To learn more about pointers I wrote just a simple test function which creates a pointer, allocates space ande after some output on the shell the space shall be freed.<\/p>\n
void main() {\n int *p = (int*) malloc(sizeof(int));\n int a = 42;\n printf(\"p: %x\\n\", p);\n printf(\"*p: %d\\n\", *p);\n printf(\"a: %s\\n\", a);\n printf(\"&a: %x\\n\", &a);\n p = &a;\n printf(\"p: %x\\n\", p);\n printf(\"*p: %x\\n\", *p);\n\n \/\/Until now everything works as expected\n free(p); \/\/ERROR\n \/\/ printf(\"p: %x\\n\", p); \/\/ Shall cause error, because space was freed\n \/\/ printf(\"*p: %x\\n\", *p); \/\/ Shall cause error, because space was freed\n}\n<\/code><\/pre>\nAt first runs everything was ok. free(p) caused no error. Then I tried the same test with a struct and got the double free error. “Ok, maybe I do something wrong, lets go to the start”, I thought and Ctrl+Z everything to this one above, like it was before. Now I still get this error. Why? Thats a newbie question, I know. The code above you can find everywhere in the web as a simple demonstration of malloc and free. Like here: http:\/\/www.cplusplus.com\/reference\/cstdlib\/free\/ Hope, someone can tell me what I do wrong.<\/p>\n
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The pointer passed to free()<\/code> must<\/em> be returned from a previous call to malloc()<\/code> (or calloc()<\/code> or realloc()<\/code>). This is not the case as p<\/code> has the address of a<\/code> after:<\/p>\np = &a;\n<\/code><\/pre>\nand p<\/code> is then passed to free()<\/code> causing undefined behaviour.<\/p>\nNote that in the example on page linked in the question nowhere is the pointer being reassigned after the call to any of the dynamic allocation functions.<\/p>\n
The comment \"Shall cause error, because space was freed\"<\/code> in the following code snippet is incorrect:<\/p>\nfree(p); \/\/ERROR\n\/\/ printf(\"p: %x\\n\", p); \/\/ Shall cause error, because space was freed\n<\/code><\/pre>\nas it is safe to print the address of a pointer, regardless of whether the memory it has been associated with has been free()<\/code> or not. It is<\/em> an error if you attempt to access memory that has been free()<\/code>d, so this comment is correct:<\/p>\n\/\/ printf(\"*p: %x\\n\", *p); \/\/ Shall cause error, because space was freed\n<\/code><\/pre>\nbecause p<\/code> is being deferenced and an attempt to read free()<\/code>d memory is occurring.<\/p>\nThis is an error:<\/p>\n
printf(\"a: %s\\n\", a); \/* Must be '%d' as 'a' is an 'int'. *\/\n<\/code><\/pre>\nNote that:<\/p>\n
printf(\"p: %x\\n\", p);\n<\/code><\/pre>\nis also undefined behaviour: use %p<\/code> format specifier for pointers. See Correct format specifier to print pointer (address)?<\/p>\nDo I cast the result of malloc?<\/p>\n<\/li>\n
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void main() {\n int *p = (int*) malloc(sizeof(int)); \/\/ here is error number one.\n \/\/ you casted your fresh malloc'd int\n \/\/ to an int*\n \/\/ which is not what you want to do\n \/\/ get rid of the cast you don't need it.\n\n \/\/ here is where your problem continues.\n int a = 42; \/\/ this variable A was created on the stack not the heap!!\n \/\/you cant free this, which is what you are doing below.\n printf(\"p: %x\\n\", p);\n printf(\"*p: %d\\n\", *p);\n printf(\"a: %s\\n\", a);\n printf(\"&a: %x\\n\", &a);\n\n p = &a; \/\/ this is a correct statement. \n \/\/ you getting the address of var a\n \/\/ and making pointer p point to A's contents.\n printf(\"p: %x\\n\", p);\n printf(\"*p: %x\\n\", *p);\n\n \/\/Until now everything works as expected\n free(p); \/\/ERROR: here you are trying to free var A \n \/\/ but you cant because it wasnt dynamically allocated.\n \/\/ printf(\"p: %x\\n\", p); \/\/ Shall cause error, because space was freed\n \/\/ printf(\"*p: %x\\n\", *p); \/\/ Shall cause error, because space was freed\n}\n<\/code><\/pre>\nhope this helps a bit. I highly recommend that you read CH 5 on pointer in K&R(Kernegie and Richie) C programming language book. its really short and you can probly find it free online somewhere. in fact i recommond that you read the whole book. best c book out there. remember you can only free what you malloced (remember each malloc gives you blocks in memory from the heap)<\/p>\n<\/li>\n<\/ol>\n
Originally posted 2013-11-09 19:43:32. <\/small><\/p>","protected":false},"excerpt":{"rendered":"
To learn more about pointers I wrote just a simple test function which creates a pointer, allocates space ande after some output on the shell the space shall be freed. void main() { int *p = (int*) malloc(sizeof(int)); int a = 42; printf(“p: %x\\n”, p); printf(“*p: %d\\n”, *p); printf(“a: %s\\n”, a); printf(“&a: %x\\n”, &a); p […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[],"class_list":["post-454","post","type-post","status-publish","format-standard","hentry","category-uncategorized"],"_links":{"self":[{"href":"https:\/\/unknownerror.org\/index.php\/wp-json\/wp\/v2\/posts\/454","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/unknownerror.org\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/unknownerror.org\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/unknownerror.org\/index.php\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/unknownerror.org\/index.php\/wp-json\/wp\/v2\/comments?post=454"}],"version-history":[{"count":0,"href":"https:\/\/unknownerror.org\/index.php\/wp-json\/wp\/v2\/posts\/454\/revisions"}],"wp:attachment":[{"href":"https:\/\/unknownerror.org\/index.php\/wp-json\/wp\/v2\/media?parent=454"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/unknownerror.org\/index.php\/wp-json\/wp\/v2\/categories?post=454"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/unknownerror.org\/index.php\/wp-json\/wp\/v2\/tags?post=454"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}