How to create a variable size 2D string array by passing variable

Question

Could anyone give me an sample how to create a 2D char array in C by passing the variable for the array length.

//Current program
int i;
int seq_cnt;    
exec sql    
        select count(0)     
        into seq_cnt     
        from    table;    
char tmp1[50][5+1];       
char tmp2[50][5+1];   

for(i=0;i < seq_cnt ; i++){   
     strcpy(tmp1[i],"something");    
     strcpy(tmp2[i],"something");    
}

Now what I want is for the array size of tmp1 and tmp2, I want to use the seq_cnt to declare the actual size of tmp1 and tmp2 instead of hardcode it (50).

like:

char tmp1[seq_cnt][5+1];     
char tmp2[seq_cnt][5+1];

I'm new to C.

char tmp1[seq_cnt][5+1]; char tmp2[seq_cnt][5+1]; should work if you are using C99+. Or you can resort to dynamically allocating memory using malloc. Note: "something" won't fit into an array of size 5 + 1 — Spikatrix
– Spikatrix, Commented Jan 18, 2019 at 9:59
@Spikatrix: Depends on the compiler. Some VC version do not support VLAs. Also C11 made VLAs optional. C++ does not know them at all. — alk
– alk, Commented Jan 18, 2019 at 10:01

alk · Accepted Answer · 2019-01-21 15:22:09Z

1

I want to use the seq_cnt to declare the actual size of tmp1

Do

char (*tmp1)[5+1] = malloc(seq_cnt * sizeof *tmp1);

Update on the three different usages of an asterisk * in C.

Types/Variable definitions

Here
```
char (*tmp1)[5+1]
```
the asterisk is used to define a pointer, a pointer to a char[5+1] array.

^{Please not that the parentheses are mandatory, as char *tmp[5+1] would defined an array of 6 pointer to char.}
Indirection (or de-referencing) operator

Here
```
sizeof *tmp1
```
the asterisk is used to tell the compiler to not take the size of tmp1 which would be the size of a pointer, but the size of what tmp1 points to, namely a char[5+1].

^{Alternatively one could write sizeof (char[5+1]). Please note that the parenthesis are not belonging to sizeof as it is not a function, but an operator.}
Multiplication operator

Here
```
seq_cnt * sizeof ...
```
the asterisk is used to indicate an ordinary multiplication, namely to calculate the product of seq_cnt and the size of something.

So all in all the top statement allocates seg_cnt times the bytes a char[5+1] needs and assigns the address of the 1st byte of the chunk allocated to tmp1, makes it point to enough memory to hold seq_cnt array of char[5+1].

edited Jan 21, 2019 at 15:22

answered Jan 18, 2019 at 10:03

alk

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1 Comment

thomas Over a year ago

Thank you so much. This is what I want exactly . Could you mind explaining it more ? I am not familiar with malloc . and there is so many * that making me confused.

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