{"id":6022,"date":"2023-09-09T00:01:04","date_gmt":"2023-09-09T07:01:04","guid":{"rendered":"https:\/\/c-for-dummies.com\/blog\/?p=6022"},"modified":"2023-09-16T08:22:00","modified_gmt":"2023-09-16T15:22:00","slug":"reading-and-writing-to-memory-files","status":"publish","type":"post","link":"https:\/\/c-for-dummies.com\/blog\/?p=6022","title":{"rendered":"Reading and Writing to Memory-Files"},"content":{"rendered":"

Opening and closing a memory-file is just academic. To make the whole shebang work, you must be able to read and write data.
\n
\nTo continue from last week’s Lesson<\/a>, I introduce the mem_getc()<\/em> and mem_putc()<\/em> functions. Both use the mem_file<\/em> structure and rely upon the mem_file<\/em> pointer being allocated. They follow the format of the fgetc()<\/em> and fputc()<\/em> functions, which are the basic file I\/O functions for cooked data. (The fread()<\/em> and frwite()<\/em> functions deal with raw data.)<\/p>\n

Here is the mem_putc()<\/em> function:<\/p>\n

\r\n\/* put a character to a memory file\r\n   input- single character, memory file handle\r\n   output- character written or\r\n           EOF on failure\r\n *\/<\/span>\r\nint mem_putc(int c, struct mem_file *m)\r\n{\r\n    int *index;\r\n\r\n    if( m->address==NULL )\r\n        return(EOF);\r\n\r\n    index = m->address+m->size;\r\n    *index = c;\r\n\r\n    m->size++;\r\n    \/* check for overflow *\/<\/span>\r\n    if( (m->size % DEFAULT_SIZE) == 0 )\r\n    {\r\n        m->address=realloc(m->address,m->size-1+DEFAULT_SIZE);\r\n        if( m->address==NULL )\r\n            return(EOF);\r\n    }\r\n\r\n    return(c);\r\n}<\/pre>\n
The first test is to ensure that the mem_file<\/em> structure passed is allocated. If not, the EOF<\/code> constant is returned. This return value is consistent with the fputc()<\/em> function.<\/p>\n
Variable index<\/code> serves as a reference into the memory-file’s storage. It’s set to reference the end of the used portion of the memory-file’s allocated space: index = m->address+m->size;<\/code> This function always appends data unless the memory-file size<\/code> value has been otherwise manipulated.<\/p>\n
The character passed, c<\/code>, is appended to the memory-file: *index = c;<\/code><\/p>\n
The size<\/code> offset is incremented: m->size++;<\/code><\/p>\n
Immediately after incrementing the size<\/code> value, a test is made to ensure that allocated storage hasn’t been exceeded: if( (m->size % DEFAULT_SIZE) == 0 )<\/code> If the result is TRUE, another DEFAULT_SIZE<\/code> chunk is allocated for the memory-file, along with appropriate error-checking.<\/p>\n
The function returns the character added, c<\/code>.<\/p>\n
Here is the mem_getc()<\/em> function:<\/p>\n
\r\n\/* fetch a character from a memory file\r\n   input- memory file handle\r\n   output- character retrieved or\r\n           EOF on failure\r\n *\/<\/span>\r\nint mem_getc(struct mem_file *m)\r\n{\r\n    int *index;\r\n\r\n    if( m->address==NULL )\r\n        return(EOF);\r\n\r\n    if( m->offset>m->size )\r\n        return(EOF);\r\n\r\n    index = m->address+m->offset;\r\n    m->offset++;\r\n\r\n    return( *index );\r\n}<\/pre>\n
If the address<\/code> is NULL<\/code>, the function returns immediately with an EOF.<\/p>\n
Next a check is done to ensure that the function isn’t reading beyond the end of the memory-file: if( m->offset>m->size )<\/code> If so, the function returns EOF<\/code>.<\/p>\n
When everything is kosher, the value of index<\/code> is set to the current offset<\/code> in the file, where data is being read: index = m->address+m->offset;<\/code><\/p>\n
The value of offset<\/code> is incremented in anticipation of the next read operation: m->offset++;<\/code><\/p>\n
Finally, the value refenced at the buffer’s original offset<\/code> is returned to the caller: return( *index );<\/code><\/p>\n
You can view the full code on GitHub<\/a>. In the main()<\/em> function, 'A'<\/code> is written to the created and opened memory-file, which is then read back in:<\/p>\n
Memory file 'memory file' created
\nTimestamp = Sat Sep  9 12:29:03 2023<\/p>\n
Put character 'A' to file 'memory file'
\nFetched character 'A' from file 'memory file'<\/p>\n
Memory file 'memory file' closed<\/code><\/p>\n
One concern I have is with the memory reallocation when the original allocated chunk size (DEFAULT_SIZE<\/code>) is surpassed. I wrote a test that sent 8K of data to the memory-file, which was read back and output with no problems. Still, I’d want to do more tests to ensure that everything works properly. And if I encounter problems as this project progresses, I’ll mention them in a future post.<\/p>\n
Next week’s Lesson<\/a> covers moving the memory-file offset pointer in preparation for reading and writing chunks of data — random memory-file access.<\/p>\n","protected":false},"excerpt":{"rendered":"
Reading and writing to anything first requires basic character I\/O. Continue reading →<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[2],"tags":[],"class_list":["post-6022","post","type-post","status-publish","format-standard","hentry","category-main"],"_links":{"self":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/6022","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=6022"}],"version-history":[{"count":4,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/6022\/revisions"}],"predecessor-version":[{"id":6042,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/6022\/revisions\/6042"}],"wp:attachment":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=6022"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=6022"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=6022"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}