CODEWITHSUNDEEP
c
malajedala
malajedala

Reputation: 907

memcpy on two-dimensional array

I am wondering if I am using the memcpy function right.

So I have a two-dim. array of strings, and when I try to fill them valgrind tells me 

==825== Invalid write of size 8
==825==    at 0x344B8E: _platform_memmove$VARIANT$Unknown (in            /usr/lib/system/libsystem_platform.dylib)
==825==    by 0x1C4D74: __memcpy_chk (in /usr/lib/system/libsystem_c.dylib)
==825==    by 0x100001328: generate_test_data (check_generate_test_data.c:4120)
==825==    by 0x100000CA6: main (check_generate_test_data.c:137)
==825==  Address 0x100030a00 is 0 bytes after a block of size 32 alloc'd
==825==    at 0x47F1: malloc (vg_replace_malloc.c:302)
==825==    by 0x100000FBB: generate_test_data (check_generate_test_data.c:4095)
==825==    by 0x100000CA6: main (check_generate_test_data.c:137)


 int i;
 char **test;
 int total = 4;
 int elements = 11;
 test = malloc(sizeof(char**)* total);
 for (i=0; i < total; i++) {
    // char *to_fill --//is filled with a method
    //  
    test[i] = malloc(sizeof(char*) * elements;    // <== here is where compiler complains
    memcpy(&test[i], &to_fill, strlen(to_fill);   // <== here is where valgrind complains
 }

When I change it to: &test[i][0]

then the strings in test stay empty :S.. I also tried to remove the & of to_fill but with this my program just crashes.. I have no Idea what I am doing wrong here.

I changed all of the suggestions but the same error of valgrind appears, I even tried to put a constant char array into to_fill but still:

 int i;
 char **test;
 char *to_fill;
 int total = 4;
 int elements = 11;
 test = malloc(sizeof(char*)* total);
 to_fill = malloc(sizeof(char)* 100);
 to_fill[0] = '\0';
 for (i=0; i < total; i++) {
   //to_fill = method_to_fill_it(); 
   strncpy(to_fill, "example", 7);
   to_fill[7] = '\0';
   test[i] = malloc(sizeof(char*) * elements;
   memcpy(&test[i], &to_fill, strlen(to_fill);   // <== here is where valgrind complains
   to_fill[0] = '\0';
 }

Upvotes: 0

Views: 1550

Answers (1)

Igor Pejic
Igor Pejic

Reputation: 3698

Your memcpy statement is wrong, you don't need to dereference a char pointer which holds the address as it's value. Do this instead:

 memcpy(test[i], to_fill, strlen(to_fill));

When you do:

 memcpy(test[i], &to_fill, strlen(to_fill));

the memory address where to_fill is stored is passed to the function.

What you want to pass is the memory to which to_fill, the pointer, points to.

Upvotes: 1

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