I had such a struggle with file I\/O when I first learned computer programming. I knew what a file was and how to create it in an application. With computer programming, however, you enter a lower-level realm that requires more knowledge of file access.
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\nThe struggle is between sequential and random file access. Before boring into the details, I present the basic file access elements in the C language:<\/p>\n
FILE<\/strong> To access a file, you use a file handle, represented by the fopen()<\/em><\/strong> The function to open a file for reading and writing is fopen()<\/em>. Its arguments are two strings, a filename and an access mode. This function is defined in the fclose()<\/em><\/strong> The function to close a file is fclose()<\/em>. It writes any left over tidbits to the file and ensures that data is safely stored. Its sole argument is the Of these three elements, fopen()<\/em> is the most involved — which is good, because it’s not that complex. Its second argument is a mode string that stipulates how the file is opened:<\/p>\n Both Appending a Following None of these options set how the data is read from the file, sequentially or via random access. So you can put off being flustered by those options until later.<\/p>\n The following code shows a file-opening skeleton. The file is opened for writing, which means it’s created. If the file indicated already exists, it’s overwritten.<\/p>\n The filename In Line 9, the fopen()<\/em> function attempts to open the file. Lines 10 through 14 handle any file errors. You must<\/em> include this type of testing in all your file-handling code. When the file pointer is Line 15 uses the fclose()<\/em> function to close the file referenced by No data is written to the file To alter the code to open a file for reading, change the In next week’s Lesson<\/a>, I cover writing to a file and reading from a file.<\/p>\n","protected":false},"excerpt":{"rendered":" Making a file is easy, providing that you know the proper incantations. Continue reading FILE<\/code> variable. FILE<\/code> is typedef’d in the stdio.h<\/code> header file as a structure pointer. FILE<\/code> variables are created with the * operator, but afterwards used without that operator. That way programmers who can’t stand pointers can work with file functions and not suffer an aneurism.<\/p>\nstdio.h<\/code> header file.<\/p>\nFILE<\/code> variable returned from a previously successful fopen()<\/em> function. It too is defined in stdio.h<\/code>.<\/p>\n\"a\"<\/code> to open or create a file for appending.
\n\"r\"<\/code> to open a file for reading only.
\n\"w\"<\/code> to open or create a file for writing.<\/p>\n\"a\"<\/code> and \"w\"<\/code> create a file if it doesn’t exist. The \"r\"<\/code> option fails if the file doesn’t exist.<\/p>\n+<\/code> character to any mode string also opens the file for reading and writing.<\/p>\n\"w\"<\/code> or \"w+\"<\/code> with the x<\/code> character causes the fopen()<\/em> function to fail when the named file already exists.<\/p>\n\r\n#include <stdio.h>\r\n\r\nint main()\r\n{\r\n const char filename[] = \"test\";\r\n FILE *f;\r\n\r\n printf(\"Creating '%s'\\n\",filename);\r\n f = fopen(filename,\"w\");\r\n if( f == NULL )\r\n {\r\n fprintf(stderr,\"Unable to create file '%s'\\n\",filename);\r\n return(1);\r\n }\r\n fclose(f);\r\n puts(\"Done\");\r\n\r\n return(0);\r\n}<\/pre>\ntest<\/code> is established a string constant at Line 5. Line 6 creates the FILE<\/code> handle (pointer variable) f<\/code>.<\/p>\nNULL<\/code>, the file cannot be opened (for whatever reason) and you should discontinue accessing it.<\/p>\nFILE<\/code> handle f<\/code>.<\/p>\ntest<\/code>. If you check the directory, you see that file test<\/code> has zero bytes. That’s exactly what the program did: Create a file for writing, not write anything, then close the file.<\/p>\n\"w\"<\/code> argument in Line 10 to \"r\"<\/code>. You can modify the printf()<\/em> statement at Line 8 and the fprintf()<\/em> statement as Line 12 to reflect that a file is being read from and opened for reading. As long as the file test exists, it can be opened and closed.<\/p>\n