{"id":4068,"date":"2020-04-11T00:01:38","date_gmt":"2020-04-11T07:01:38","guid":{"rendered":"https:\/\/c-for-dummies.com\/blog\/?p=4068"},"modified":"2020-04-04T09:17:01","modified_gmt":"2020-04-04T16:17:01","slug":"crossing-between-formatted-and-unformatted-file-functions","status":"publish","type":"post","link":"https:\/\/c-for-dummies.com\/blog\/?p=4068","title":{"rendered":"Crossing Between Formatted and Unformatted File Functions"},"content":{"rendered":"

One of the reasons behind the low-level open()<\/em> function is to access non-traditional files. This stems from the UNIX environment’s treatment of every device as a file. Sometimes you need low-level access to accomplish specific tasks, such as accessing a device driver. Yet, in the C language universe, an interesting crossover is provided between low-level raw file access and formatted file access.
\n
\nThe function that bridges the gap is fdopen()<\/em>. Here is its format:<\/p>\n

FILE * fdopen(int fildes, const char *mode);<\/code><\/p>\n

This function works like the fopen()<\/em> function, but substituting a file descriptor integer, fildes<\/code>, for the filename (first) argument. The fildes<\/code> is the integer value returned from an open()<\/em> function<\/a>. The second argument is a string representing the file-opening mode. The value returned is a FILE<\/em> pointer.<\/p>\n

After the fdopen()<\/em> function is successfully used on a file opened by using the open()<\/em> function, you can use employ the familiar formatted file functions to work with the file. When you’re done, you first fclose()<\/em> the file that fdopen()<\/em> opened, then close()<\/em> the file the open()<\/em> function opened. Yes, the process involves more overhead, but that’s how it works. Sample code:<\/p>\n

\r\n#include <stdio.h>\r\n#include <stdlib.h>\r\n#include <fcntl.h>\r\n#include <unistd.h>\r\n\r\nint main()\r\n{\r\n    const char filename[] = \"gettysburg.txt\";\r\n    FILE *fp;\r\n    int fdes,ch;\r\n\r\n    \/* open the file to get a file descriptor *\/<\/span>\r\n    fdes = open(filename,O_RDONLY);\r\n    if( fdes==-1 )\r\n    {\r\n        fprintf(stderr,\"Unable to open %s for unformatted input\\n\",filename);\r\n        exit(1);\r\n    }\r\n\r\n    \/* use the descriptor to open the file for streamed input *\/<\/span>\r\n    fp = fdopen(fdes,\"r\");\r\n    if( fp==NULL )\r\n    {\r\n        fprintf(stderr,\"Unable to open %s for streamed input\\n\",filename);\r\n        exit(1);\r\n    }\r\n\r\n    \/* read the file using formatted input *\/<\/span>\r\n    while( !feof(fp) )\r\n    {\r\n        ch = fgetc(fp);\r\n        if( ch==EOF )\r\n            break;\r\n        putchar(ch);\r\n    }\r\n\r\n    \/* close the open files *\/<\/span>\r\n    fclose(fp);\r\n    close(fdes);\r\n\r\n    return(0);\r\n}<\/pre>\n
Next, the fdopen()<\/em> function opens the file, already open for unformatted reading, for formatted reading:<\/p>\n
fp = fdopen(fdes,\"r\");<\/code><\/p>\n
The fdes<\/code> variable is used instead of a filename. The result is saved in the fp<\/code> FILE<\/em> pointer, which is immediately tested. If everything is kosher, the fgetc()<\/em> function reads the file’s contents and outputs its text:<\/p>\n
while( !feof(fp) )
\n{
\n    ch = fgetc(fp);
\n    if( ch==EOF )
\n        break;
\n    putchar(ch);
\n}<\/code><\/p>\n
The feof()<\/em> function and a test for the EOF<\/code> character are used to determine when all the text is read from the file. These are formatted file-reading techniques, yet applied to a file originally opened for low-level access.<\/p>\n
Two statements are required to close-up when done:<\/p>\n
fclose(fp);
\nclose(fdes);<\/code><\/p>\n
A sample run output text from the gettysburg.txt<\/code> file.<\/p>\n
The fdopen()<\/em> function provides a useful way to use your favorite file access functions on files opened for low-level access. Do be aware that the fdopen()<\/em> function must be used in the same mode as the file was opened with in the open()<\/em> function. Further, some file-opening mode strings don’t work on files opened in-the-raw: The \"x\"<\/code> mode causes fdopen()<\/em> to fail if the file opened doesn’t exist.<\/p>\n","protected":false},"excerpt":{"rendered":"
It’s possible to use the raw-access open()<\/em> function to open a file and then use formatted file functions. Weird, but possible. 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-4068","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\/4068","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=4068"}],"version-history":[{"count":3,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/4068\/revisions"}],"predecessor-version":[{"id":4073,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/4068\/revisions\/4073"}],"wp:attachment":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=4068"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=4068"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=4068"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}
The code first uses the open()<\/em> function at Line 13 to open gettysburg.txt<\/a>. The file descriptor is stored in int<\/em> variable fdes<\/code>.<\/p>\n