Data Files - C Interview Questions VII

Q. How can I read and write comma-delimited text?

Many of today's popular programs use comma-delimited text as a means of transferring data from one program to another, such as the exported data from a spreadsheet program that is to be imported by a database program. Comma-delimited means that all data (with the exception of numeric data) is surrounded by double quotation marks ("") followed by a comma. Numeric data appears as-is, with no surrounding double quotation marks. At the end of each line of text, the comma is omitted and a newline is used.

To read and write the text to a file, you would use the fprintf() and fscanf() standard C library functions. The following example shows how a program can write out comma-delimited text and then read it back in.

#include <stdio.h>

#include <string.h>

typedef struct name_str

{

     char       first_name[15];

     char       nick_name[30];

     unsigned years_known;

} NICKNAME;

NICKNAME nick_names[5];

void main(void);

void set_name(unsigned, char*, char*, unsigned);

void main(void)

{

     FILE*     name_file;

     int       x;

     NICKNAME tmp_name;

     printf("\nWriting data to NICKNAME.DAT, one moment please...\n");

     /* Initialize the data with some values... */

     set_name(0,    "Sheryl",      "Basset",      26);

     set_name(1,    "Joel",        "Elkinator",    1);

     set_name(2,    "Cliff",       "Shayface",    12);

     set_name(3,    "Lloyd",       "Lloydage",    28);

     set_name(4,    "Scott",       "Pie",          9);

     /* Open the NICKNAME.DAT file for output in text mode. */

     name_file = fopen("NICKNAME.DAT", "wt");

     /* Iterate through all the data and use the fprintf() function

        to write the data to a file. */

     for (x=0; x<5; x++)

     {

          fprintf(name_file, "\"%s\", \"%s\", %u\n",

                      nick_names[x].first_name,

                      nick_names[x].nick_name,

                      nick_names[x].years_known);

     }

     /* Close the file and reopen it for input. */

     fclose(name_file);

     printf("\nClosed NICKNAME.DAT, reopening for input...\n");

     name_file = fopen("NICKNAME.DAT", "rt");

     printf("\nContents of the file NICKNAME.DAT:\n\n");

     /* Read each line in the file using the scanf() function

        and print the file's contents. */

     while (1)

     {

          fscanf(name_file, "%s %s %u",

                     tmp_name.first_name,

                     tmp_name.nick_name,

                     &tmp_name.years_known);

          if (feof(name_file))

               break;

          printf("%-15s %-30s %u\n",

                     tmp_name.first_name,

                     tmp_name.nick_name,

                     tmp_name.years_known);

     }

     fclose(name_file);

}

void set_name(unsigned name_num, char* f_name, char* n_name, unsigned years)

{

     strcpy(nick_names[name_num].first_name, f_name);

     strcpy(nick_names[name_num].nick_name,  n_name);

     nick_names[name_num].years_known = years;

}

p � - r �� Ѐ

EACCES

-

Permission denied to access file

EINVACC

-

Invalid access code

The following example shows how to open a file in shared mode:

#include <stdio.h>

#include <fcntl.h>

#include <sys\stat.h>

#include <io.h>

#include <share.h>

void main(void);

void main(void)

{

     int file_handle;

     /* Note that sopen() is not ANSI compliant */

     file_handle = sopen("C:\\DATA\\TEST.DAT", O_RDWR, SH_DENYNO);

     close(file_handle);

}

Whenever you are sharing a file's contents with other programs, you should be sure to use the standard C library function named locking() to lock a portion of your file when you are updating it.