Whats wrong with fread in this program? - c

I'm intermediate student of C. I'm trying to make a bank management program but first I need to make a login program, so I created one of the following. As I've recently learned about file I/O in C and don't know much about fread and fwrite. I have a file (data.txt) which format if as following.
user1 1124
user2 3215
user3 5431
In the following program I've asked user to input user name and pin(4-digit password) and copy file data into a structure then compare these two for verifying information.
What is wrong with my program and how to make fread work properly. And is the formating in data.txt file all right or should I change it.
Thanks in advance...
#include<stdio.h>
#include<ctype.h>
#include<string.h>
struct user_account {
char u_name[30];
int u_pin;
} log_in;
int login()
{
int start;
int i, n;
int t_pin[4]; // TEMPORARY INT PIN for storing pin inputed by user
char t_name[30]; // TEMPORARY STRING for storing name inputed by user
FILE *fp;
fp = fopen("data.txt","rb"); // Opening record file
if(fp == NULL)
{
puts("Unable to open file!");
return 1;
}
start : {
printf("User Name : ");
scanf("%s",&t_name);
printf("Pin Code : ");
for(i = 0; i < 4; i++) { // This loop is for hiding input pin
n = getch();
if(isdigit(n)) {
t_pin[i] = n;
printf("*"); }
else {
printf("\b");
i--;
}
}
fread(&log_in,sizeof(log_in),1,fp);
// Comparing user name and pin with info in the structure copied from the file
if(strcmp(log_in.u_name, t_name) == 0 && log_in.u_pin == t_pin)
puts("Login successful! Welcome User");
else {
printf("\nIncorrect Information!\n");
printf("Press any key to log in again...");
getch();
system("cls");
goto start; }
}
}
int main()
{
int login();
return 0;
}

The problem is that you have an ASCII/text file but you are trying to use fread to read directly into a structure; this requires a binary formatted file. fread cannot do format conversion. Use fscanf instead:
fscanf(fp, "%s %d", &log_in.u_name, &log_in.u_pin);

Problems that I see:
The following line is incorrect.
scanf("%s",&t_name);
You need to use:
scanf("%29s",t_name);
fread is not the right solution given the file format. fread works when the file is in binary format.
Given the format of your input file, you need to use:
scanf("%29s %d", log_in.uname, &log_in.u_pin);
Comparing the pins using log_in.u_pin == t_pin should produce a compiler error. The type of log_in.u_pin is int while the type of t_pin is int [4]. You will have to change your strategy for getting the integer value from t_pin.
You can use something like:
int pin = 0;
for (i = 0; i < 4; ++i )
{
pin = 10*pin + t_pin[i];
}
However, for that to work, you have store the proper integer values in t_pin. When the character you read is '1', you have to store 1. Either that, or you will have to change the above for loop to:
pin = 10*pin + t_pin[i]-'0';
The way are using goto start to loop in your function is not correct. You haven't read all the user ID and pins to from the input file and compared them. The way you have it, you will end up doing:
Read the user name and pin
Check against the first entry in the file. If they don't match...
Read the user name and pin again.
Check against the next entry in the file. If they don't match...
etc.
You want to:
Read the user name and pin
Check against the first entry in the file. If they don't match...
Check against the next entry in the file. If they don't match...
Check against the next entry in the file. If they don't match...
etc.

a) fread is used to read fixed sized (in bytes) records. While that is an option, you might find that fscanf is more suited to your use case.
b) goto has its uses, arguably, but in your particular case a while loop is far more appropriate as it make the intention clearer and is less susceptible to misuse.
c) The crux of your issue is reading the file. You only read in and check against a single record from your file per pass and once all records are read you will be getting EOF rather than additional records.
You have a few options. You might read the entire user table into a list prior to your login loop. When a user logs in you must iterate through the list until a match or end of list is found. Alternatively you might loop through the entire file on each user attempt looking for a match and seek to the back to the beginning of the file when you are done. In either case you must check for and handle read errors.
d) Finally when a non-digit is entered you will probably find that backspace isn't what you had in mind. Also you might consider allowing the user to press backspace while entering the pin.

Related

How to read a .csv file in C

I know that there are other questions on this topic but I still can't find the solution to the problem.
I'm trying to read a .csv file in C using the fscanf() function.
If I open the csv file with a text editor it looks like this:
578,2.2212e+05,223,0,243,0,0,0.09,0,0,0,3
633,2.2222e+05,223,0,243,0,0,-0.04,0,0,0,3
688,2.2232e+05,223,0,243,0,0,0.07,0,0,0,3
740,2.2242e+05,223,0,243,0,0,0.04,0,0,0,3
793,2.2252e+05,224,0,244,0,0.01,0.16,0,0,0,3
848,2.2262e+05,223,0,717,0.060985,0.02,0.08,0,0,0,4
902,2.2272e+05,223,0,721,0.084618,0.03,0.24,0,0,0,5
955,2.2282e+05,223,0,730,0.12825,0.05,0.34,0,0,0,4
I just reported the first few rows but it contains a lot more.
Then I used this code to read the file:
FILE* stream = fopen("./filelog.csv", "r");
if(stream == NULL) {
printf("\n file opening failed ");
return -1 ;
}
int values[8];
int count;
for(count = 0; count < 8; count++) {
int u = fscanf(stream, "%i", &values[count]);
printf("%i\n", values[count]);
}
The output I get is the following:
578
32697
0
0
1
0
4199901
0
We can see that only the first value is read correctly. How could I read the whole file and store it into a matrix? I can't find any solution anywhere.
Thank you so much for your replies in advance!
Your format string %i parses a single integer, but the next thing after the integer is a comma, and you're not parsing that. You should always check the return value, to make sure parsing succeeded, before using the value(s).
You need a format string that matches the actual content of the file, something like:
if (fscanf(stream, " %d,%f,%d,%d,%d,%d,%d,%f,%d,%d,%d,%d", ...) == 12)
{
}
in the code above the ... means "put pointers to 12 variables of the proper types, here". The space at the front of the format string is intended to help "consume" whitespace (like linefeeds) in the stream.

Check if the user input an empty string in C using char array

I'm trying to have the program check, that, if a user inputs nothing the print statement will say it cant find the file name, but the issue I'm having is that the command line will just go to a new line after hitting enter instead of saying the print statement.
This is the code here. I was told that Null is the place holder for if nothing is put in so I thought it would work.
int main()
{
FILE *fin;
FILE *fout;
char fInName[50];
char fOutName[50];
printf("pleas type input file, and output file please type legibly\n ");
scanf("%s %s", &fInName, &fOutName);
fin = fopen(fInName, "r");
fout = fopen(fOutName, "r");
if (fInName == NULL && fOutName == NULL)
{
printf("Error: Cannot open input file %s.", fInName);
}
else if (fInName != NULL && fOutName == NULL)
{
printf("file found");
}
}
What im trying to test is if a first file name is entered and the second isnt then print the statement. If both arent entered then print file does not exist.
there is more to the code to see if the file exists or not, but thst would be a bit much, now Im just trying to understand why it wont read unentered data.
Ive tried looking at examples such as: How to detect empty string from fgets
and tried to alter the code to fit that type of style but it didnt work for me so Im giving you the code it was originally so that anything helpful wouldnt confuse me more.
Edit:
okay so I tried to do a simple code in order to see what may be the cause of this issue:
int main()
{
char firstname[50];
char lastname[50];
char nothing [0];
printf("pleas type input file, and output file please type legibly pwease\n ");
scanf("%s" "%s", firstname, lastname);
if (firstname == lastname )
{
printf("Error: Cannot open input file %s.", firstname);
}
else
{
printf("file found");
}
}
I ran the code using adam and either if I typed adam (space) adam or adam(enter) adam the program thinks that the input is not the same, I feel like that would help identify why it doesnt know why nothing is typed in.
The problem is occurring when you try to check if fInName == NULL.
The problem is that fInName is just a variable that you're using to store the name of the file that you want to open. What you actually want to check is that the user gave you a valid filename, and to do so you will want to understand what the return value of functions are.
For example, when you try to open a file using fopen(), if fopen() is unable to successfully open the file, say because the user didn't input anything or misspelled the filename, then fopen() will return NULL, storing it in whatever variable you assigned it to (in your case, *fin and *fout).
Also, scanf() is not recommended for char arrays because if the user inputs more data than you allocated for the array, which in this case is enough space for 50 characters, then scanf() will try to write data to memory that's not yours, causing a buffer overflow.
A much safer option is to use fgets() because you can choose exactly how much data is written into your char array, with the only downside being that fgets() will write newline characters \n (caused by hitting the enter key) into the array, though the simple solution is to overwrite the newline character with '\0'.
Therefore, I would propose:
int main(void)
{
char fInName[50];
char fOutName[50];
// ensure proper usage
do
{
printf("What file would you like to open? ");
// get infile from user and remove trailing newline '\n' character
fgets(fInName, 50, stdin);
fInName[strcspn(fInName, "\n")] = '\0';
}
// prompt for input until user enters something
while (strlen(fInName) < 1);
do
{
printf("What file would you like to output to? ");
// get outfile from user and remove trailing newline '\n' character
fgets(fOutName, 50, stdin);
fOutName[strcspn(fOutName, "\n")] = '\0';
}
// prompt for input until user enters something
while (strlen(fOutName) < 1);
FILE *fin = fopen(fInName, "r");
if (fin == NULL)
{
printf("Error: Cannot open input file %s.", fInName);
return 1;
}
else
{
printf("file found");
}
}

Reading and Displaying content of the Text File via C coding

I am trying to code a game in C that deals with selection of races.
Each races has their own "stories" and when the user chooses to read one of their stories,
what I want to happen is,
While the program is running on Command Prompt, it will display the content I have typed in that specific text file about the story of the selected race.
This is what I have done so far.
void Race(char nameRace[20])
{
int race_choice,race_choice2,race_story;
FILE *race;
FILE *race1;
FILE *race2;
FILE *race3;
printf("The Races: 1.Human 2.Elf 3.Orc\n");
printf("Press 1 for Details of Each Races or 2 for selection: ");
scanf("%d",&race_choice);
if (race_choice==1)
{
printf("Which Race do you wish to know about?\n\t1.The Human\n\t2.The Elf\n\t3.The Orc\n\t: ");
scanf("%d",&race_story);
if (race_story==1)
{
race1=fopen("race1.txt","r");
fgetc(race1); // This does not display what I have typed on the race1.txt file on Command prompt.
// And I plan to write 2~3 paragraphs on the race1.txt file.
printf("\nGo Back to the Selection?(1 to Proceed)\n ");
scanf("%d",&race_choice2);
if (race_choice2==1)
{
printf("\n\n");
Race(nameRace);
}
else
{
wrongInput(race_choice2);// This is part of the entire code I have created. This works perfectly.
}
}
}
}
Please help me? :) Please!
The functionality you seem to be lacking is the ability to read a text file and output it. So it might be a good idea to code up a function which does just this, and then you whenever you need to display the contents of a file you can just pass a file name to our function and let it take care of the work, e.g.
static void display_file(const char *file_name)
{
FILE *f = fopen(file_name, "r"); // open the specified file
if (f != NULL)
{
INT c;
while ((c = fgetc(f)) != EOF) // read character from file until EOF
{
putchar(c); // output character
}
fclose(f);
}
}
and then within your code would just call this as e.g.
display_file("orcs.txt");
fgetc function reads, and returns single character from file, it doesn't prints it.
So you will need to do following:
while(!feof(race1)) { // Following code will be executed until end of file is reached
char c = fgetc(race1); // Get char from file
printf("%c",c); // Print it
}
It will print contents of race1 char-by-char.
I think you'll probably want to read the file line by line, so it's best to use fgets() instead of fgetc().
Example:
while(!feof(race1)) // checks to see if end of file has been reached for race1
{
char line[255]; // temporarily store line from text file here
fgets(line,255,race1); // get string from race1 and store it in line, max 255 chars
printf("%s",line); // print the line from the text file to the screen.
}
If you replace fgetc(race1) with the chunk of code above, it may work. I have not tried running it but it should work.

C Program unable to create output text file

A friend of mine needs to use MATLAB for one of his classes, so he called me up (a Computer Science Major) and asked if I could teach him C. I am familiar with C++, so I am also familiar with the general syntax, but had to read up on the IO library for C.
I was creating some simple IO programs to show my friend, but my third program is causing me trouble. When I run the program on my machine using Eclipse (with the CDT) Eclipse's console produces a glitchy output where instead of prompting me for the data, it gets the input and then prints it all at once with FAILURE.
The program is supposed to get a filename from user, create the file, and write to it until the user enters a blank line.
When I compile/run it on my machine via console (g++ files2.c) I am prompted for the data properly, but FAILURE shows up, and there is no output file.
I think the error lies with how I am using the char arrays, since using scanf to get the filename will create a functional file (probably since it ignores whitespace), but not enter the while loop.
#include <stdio.h>
#define name_length 20
#define line_size 80
int main() {
FILE * write_file; // pointer to file you will write to
char filename[name_length]; // variable to hold the name of file
char string_buffer[line_size]; // buffer to hold your text
printf("Filename: "); // prompt for filename
fgets(filename, name_length, stdin); // get filename from user
if (filename[name_length-1] == '\n') // if last char in stream is newline,
{filename[name_length-1] = '\0';} // remove it
write_file = fopen(filename, "w"); // create/overwrite file user named
if (!write_file) {printf("FAILURE");} // failed to create FILE *
// inform user how to exit
printf("To exit, enter a blank line (no spaces)\n");
// while getting input, print to file
while (fgets(string_buffer, line_size, stdin) != NULL) {
fputs(string_buffer, write_file);
if (string_buffer[0] == '\n') {break;}
}
fclose(write_file);
return 0;
}
How should I go about fixing the program? I have found next to nothing on user-terminated input being written to file.
Now if you will excuse me, I have a couple of files to delete off of my University's UNIX server, and I cannot specify them by name since they were created with convoluted filenames...
EDIT------
Like I said, I was able to use
scanf("%s", filename);
to get a working filename (without the newline char). But regardless of if I use scanf or fgets for my while loop, if I use them in conjunction with scanf for the filename, I am not able to write anything to file, as it does not enter the while loop.
How should I restructure my writing to file and my while loop?
Your check for the newline is wrong; you're looking at the last character in filename but it may be before that if the user enters a filename that's shorter than the maximum. You're then trying to open a file that has a newline in it's name.
These lines seem to be incorrect:
if (filename[name_length-1] == '\n') // if last char in stream is newline,
{filename[name_length-1] = '\0';} // remove it
You verify the name_length - 1 character,, which is 19 in your case without any regard of the introduced filename's length. So if your file name's length is less then 18 you won't replace the '\n' character at the end of your string. Obviously the file name can't contain '\n' character.
You need to get the size of you file name first with strlen() as an example.
if (filename[strlen(filename) - 1] == '\n')
{
filename[strlen(filename) - 1] = '\0';
}
(Don't forget to include the string.h header)
I hope I was able to help with my weak english.

easy way to parse a text file?

I'm making a load balancer (a very simple one). It looks at how long the user has been idle, and the load on the system to determine if a process can run, and it goes through processes in a round-robin fashion.
All of the data needed to control the processes are stored in a text file.
The file might look like this:
PID=4390 IDLE=0.000000 BUSY=2.000000 USER=2.000000
PID=4397 IDLE=3.000000 BUSY=1.500000 USER=4.000000
PID=4405 IDLE=0.000000 BUSY=2.000000 USER=2.000000
PID=4412 IDLE=0.000000 BUSY=2.000000 USER=2.000000
PID=4420 IDLE=3.000000 BUSY=1.500000 USER=4.000000
This is a university assignment, however parsing the text file isn't supposed to be a big part of it, which means I can use whatever way is the quickest for me to implement.
Entries in this file will be added and removed as processes finish or are added under control.
Any ideas on how to parse this?
Thanks.
Here is a code that will parse your file, and also account for the fact that your file might be unavailable (that is, fopen might fail), or being written while you read it (that is, fscanf might fail). Note that infinite loop, which you might not want to use (that's more pseudo-code than actual code to be copy-pasted in your project, I didn't try to run it). Note also that it might be quite slow given the duration of the sleep there: you might want to use a more advanced approach, that's more sort of a hack.
int pid;
float idle, busy, user;
FILE* fid;
fpos_t pos;
int pos_init = 0;
while (1)
{
// try to open the file
if ((fid = fopen("myfile.txt","rw+")) == NULL)
{
sleep(1); // sleep for a little while, and try again
continue;
}
// reset position in file (if initialized)
if (pos_init)
fsetpos (pFile,&pos);
// read as many line as you can
while (!feof(fid))
{
if (fscanf(fid,"PID=%d IDLE=%f BUSY=%f USER=%f",&pid, &idle, &busy, &user))
{
// found a line that does match this pattern: try again later, the file might be currently written
break;
}
// add here your code processing data
fgetpos (pFile,&pos); // remember current position
pos_init = 1; // position has been initialized
}
fclose(fid);
}
As far as just parsing is concerned, something like this in a loop:
int pid;
float idle, busy, user;
if(fscanf(inputStream, "PID=%d IDLE=%f BUSY=%f USER=%f", %pid, &idle, &busy, &user)!=4)
{
/* handle the error */
}
But as #Blrfl pointed out, the big problem is to avoid mixups when your application is reading the file and the others are writing to it. To solve this problem you should use a lock or something like that; see e.g. the flock syscall.
Use fscanf in a loop. Here's a GNU C tutorial on using fscanf.
/* fscanf example */
#include <stdio.h>
typedef struct lbCfgData {
int pid;
double idle;
double busy;
double user;
} lbCfgData_t ;
int main ()
{
// PID=4390 IDLE=0.000000 BUSY=2.000000 USER=2.000000
lbCfgData_t cfgData[128];
FILE *f;
f = fopen ("myfile.txt","rw+");
for ( int i = 0;
i != 128 // Make sure we don't overflow the array
&& fscanf(f, "PID=%u IDLE=%f BUSY=%f USER=%f", &cfgData[i].pid,
&cfgData[i].idle, &cfgData[i].busy, cfgData[i].user ) != EOF;
i++
);
fclose (f);
return 0;
}

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