*string_ptr[4], pointer of chars

Hi there,

I have been using pointers of "strings" (formally chars) before, but assigning the values with a direct asignation (see ex. below); now though, I need to assign the values only once they are available, that means that I'd like to use a strcpy to do that.

Well, I get a bus error and I can't explain why.
I'd appreciate it if someone were able to help
thanks in advance

1) Exemple 1: Direct assignment of strings: This works fine

Code:

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const char *varnames[] = {"HOLA", "HAYA", "CIAO"};

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2) Case that gives BUS error:

Code:

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//DEclaration
char *FLAG[nbdy_nodes];

//Allocation
for(i=0; i<3; i++)
        FLAG[i] = (char*) malloc(4 * sizeof(char));

//Assignment:
strcpy(bdy_FLAG[1], "1111");

//or strcpy(*FLAG[1], "1111");

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Thank you

"1111"

needs buffer of 5 chars - 4 letters and nul-terminator

One mistake here (I don't know if this will be a bus error, but it will cause an overwrite), is that strcpy adds a byte to the string (the null terminator). So if you want to put "1111" into FLAG[x], FLAG[x] needs to be allocated 5 bytes, not 4.

You first use of strcpy (without the asterisk) is correct.

Quote:

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Originally Posted by MK27

You first use of strcpy (without the asterisk) is correct.

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For that statement you need to know what the bdy_FLAG is

Hi, thanks for replying;

I actually first used

Code:

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strncpy(FLAG[1], "1111", 4);

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after allocating it in this way:

Code:

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//DEclaration
char *FLAG[nbdy_nodes];

//Allocation
for(i=0; i<3; i++)
        FLAG[i] = (char*) malloc(4 * sizeof(char));

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exactly for that reason, but still get a BUS; so I wonder if I can use strcpy/strncpy with a pointer to chars

thank you again

Quote:

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Originally Posted by cfdprogrammer

Hi, thanks for replying;

I actually first used

Code:

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strncpy(FLAG[1], "1111", 4);

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exactly for that reason, but still get a BUS; so I wonder if I can use strcpy/strncpy with a pointer to chars

thank you again

---

This leaves FLAG[1] not nul-terminated - so it is not a C-string and you cannot pass it to any function that expects one.

I assume you use those strings with other functions, too? Such as printf, or stuff?
Since there's no null in there, they will read past the array into oblivion.

Quote:

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Originally Posted by vart

This leaves FLAG[1] not nul-terminated - so it is not a C-string and you cannot pass it to any function that expects one.

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HI, how do I make it a C string then using strcpy?

best,
cfd

As usual - strcpy(target, "mystring");

Quote:

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Originally Posted by Elysia

As usual - strcpy(target, "mystring");

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hi,

but this is why I had to write here, because done in that way, I get a bus error, so that I dont udnerstand how to use strcpy with a pointer to chars

thanks

Just make sure there's enough room for the string (the number of chars in the string + 1 for null) and it will be fine.
If not, then you've done something bad elsewhere.

Quote:

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Originally Posted by cfdprogrammer

hi,

but this is why I had to write here, because done in that way, I get a bus error, so that I dont udnerstand how to use strcpy with a pointer to chars

thanks

---

Did you? Or were you using strncpy at that time?

All of these functions work with a pointer to a chars. In fact, the return value of all these function is: A pointer to chars (a "char pointer").

But you must use the null terminator. Remember, those chars are values in memory. A string function DOES NOT KNOW how long the string pointed to by a pointer is before it reads it from memory; the way they work is they beginning reading from memory at the location pointed to by the pointer and they continue, sequentially, until they hit a zero byte (a char with a value of '\0'). You must use a null terminated string of chars with the "string" functions (the C definition of a string is actually "a null terminated sequence of chars").

I was indeed allocating it wrongly ( I was not allocating the correct pointer, I wonder how I did that wrong!), anyhow,

now I get into an even worse problem:


malloc: *** error for object 0xbffff538: Non-aligned pointer being freed
*** set a breakpoint in malloc_error_break to debug

These are the calls I am doing:

Code:

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char *bdy_FLAG[10];
        for(i=0; i<10; i++)
                bdy_FLAG[i] = (char*) malloc(8 * sizeof(char));
   

      //I call the function that needs to assign bdy_FLAG[] with the strings
    BDY_FLAG(bdy_FLAG);

//Free dynamic memory:
free(bdy_FLAG);

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am I freeing it wrong?

Yes, all calls to malloc must be paired with one to free. Since you malloc memory to each element in the array, then you need to free each element of the array, not the array itself (because you did NOT malloc the array itself).
Nothing is automatic in C.

This what I did then, but still, the BUS error keeps persecuting me!

Code:

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for(i=0; i<10; i++)
                        free(bdy_FLAG[i]);\

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You only have 10. What's the value of nbdy_nodes?

Quote:

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Originally Posted by Elysia

You only have 10. What's the value of nbdy_nodes?

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Sorry, I meant to put 10 for this thread but copied my code directly. The nbdy_nodes is coherent with my allocations in the real code; let's pretend it were 10 as in this forum though

cfd

Alright, then it's likely that the memory is corrupted somehow by BDY_FLAG.
Comment it out, see if it fixes anything.
If it does, then the easiest way to catch your error is to use a debugger or check for buffer overflows.

Yes, indeed when I do not call the function BDY_FLAG I get all running fine;

I debugged it and I was entering a part of memory not allocated!

Man, this can give you a lot of headaches!

thank you very much for helping

Best
CFD