{"id":34567,"date":"2023-03-31T00:47:36","date_gmt":"2023-03-30T19:17:36","guid":{"rendered":"https:\/\/jassweb.com\/solved\/solved-error-when-coding-my-own-implementation-of-realloc\/"},"modified":"2023-03-31T00:47:36","modified_gmt":"2023-03-30T19:17:36","slug":"solved-error-when-coding-my-own-implementation-of-realloc","status":"publish","type":"post","link":"https:\/\/jassweb.com\/solved\/solved-error-when-coding-my-own-implementation-of-realloc\/","title":{"rendered":"[Solved] Error when coding my own implementation of realloc()"},"content":{"rendered":"

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Invalid assumption:<\/strong><\/p>\n

void *my_realloc(void *ptr, size_t size)\n{\n    unsigned char *old_ptr = (unsigned char *)ptr;\n<\/code><\/pre>\n
You’re breaking the realloc()<\/code> specification here. realloc()<\/code> doesn’t assume that the ptr<\/code> would always be pointing to a string. It is type-agnostic.<\/p>\n
And the cast is redundant. There’s an implicit conversion from void *<\/code> to any other pointer type.<\/p>\n
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Undefined behaviour:<\/strong><\/p>\n
old_size = my_strlen_unsigned(old_ptr) + 1;\n<\/code><\/pre>\n
The standard, or your my_strlen_unsigned<\/code> version of it, works with strings, i.e. the pointer is assumed to be pointing to an array of char<\/code> terminated by a null-byte. An array of int<\/code>s will not be terminated with a null-byte. Your my_realloc()<\/code> thus invokes undefined behaviour.<\/p>\n
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Possible fix:<\/strong><\/p>\n
You can’t determine the old size of the block pointed to by ptr<\/code> and reimplement realloc()<\/code>, without at least reimplementing your own malloc()<\/code> and free()<\/code>, portably. But you can take the old size as the third argument. (But the standard realloc()<\/code> only takes two. So would this be a conforming implementation?)<\/p>\n
Here’s glibc’s implementation of it: malloc().c<\/code><\/a><\/p>\n
Here’s musl C implementation of it: malloc().c<\/code><\/a><\/p>\n
And here’s my toy example that attempts to emulate the standard realloc()<\/code> to some extent:<\/p>\n
\/** \n*    @brief The my_realloc() function shall deallocate the old object pointed to\n*           by ptr and return a pointer to a new object that has the size specified by new_size.\n*\n*    @param ptr - A pointer to a block of memory to resize.\n*    @param old_size - The size of the block pointed to by ptr.\n*    @param new_size - The size to resize by.\n*\n*    If ptr is a null pointer, my_realloc() shall be equivalent to\n*    malloc() for the specified new_size.\n*\n*    If ptr does not match a pointer returned earlier by calloc(),\n*    malloc(), or realloc() or if the space has previously been\n*    deallocated by a call to free() or realloc(), the behavior is undefined.\n*    \n*    @return Upon successful completion, my_realloc() shall return a pointer to the moved allocated space.  \n*            If size and ptr both evaluate to 0, my_realloc() shall return a \n*            NULL pointer with errno set to [EINVAL].\n*            If there is not enough available memory, my_realloc() shall return a\n*            NULL pointer and set errno to [ENOMEM].\n*            \n*            If my_realloc() returns a NULL pointer, the memory referenced by ptr shall not be changed.\n*\n*    @warning my_realloc() may return NULL to indicate an error. For that reason, a different pointer variable \n*             must be used to hold it's return value. Otherwise, you risk overwriting the original ptr with NULL and \n*             losing your only reference to the original block of memory.\n*\/ \n              \nvoid *my_realloc (void *ptr, size_t new_size, size_t old size) \n{\n    if (!ptr) {\n        return malloc (new_size);\n    }\n    \n    if (!new_size) {\n        errno = EINVAL;\n        return 0;\n    }\n    \n    if (new_size <= old_size) {\n        return ptr;\n    }\n\n   \/* As a last resort, allocate a new chunk and copy to it. \n    *\/\n    void *new = 0;\n    if (new_size > old_size) {\n        new = malloc (new_size);\n        if (!new) {\n            return 0;\n        }\n        memcpy (new, ptr, old_size);\n        free (ptr);\n    }\n    return new;\n}\n<\/code><\/pre>\n
You can also find a sample implementation in chapter 8 of K&R.<\/p>\n
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Side-notes:<\/strong><\/p>\n
char *new_str = malloc(1);\nnew_str = my_realloc(..);\n<\/code><\/pre>\n
You risk losing access to the original memory allocated through malloc()<\/code> here. If my_realloc()<\/code> returned NULL<\/code>, new_str<\/code> would be assigned it’s result and you would cause a memory leak in your program.<\/p>\n
Furthermore, the memory returned by malloc()<\/code> is uninitialized. And your code invokes undefined behaviour by calling my_strlen()<\/code> underneath my_realloc()<\/code> on a pointer that is uninitialized. Hence the warnings.<\/p>\n
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solved Error when coding my own implementation of realloc() <\/p>\n","protected":false},"excerpt":{"rendered":"
[ad_1] Invalid assumption: void *my_realloc(void *ptr, size_t size) { unsigned char *old_ptr = (unsigned char *)ptr; You’re breaking the realloc() specification here. realloc() doesn’t assume that the ptr would always be pointing to a string. It is type-agnostic. And the cast is redundant. There’s an implicit conversion from void * to any other pointer type. … Read more<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[320],"tags":[324,1588,2162,913],"class_list":["post-34567","post","type-post","status-publish","format-standard","hentry","category-solved","tag-c","tag-libc","tag-realloc","tag-valgrind"],"yoast_head":"\n[Solved] Error when coding my own implementation of realloc() - JassWeb<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/jassweb.com\/solved\/solved-error-when-coding-my-own-implementation-of-realloc\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"[Solved] Error when coding my own implementation of realloc() - JassWeb\" \/>\n<meta property=\"og:description\" content=\"[ad_1] Invalid assumption: void *my_realloc(void *ptr, size_t size) { unsigned char *old_ptr = (unsigned char *)ptr; You’re breaking the realloc() specification here. realloc() doesn’t assume that the ptr would always be pointing to a string. 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