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Full Discussion: Malloc to void pointer fails
Top Forums Programming Malloc to void pointer fails Post 302906559 by rupeshkp728 on Friday 20th of June 2014 05:03:37 PM
Old 06-20-2014
Malloc to void pointer fails

I have a function to which I will pass a struct ID and it will return me a string.
I will pass a pointer to store the name string and that pointer will be allocated memory by the function called.
Code:
int ConvertIDToName(void *id, void *name, size_t *size)
{
    int         status = 0;
    unsigned char *xIDname = "user4.microsoft.com";

    *name = (unsigned char*)malloc(30);
    memcpy(*name, xIDName, *size);

    ...

    return(0);
}

Code:
main(int argc, char* argv[])
{
    struct ID_t idObj ={1,5, {0,0,0,0,0,5}};
    unsigned char*        IDName = NULL;
    UINT32            IDNameSize = MAX_CHAR;

    ConvertIDToName(&idObj, &IDName, (size_t *)&IDNameSize);

    return(0);
}

The function ConvertIDToName() fails to store the allocated memory address in the void pointer.
I am unable to assign memory in the pointer and it gives me error from 3rd statement of function ConvertIDToName():
Code:
 
   warning: dereferencing âvoid *â pointer
    test_code.c:683: error: invalid use of void expression

What am I doing wrong?

---------- Post updated at 02:33 AM ---------- Previous update was at 02:32 AM ----------

I missed the simple thing of typecasting the void pointer before using it.
This solves the issue:
Code:
*(unsigned char **)name = malloc(30);
memcpy(*(unsigned char **)name, xIDName, *size);

 

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memory(3C)						   Standard C Library Functions 						memory(3C)

NAME
memory, memccpy, memchr, memcmp, memcpy, memmove, memset - memory operations SYNOPSIS
#include <string.h> void *memccpy(void *restrict s1, const void *restrict s2, int c, size_t n); void *memchr(const void *s, int c, size_t n); int memcmp(const void *s1, const void *s2, size_t n); void *memcpy(void *restrict s1, const void *restrict s2, size_t n); void *memmove(void *s1, const void *s2, size_t n); void *memset(void *s, int c, size_t n); ISO C++ #include <string.h> const void *memchr(const void *s, int c, size_t n); #include <cstring> void *std::memchr(void *s, int c, size_t n); DESCRIPTION
These functions operate as efficiently as possible on memory areas (arrays of bytes bounded by a count, not terminated by a null charac- ter). They do not check for the overflow of any receiving memory area. The memccpy() function copies bytes from memory area s2 into s1, stopping after the first occurrence of c (converted to an unsigned char) has been copied, or after n bytes have been copied, whichever comes first. It returns a pointer to the byte after the copy of c in s1, or a null pointer if c was not found in the first n bytes of s2. The memchr() function returns a pointer to the first occurrence of c (converted to an unsigned char) in the first n bytes (each interpreted as an unsigned char) of memory area s, or a null pointer if c does not occur. The memcmp() function compares its arguments, looking at the first n bytes (each interpreted as an unsigned char), and returns an integer less than, equal to, or greater than 0, according as s1 is lexicographically less than, equal to, or greater than s2 when taken to be unsigned characters. The memcpy() function copies n bytes from memory area s2 to s1. It returns s1. If copying takes place between objects that overlap, the behavior is undefined. The memmove() function copies n bytes from memory area s2 to memory area s1. Copying between objects that overlap will take place cor- rectly. It returns s1. The memset() function sets the first n bytes in memory area s to the value of c (converted to an unsigned char). It returns s. USAGE
Using memcpy() might be faster than using memmove() if the application knows that the objects being copied do not overlap. ATTRIBUTES
See attributes(5) for descriptions of the following attributes: +-----------------------------+-----------------------------+ | ATTRIBUTE TYPE | ATTRIBUTE VALUE | +-----------------------------+-----------------------------+ |Interface Stability |Stable | +-----------------------------+-----------------------------+ |MT-Level |MT-Safe | +-----------------------------+-----------------------------+ |Standard |See standards(5). | +-----------------------------+-----------------------------+ SEE ALSO
string(3C), attributes(5), standards(5) NOTES
Overlap between objects being copied can arise even when their (virtual) address ranges appear to be disjoint; for example, as a result of memory-mapping overlapping portions of the same underlying file, or of attaching the same shared memory segment more than once. SunOS 5.11 4 Feb 2009 memory(3C)
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