Strange behavior by setting NULL in array

Question

I've a strange behavior while i'm trying to set a NULL value at the end of array of string (array of char) in c. I need to pass parameters to a process with execv, and i've all parameters with int type, so i'm trying to get an array of strings with all parameters casted in char array. I made a function that returns this array of strings, but when I set the last value of array to NULL (to have a terminating value in parameters) I lost the value stored in second position. If I remove the NULL value, no issue happens. This is the code, hope someone can help me

char **getParams(int offset, short isNode, int first) {

    printf("Get params\n");

    int args = 8;
    int totalSize = 0;
    int tmp[args];

    tmp[0] = sizeof(char) * 5; // user / node
    tmp[1] = snprintf(NULL, 0, "%d", x);
    tmp[2] = snprintf(NULL, 0, "%d", x2);
    tmp[3] = snprintf(NULL, 0, "%d", x3);
    tmp[4] = snprintf(NULL, 0, "%d", x4);
    tmp[5] = snprintf(NULL, 0, "%d", x5);
    tmp[6] = snprintf(NULL, 0, "%d", x6);
    tmp[7] = snprintf(NULL, 0, "%d", x7);

    int i = 0;
    for (i = 0; i < args; i++) totalSize += tmp[i];

    char **_pars = malloc(totalSize + sizeof(NULL));
    
    i = 0;
    for (i = 0; i < args; i++) _pars[i] = malloc(tmp[i]);
    
    _pars[0] = isNode == 1 ? "aaaa" : "bbbb" ;
    sprintf(_pars[1], "%d", x);
    sprintf(_pars[2], "%d", x2);
    sprintf(_pars[3], "%d", x3);
    sprintf(_pars[4], "%d", x4);
    sprintf(_pars[5], "%d", x5);
    sprintf(_pars[6], "%d", x6);
    sprintf(_pars[7], "%d", x7);
    printf("Value: %s", _pars[1]); /// Print without problem value
    _pars[8] = NULL;   //// ,--- Here happens the issue
    printf("Value: %s", _pars[1]); /// Doesnt print any value

    return _pars;
}

I also tried to put this after the for in which i malloc array position, but without result

_pars[8] = malloc(sizeof(NULL));

Some programmer dude · Accepted Answer · 2022-01-16 16:03:00Z

3

All your allocations are wrong.

First of all you want to create an array of 8 elements, so the first allocation should be

char **_pars = malloc(8 * sizeof *_pars);

Or if you want the array to be terminated by a null-pointer you need to allocate 9 elements (exchange the 8 for a 9 above).

Secondly the size returned by snprintf does not include the null-terminator, so all your calls

_pars[i] = malloc(tmp[i]);

should be

_pars[i] = malloc(tmp[i] + 1);

to fit the null-terminator.

On another note, all your code could be simplified quite a lot if you put all the x values in an array:

int exes[8] = { x, x2, x3, x4, x5, x6, x7 };

Because then you could use loops for all repetitive code:

for (unsigned i = 1; i < 8; ++i)
{
    tmp[i] = snprintf(NULL, 0, "%d", exes[i]);
}

edited Jan 16, 2022 at 16:03

answered Jan 16, 2022 at 15:57

Some programmer dude

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3 Comments

Emme Developer Over a year ago

Hi, many thanks for your answer, is really helpful. But I don't understand what you mean when you say char **_pars = malloc(8 * sizeof *_pars);, if i change the totalSize by assigning it correctly (by using + 1 to fit null terminator) is not the same thing? I don't understand how i can use sizeof (*_pars) if i've not defined a size for a single parameter

Some programmer dude Over a year ago

@EmmeDeveloper The size in totalSize is the size of the strings themselves (without the eight null-terminators), not the number of elements for the array _pars. Think of _pars as an array of pointers (equivalent to char *_pars[8];) and it would make more sense.

Emme Developer Over a year ago

Oh ok got it thanks, i tried to allocate totalSize to use less memory, but i forget that each element of array must have the same size, so i cannot use totalSize but i need size of one element * number of element in the array. Many thanks

Olaf Dietsche · Accepted Answer · 2022-01-16 15:57:34Z

0

_pars is assumed to be an array of pointers to char, which implies

_pars = malloc(args * sizeof(char*));

But you allocate the memory for the strings, the pointers should point to.

answered Jan 16, 2022 at 15:57

Olaf Dietsche

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