Question

I’m having problems writing a structure to shared memory because when I do, the code only writes the first part of the structure.

Here’s my data structure:

struct shmData{
    char number[4];
    char description[1020];
};

Here’s my code:

//Structure Pointer - placed on SHM
struct shmstruct *ptr;
char description_to_write[1024] = {0};
struct shmData *data_ptr;
data_ptr = (struct shmData*) description_to_write;

//Shared Memory init
create_shm(&ptr);
printf("Started\n");

//Define Offsets
//init_shm(ptr);

//#########################

//Check if number exist
    if (!(strcmp("NO SUCH CODE\0", get_description(ptr, 6)))) {
        //-> If not, get next offset (nput) where to write new data
//      sem_wait(&ptr->mutex);
//      long offset = ptr->msgoff[ptr->nput];
//      printf("OFFSET: %li\n", offset);
//      if (++(ptr->nput) > NMESG)
//          ptr->nput = 0; /* circular buffer */
//      sem_post(&ptr->mutex);
//      printf("Buffer: %s", argv[2]);

    snprintf(data_ptr->number, 4, "%s", "6");
    snprintf(data_ptr->description, 1020, "%s\n", argv[2]);

    printf("DATA: %s\n", data_ptr->number);
    printf("DATA: %s\n", data_ptr->description);

    printf("description_to_write: %i\n", description_to_write);

    //strcpy(&ptr->msgdata[offset], );
    //sem_post(&ptr->nstored);
    //printf("Wrote...\n\n");
} else {
    //-> Else get offset of this number and put data to it
}

How should I write this? In the code above, I tried simply to display the description array but I only get the number back.

I need it to be written to shared memory as follows:

Number(space)(space)(space)Description

Shared Memory Dump looks like:

00003b0: 3120 2020 496e 7075 7420 4572 726f 720a  1   Input Error.
00003c0: 0000 0000 0000 0000 0000 0000 0000 0000  ................
*
00007b0: 3220 2020 4f75 7470 7574 2045 7272 6f72  2   Output Error
00007c0: 0a00 0000 0000 0000 0000 0000 0000 0000  ................
00007d0: 0000 0000 0000 0000 0000 0000 0000 0000  ................

From debugger:

description_to_write[0] char    54 '6'  
description_to_write[1] char    0 '\000'    
description_to_write[2] char    0 '\000'    
description_to_write[3] char    0 '\000'
description_to_write[4] char    83 'S'  
description_to_write[5] char    111 'o' 
description_to_write[6] char    109 'm' 
description_to_write[7] char    101 'e' 
description_to_write[8] char    84 'T'  
description_to_write[9] char    101 'e' 
description_to_write[10]char    120 'x' 
description_to_write[11]char    116 't' 
description_to_write[12]char    10 '\n' 

Ugly solution:

int i = 0;
        while(i < 4){
            if(data_ptr->number[i] == '\000'){
                data_ptr->number[i] = 32;
            }
            i++;
        }

Answer 1

The 4 passsed to snprintf specifies the MAXIMUM number of chars to write. Not the amount to write. snprtinf retursn the number of chars it did write. So you are only printing 1 char, which leaves 3 nulls after it.

Just do:

snprintf(data->number,4,"%s    ","6");

That way it will try and print the string followed by 4 spaces, and print a max of 4 charcacters.