{"id":5699,"date":"2023-01-08T00:01:05","date_gmt":"2023-01-08T08:01:05","guid":{"rendered":"https:\/\/c-for-dummies.com\/blog\/?p=5699"},"modified":"2023-01-07T11:11:29","modified_gmt":"2023-01-07T19:11:29","slug":"how-big-is-that-file-solution","status":"publish","type":"post","link":"https:\/\/c-for-dummies.com\/blog\/?p=5699","title":{"rendered":"How Big is That File? – Solution"},"content":{"rendered":"

The challenge for this month’s Exercise<\/a> is to return a file’s size without using the stat()<\/em> function. My goal is to get you to think about various file tools and how they can be useful beyond their intended purpose.
\n
\nThe hint for this solution is random file access. This approach manipulates a “file pointer,” which has nothing to do with C language pointers. No, a file pointer is an index into the file, an offset. I prefer to use the term file position indicator<\/em>, which is how I describe it in an older post<\/a>. The functions required are fseek()<\/em> and ftell()<\/em>.<\/p>\n

The fseek()<\/em> function moves the file position indicator. Here is the man<\/em> page format:<\/p>\n

int fseek(FILE *stream, long offset, int whence);<\/code><\/p>\n

Given the open file handle *stream<\/code>, you can reset the file position indicator’s location to the end of the file. Use the whence<\/code> value SEEK_END<\/code> and set the offset<\/code> to zero. Upon success, the file position indicator references the file’s final byte. You then use the ftell()<\/em> function to report this offset:<\/p>\n

long ftell(FILE *stream);<\/code><\/p>\n

The stream<\/code> argument is the open file handle. The value returned is a long<\/em> integer representing the file position indicator’s location — its offset from the start of the file. Presto! You have the file’s size, as shown in my solution:<\/p>\n

2023_01-Exercise.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n\r\nint main(int argc, char *argv[])\r\n{\r\n    char *filename;\r\n    FILE *fh;\r\n    long size;\r\n    \r\n    \/* check for filename argument *\/<\/span>\r\n    if( argc<2 )\r\n    {\r\n        puts(\"Specify a filename\");\r\n        return(1);\r\n    }\r\n\r\n    \/* operate from the first argument *\/<\/span>\r\n    filename = argv[1];\r\n\r\n    fh = fopen(filename,\"r\");\r\n    if( fh==NULL )\r\n    {\r\n        fprintf(stderr,\"Unable to open %s\\n\",filename);\r\n        return(1);\r\n    }\r\n\r\n    fseek( fh, 0, SEEK_END );\r\n    size = ftell(fh);\r\n    fclose(fh);\r\n\r\n    printf(\"%s is %ld bytes long\\n\",filename,size);\r\n\r\n    return(0);\r\n}<\/pre>\n
The code starts the same as the Exercise post that used the stat()<\/em> function: An argument is tested for and, when found, the named file is opened for read access.<\/p>\n
At Line 26, the fseek()<\/em> function moves the file position indicator to the last byte in the file. Then ftell()<\/em> reports that value, saved in long<\/em> variable size<\/code>. The file is closed, then the size is output.<\/p>\n
Of course, all this file position indicator manipulation isn’t necessary if you just use the stat()<\/em> function. Still, I find that random file access is something many C programmers don’t use often. To think of reading data at a certain offset in a file isn’t something you normally do. Yet it’s something you can do and something you can use to obtain information about a file, such as its size.<\/p>\n
I hope your solution met with success. If you figured out a way to do it without manipulating the file position indicator, please let me know. Thanks!<\/p>\n","protected":false},"excerpt":{"rendered":"
The challenge for this month’s Exercise is to return a file’s size without using the stat() function. My goal is to get you to think about various file tools and how they can be useful beyond their intended purpose.<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[5],"tags":[],"class_list":["post-5699","post","type-post","status-publish","format-standard","hentry","category-solution"],"_links":{"self":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/5699","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=5699"}],"version-history":[{"count":4,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/5699\/revisions"}],"predecessor-version":[{"id":5717,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/5699\/revisions\/5717"}],"wp:attachment":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=5699"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=5699"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=5699"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}