{"id":24151,"date":"2022-11-30T18:17:52","date_gmt":"2022-11-30T12:47:52","guid":{"rendered":"https:\/\/jassweb.com\/solved\/solved-string-literal-pointer-issue-passing-to-nested-functions\/"},"modified":"2022-11-30T18:17:52","modified_gmt":"2022-11-30T12:47:52","slug":"solved-string-literal-pointer-issue-passing-to-nested-functions","status":"publish","type":"post","link":"https:\/\/jassweb.com\/solved\/solved-string-literal-pointer-issue-passing-to-nested-functions\/","title":{"rendered":"[Solved] string literal pointer issue passing to nested functions"},"content":{"rendered":"<p> [ad_1]<br \/>\n<\/p>\n<div id=\"answer-39900138\" class=\"answer js-answer accepted-answer js-accepted-answer\" data-answerid=\"39900138\" data-parentid=\"39896649\" data-score=\"1\" data-position-on-page=\"1\" data-highest-scored=\"1\" data-question-has-accepted-highest-score=\"1\" itemprop=\"acceptedAnswer\" itemscope itemtype=\"https:\/\/schema.org\/Answer\">\n<div class=\"post-layout\">\n<div class=\"votecell post-layout--left\"><\/div>\n<div class=\"answercell post-layout--right\">\n<div class=\"s-prose js-post-body\" itemprop=\"text\">\n<p>In C, string operations cannot have a convenient interface, because of memory management. If a function receives a string, and converts it to another string, you should decide how to declare its interface.<\/p>\n<p>The simplest interface is in-place (<code>strtok<\/code> uses it); you can use it only if the output is somehow smaller than input:<\/p>\n<pre><code>void func(char* input_and_output);\n<\/code><\/pre>\n<p>A more traditional interface is &#8220;output in pre-allocated buffer&#8221; (<code>sprintf<\/code> uses it); you can use it only if it&#8217;s possible to calculate or somehow constrain the size of the output:<\/p>\n<pre><code>void func(const char* input, char* output);\n<\/code><\/pre>\n<p>Another idea is &#8220;allocate the output dynamically&#8221; (<code>asprintf<\/code> uses it); it doesn&#8217;t have limitations but is harder to use:<\/p>\n<pre><code>char* func(const char* input);\n<\/code><\/pre>\n<p>Its disadvantage is the responsibility that it imposes on the caller &#8211; it must <code>free<\/code> the allocated string at some time in the future.<\/p>\n<hr>\n<p>Let&#8217;s use the third interface:<\/p>\n<pre><code>char* transFormPath(const char* filePath)\n{\n    char* newPath;\n    newPath = malloc(100); \/\/ is 100 enough? no idea\n    strcpy(newPath, \"haha\"); \/\/ just an example\n    return newPath;\n}\n\nint main()\n{\n    ...\n    char* filePath = \"path\";\n    char* newPath = transFormPath(filePath);\n    ...\n    free(newPath);\n}\n<\/code><\/pre>\n<p>If you decided to use this interface, your <code>transFormPath<\/code> function <em>should<\/em> allocate the string dynamically, and the caller <em>should<\/em> free the string at some time when it&#8217;s not needed anymore. This is not trivial to accomplish &#8211; usually the &#8220;time when the string is not needed&#8221; is not easy to define, and if you change your code, the time when it&#8217;s safe to call <code>free<\/code> can change without you realizing it. As a quick-and-dirty hack, you can write your code without freeing memory (i.e. introduce a memory leak on purpose).<\/p>\n<hr>\n<p>Let&#8217;s use the second interface:<\/p>\n<pre><code>void transFormPath(const char* filePath, char* newPath)\n{\n    \/\/ Here we assume newPath has enough space to hold the output\n    strcpy(newPath, \"haha\"); \/\/ just an example\n}\n\nint main()\n{\n    ...\n    char* filePath = \"path\";\n    char* newPath = malloc(100); \/\/ is 100 enough? no idea\n    transFormPath(filePath, newPath);\n    ...\n    free(newPath);\n    ...\n    char newPath2[100]; \/\/ is 100 enough? no idea\n    transFormPath(filePath, newPath2);\n}\n<\/code><\/pre>\n<p>A handy rule of thumb is &#8220;<code>malloc<\/code> and <code>free<\/code> should be called in the same function&#8221; &#8211; this interface makes it possible not to violate it.<\/p>\n<p>Also, this interface makes it possible to use stack (automatic) memory allocation &#8211; if there is some limit on the size of the output, just allocate the maximum possible buffer on stack.<\/p>\n<\/p><\/div>\n<div class=\"mt24\"><\/div>\n<\/div>\n<p>            <span class=\"d-none\" itemprop=\"commentCount\">1<\/span> <\/p><\/div>\n<\/div>\n<p>[ad_2]<\/p>\n<p>solved string literal pointer issue passing to nested functions <\/p>\n","protected":false},"excerpt":{"rendered":"<p>[ad_1] In C, string operations cannot have a convenient interface, because of memory management. If a function receives a string, and converts it to another string, you should decide how to declare its interface. The simplest interface is in-place (strtok uses it); you can use it only if the output is somehow smaller than input: &#8230; <a title=\"[Solved] string literal pointer issue passing to nested functions\" class=\"read-more\" href=\"https:\/\/jassweb.com\/solved\/solved-string-literal-pointer-issue-passing-to-nested-functions\/\" aria-label=\"More on [Solved] string literal pointer issue passing to nested functions\">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":[324,413,712,362,1662],"class_list":["post-24151","post","type-post","status-publish","format-standard","hentry","category-solved","tag-c","tag-function","tag-pointers","tag-string","tag-string-literals"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.4 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>[Solved] string literal pointer issue passing to nested functions - 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-string-literal-pointer-issue-passing-to-nested-functions\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"[Solved] string literal pointer issue passing to nested functions - JassWeb\" \/>\n<meta property=\"og:description\" content=\"[ad_1] In C, string operations cannot have a convenient interface, because of memory management. If a function receives a string, and converts it to another string, you should decide how to declare its interface. The simplest interface is in-place (strtok uses it); you can use it only if the output is somehow smaller than input: ... 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