How thread acquires mutex lock ?

Question

Here my doubt is about acquire mutex lock.

Here pthread_mutex_t lock ; is also a global variable shared to threads. Accessing of this global variable (lock) will it be shame as accessing of other global variables ? If same, then don't we face same problem what we will face for other global variables ? if not ,how this is discriminated from other global variables ?

Answer 1

In the first go I saw the issue as real issue then I checked the structure of pthread_mutex_t which is as follows -

typedef struct {
    int __m_reserved;
    int __m_count;
    _pthread_descr __m_owner;
    int __m_kind;
    struct _pthread_fastlock __m_lock;
} pthread_mutex_t;

Now see the third field only owner is suppose to modify this variable to avoid the contention (this is my interpretation only not verified)

Comments

thanks salil. Ok still my doubt is say  pthread_mutex_t lock is global variable it will be stored in data segment .Just to change third field first we need to access lock variable right ? it seems basic but getting confused and cuerocity to know.thanks once again.

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My interpretation is - this is a lock variable and you can change it only if you are __m_owner, if noone is the owner you can claim it and become the owner and then can change the value. So there would be no contention.

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ok thank you .