How to pass array index into another function in c

Question

I am trying to figure out how to pass an array index into another function.

Here I have:

 for(i=0; i<file_cnt; i++){

          iret1 = pthread_create(&(file[i]), NULL, get_checksum, (void*)&filenames[i]);
          printf("%s\n", filenames[i]);
        }

In the function get_checksum, I eventually need the index of filenames, but i is not available in that function.

In the get_checksum function,

void* get_checksum(void* a){

            char *filename = (char *) a;
.....
}

I passed in a as a char pointer in relation to filenames[i].

However, I need to make computations later in the function: get_checksumthat involve getting the index of an array of sums. Therefore, I really need the index of filenames to be passed into the function get_checksum instead.

Any suggestions how to pass an array index into another function?

Create a structure, populate the structure with all the relevant parameters, and pass that in to pthread_create - amparo.net/ce155/thread-ex.html — OldProgrammer
– OldProgrammer, Commented Mar 26, 2016 at 20:26

Joshua · Accepted Answer · 2016-03-26 20:52:12Z

2

Normally you'd just pass 2 parameters but you can't here because pthread_create so you pass a pointer to a heap-allocated struct.

struct params {
    char *filename;
    int i;
};

/* ... */

for(i=0; i<file_cnt; i++){
      struct params *p = malloc(sizeof(struct params));
      p->filename = filenames[i];
      p->i = i;
      iret1 = pthread_create(&(file[i]), NULL, get_checksum, (void*)p);
      if (iret1) {
          free(p);
          printf("OOPS %s\n", filenames[i]);
      } else {
          printf("%s\n", filenames[i]);
      }
    }

/* ... */

void* get_checksum(void* p){
    char *filename = ((struct params *)p)->filename;
    int i = ((struct params *)p)->i;
    free(p);
    /* ... */
}

edited Mar 26, 2016 at 20:52

answered Mar 26, 2016 at 20:26

Joshua

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3 Comments

SamSmith Over a year ago

Assignment from incompatible pointer type at p->filename = &filenames[i]

Joshua Over a year ago

Possible filenames is const char filenames[] ? If so than params.filename needs to be const char *.

SamSmith Over a year ago

char **filenames so would that syntax still apply?

Alex Lop. · Accepted Answer · 2016-03-26 21:03:18Z

2

You can define a struct to pass all the parameters you need:

typedef struct params_s
{
    char * filename;
    int index;
    // additional params
} params_t;

Then either create static or allocate dinamically a variable of type params_t, initialize it and pass its address to the get_checksum:

EXAMPLE

params_t params[NUM_OF_THREADS];
...
for(i=0; i<file_cnt; i++){
    params[i].filename = &filenames[i];
    params[i].index = i;
    iret1 = pthread_create(&(file[i]), NULL, get_checksum, (void*)&params[i]);
    printf("%s\n", filenames[i]);
}

edited Mar 26, 2016 at 21:03

answered Mar 26, 2016 at 20:28

Alex Lop.

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2 Comments

Joshua Over a year ago

So I suppose you have to decide whether heap or stack allocation works for you. If the threads live past the scope of the function, bad things happen, but they might not in this case. Please note that NUMBER_OF_THREADS is not a constant so global allocation won't work.

Alex Lop. Over a year ago

@Joshua. You are right. But what I wrote is just an example. In my answer I wrote that the OP should either use it statically or allocate it dynamically. It's OPs choice according to the design.

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