fifo;

fifo* fifo_construct(fifo* self)
{
        *self = (fifo){
            {0}, /* head_mutex */
            {0}, /* cond_add */
        };

        pthread_mutex_init(&self->head_mutex, NULL);
        pthread_cond_init(&self->cond_add, NULL);

        return self;
}

int main(int argc, char** argv)
{
        fifo f;
        fifo_construct(&f);

        pthread_mutex_t mut = {0}; // No complaint here!

        fprintf(stderr, "addr: %p, %p\n", &mut, &f.head_mutex);
}

Using gcc -Wmissing-braces (10.2.0) produces the following warning:

fifo.c: In function ‘fifo_construct’:
fifo.c:12:10: warning: missing braces around initializer [-Wmissing-braces]
   12 |  *self = (fifo){
      |          ^
   13 |      {0}, /* head_mutex */
      |       {}
   14 |      {0}, /* cond_add */
      |       {}

But it doesn't complain about the initialization in main. Am I missing something or is the compiler just being picky?

Answer 1

I have a struct fifo that contains types pthread_mutex_t and pthread_cond_t . Is there something special about these types that I shouldn't initialize them with {0} ?

Not as far as the C language is concerned. {0} is a valid initializer for an object of any type. That does not imply, however, that the resulting mutex or CV value is a valid one as far at the pthreads level.

Using gcc -Wmissing-braces (10.2.0) produces the following warning: [...]

This is a style warning, which you could obtain from similar initializers for any number of similar structure types. It does not necessarily imply that there is anything is wrong with your code. GCC just thinks that it's a risky approach, notwithstanding that the special case of the {0} initializer form is utterly idiomatic.

Other options include

• Using the initializer macros that pthreads provides for these types ^* :

   *self = (fifo) { PTHREAD_MUTEX_INITIALIZER, PTHREAD_COND_INITIALIZER };

  In this case, you do not need to also call the initialization functions unless you want non-default properties.

• Omit any explicit initializers for those members.

• If, as in the example code, the mutex and CV are the only members, or if you do not care to initialize any others, then drop the whole assignment statement with its compound literal. You don't need to pre-initialize the mutex and CV when you are anyway going to initialize them via their initialization functions before using them.

• Turn off the warning with -Wno-missing-braces

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^* Older versions of the POSIX specifications only promise that these can be used for objects having static storage duration, but that caveat has been dropped from recent versions.