C: PThread_create Parsing Char[] parameter to function

Question

Hallo All,

I have this method:

void *readFileLocal(char filename[]){
       .....
}

Now i want to start this method a a thread:

char input[strlen(argv[1])];
strcpy(input,argv[1]);
pthread_t read,write;
pthread_create(&read, NULL, &readFileLocal, &input);

But during compilation it gives this warning:

file.c:29: warning: passing argument 3 of ‘pthread_create’ from incompatible pointer type

/usr/include/pthread.h:227: note: expected ‘void * (*)(void *)’ but argument is of type ‘void * (*)(char *)’

How can I parse an char array to my funcation over pthread_create without this warning ? Thanks for helpt

Pablo Santa Cruz · Accepted Answer · 2011-05-08 13:13:39Z

3

Just use this:

pthread_create(&read, NULL, readFileLocal, input);

And consider changing your function's signature to:

void *readFileLocal(void *fileName) { }

When you are passing a pointer to function (like the one you are using in readFileLocal parameter) you don't need to put &.

Also, when you have an array (like input in your case) you don't need & since in C arrays can be used as pointers already.

edited May 8, 2011 at 13:13

answered May 8, 2011 at 12:49

Pablo Santa Cruz

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3 Comments

Hasturkun Over a year ago

C arrays are not pointers, they just tend to decay into pointers at the slightest provocation.

Pablo Santa Cruz Over a year ago

@Hasturkun: Agree. I don't think it is relevant to the question though.

Hasturkun Over a year ago

It isn't really, except for furthering the strange myth claiming that arrays are pointers

Mel · Accepted Answer · 2011-05-08 12:56:47Z

2

Functions for threads need to be prototyped:

void *func(void *argv);

As with all void pointers you then need to interpret ("cast") the pointer to a meaningful entity. You readFileLocal functions then becomes:

void *readFileLocal(void *argv)
{
    char *fname = argv; // Cast to string
    // Rest of func
}

answered May 8, 2011 at 12:56

Mel

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1 Comment

pmg Over a year ago

+1 for "casting" without the cast operator. The operation is called "convert": there are implicit conversions (done by the compiler without the cast) and explicit conversions (done with the cast operator).

Azure · Accepted Answer · 2024-07-17 02:56:11Z

0

It just needs to look like this.

Don't forget to force conversion of types.

void *task(void *arg) {
    // printf("My value is %s", (char *) arg);
    return 0;
}


int main(){
    char* value = "StackOverflow";
    
    pthread_t t;
    pthread_create(&t, NULL, task, value);
}

answered Jul 17, 2024 at 2:56

Azure

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