c: issues when allocating 2d char array dynamically?

Question

I'm trying to allocate a 2D char array to be accessed like ary[i][j], using this code:

#define stringmaxlen 20

void do_alloc( char ***vals, int valscount ){
    *vals = (char**) calloc( sizeof( char** ), valscount );
    int i = 0;
    for ( ; i<valscount; i++ )
        *vals[i] = (char*) calloc( sizeof( char* ), stringmaxlen );
}

int main( ){
    //......
    char** ary;
    do_alloc( &ary, 10 );
    strcpy( ary[0], "test" );
    //......
}

Unfortunately, this causes an overflow somewhere and the program have a bug in execution, I got some references from here for the dynamic allocation: http://staff.science.nus.edu.sg/~phywjs/CZ1102/lecture20/sld014.htm.

I like to know what's wrong here and how to solve the issue, thanks.

The parameters you use for calloc are in reversed order as you can see from here and there; although I don't really know whether it would cause an erroneous behaviour. — Utkan Gezer
– Utkan Gezer, Commented Mar 24, 2014 at 21:16
@ThoAppelsin The order of parameters to calloc is irrelevant on any platform that I know of. — cmaster - reinstate monica
– cmaster - reinstate monica, Commented Mar 24, 2014 at 21:17

cmaster - reinstate monica · Accepted Answer · 2014-03-24 21:22:15Z

7

You got the operator precedence wrong: *vals[i] evaluates as *(vals[i]), not as (*vals)[i]. See http://en.wikipedia.org/wiki/Operators_in_C_and_C%2B%2B#Operator_precedence for details.

The fix is to change *vals[i] to (*vals)[i].

Also, the allocation *vals[i] = (char*) calloc( sizeof( char* ), stringmaxlen ); is wrong. It allocates way too much memory because it allocates space for stringmaxlen pointers, but you need only stringmaxlen characters.

edited Mar 24, 2014 at 21:22

answered Mar 24, 2014 at 21:16

cmaster - reinstate monica

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1 Comment

user3457200 Over a year ago

yeah! that's it, now the issue is fixed, the precision is important! thank you man!

R Sahu · Accepted Answer · 2014-03-24 23:14:35Z

3

I want to add the following to cmaster's answer.

Instead of

*vals = (char**) calloc( sizeof( char** ), valscount );

use

*vals = (char**) calloc( sizeof( char* ), valscount );

Instead of

    (*vals)[i] = (char*) calloc( sizeof(char*), stringmaxlen );

use

    (*vals)[i] = (char*) calloc( sizeof(char), stringmaxlen );

In the first case, the size of memory allocated doesn't change since sizeof(char**) is the same as sizeof(char*). However that's not true with the second case. sizeof(char) is 1 while sizeof(char*) is larger -- 4 for 32 bit hardware, 8 for 64 bit hardware.

More importantly, it clarifies the intention -- that you would like to allocate memory for stringmaxlen characters, not stringmaxlen pointers to characters.

edited Mar 24, 2014 at 23:14

answered Mar 24, 2014 at 21:36

R Sahu

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1 Comment

M.M Over a year ago

You could have avoided these errors by using the idiom: P = calloc(N, sizeof *P);, e.g. *vals = calloc( valscount, sizeof **vals ); etc.

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