I'm trying to do a relatively simple library project in C, but I'm stuck. I'm using fscanf to set a variable (cote which is char[5]) and then fgets to set another variable (titre which is a char[50]). Both of these variables belong to a structure called Ouvrage.
The problem is that fgets seems to add the string it is reading to cote, before the fgets : strlen(o.cote) returns 5 yet afterwards it returns 31.
Here is the code for test.c :
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
typedef struct {
char cote[5];
char titre[50];
} Ouvrage;
void main(void)
{
FILE *flot;
Ouvrage o;
// Opening the file
flot = fopen("ouvrages.don", "r");
if(flot == NULL)
{
printf("Error opening file.\n");
exit(1);
}
// Reading the cote
fscanf(flot, "%s%*c", o.cote);
// Printing the length of cote
printf("STRLEN: %d\t", strlen(o.cote));
// Reading the titre
fgets(o.titre, 50, flot);
o.titre[strlen(o.titre) - 1] = '\0';
// Printing the length of cote again.... Different.
printf("STRLEN: %d\n", strlen(o.cote));
}
And here is the ouvrage.don file :
NJDUI
Charlie et la chocolaterie
ROM
Rhoal Doal
So how is fgets affecting a previous variable and how to stop it ? Any help is greatly appreciated.
Welcome to the world of c-strings. Assuming cote is supposed to be 5 characters long, you actually need to reserve 6 characters in order to have space for the end of string character ('\0'); what happened was that fscanf wrote that string character as the first character of titre,but then fgets overwrote that, which is why you saw what you saw. Also note that as far as I'm concerned, scanf and related is the tool of the devil; why not just use fgets (which allows you to specify the maximum number of bytes to read [read closely the manpage to avoid an off by one]) for both reads?
Related
Here's the file I want to read.
single
splash
single
V-Line
h-line
Macro for checking if string is equal.
#define STR_MATCH(a,b) (strncmp((a),(b),strlen(b)+1) == 0)
Here's what i'm using to read it.
void readMissilesFile(char* fileName)
{
FILE* mFile;
char missile[7];
/* Open the file. */
mFile = fopen(fileName, "r");
if (mFile != NULL)
{
while (!feof(mFile))
{
fgets(missile, 7, mFile);
if (!(STR_MATCH(missile, "\n")))
{
printf("Missile: %s", missile);
}
}
fclose(mFile);
}
else
{
perror("Could not open the file.");
}
}
So i'm having difficulties as its printing out spaces when I read the line. I tried to ignore this by ensuring it only reads 7 characters which is the max length of each missile. Then I made a macro called strcmp which just checks if they are equal(to hopefully not print it).
Please find the macro attached as well.
Thanks in advance and any help is great. :)
If I understand your question correctly you can replace the newline characters by using strcspn.
You should not use feof like this, this post explains why. A safe way to read the file till the end is to use fgets as stop condition in the while loop.
The container, missile should be one char bigger than the max size of the largest string to accomodate for '\0'.
Live sample
#include <string.h>
//...
char missile[10];
//...
if (mFile != NULL)
{
while (fgets(missile, 10, mFile)) //will read till there are no more lines
{
missile[strcspn(missile, "\r\n")] = '\0'; //remove newline characters
printf("Missile: %s ", missile);
}
}
//...
I would advise the reading of this post which has detailed info about fgets, namely the issue of newline characters consumption.
There is getline function in stdio.h which reads line until delimiter. Its a POSIX though, so if you are on Windows you may lack it.
Here is example implementation:
https://github.com/ivanrad/getline/blob/master/getline.c
I have seen programs for file handling and in one of the program using fseek as shown below:
/* This example opens a file myfile.dat for reading.
After performing input operations (not shown), it moves the file
pointer to the beginning of the file.
*/
#include <stdio.h>
int main(void)
{
FILE *stream;
int result;
if (stream = fopen("myfile.dat", "r"))
{ /* successful */
if (fseek(stream, 0L, SEEK_SET)); /* moves pointer to */
/* the beginning of the file */
{ /* if not equal to 0
then error ... */
}
else {
/* fseek() successful */
}
}
Like this can one move the file pointer to the next line immediately after that line
BO_ 377 FC_DM_MISC: 8 FC
SG_ DATA3 m11 : 31|8#0+ (1,0) [0|0] "" DM
These are the two lines and I want to program in a way that when one identifies the number 377 the pointer should now go to the next line i.e., to the line SG_ DATA3 inspite of the white spaces after 8 FC. How can one do that using fseek in C?
Try this code . It may help you .Here the Each line of the Input file is converted to string ,since string manipulation is very simple comparing to complex fseek() function.This may not be perfect answer but this will be very simple solution.
#include <stdio.h>
#include <string.h>
int main(void)
{
FILE *stream;
int result;
char tmp[100]; // assuming that max length of a line in myfile.dat is 100.
if (stream = fopen("myfile.dat", "r"))
{ /* successful */
fscanf(stream, "%100[^\n]", tmp); // assuming that max length of a line in myfile.dat is 100.
printf("%s", tmp);
if (strstr(tmp, "377"))
{ // check for 337
fscanf(stream, "%100[^\n]", tmp); // next line is in the string tmp .
// continue your program.
//printf("%s", tmp);
}
}
}
fseek is used for binary data, if you work on a text file you should use either fgets or getline(recommended to use getline).
There's an open discussion of "fgets() vs getline" and many say that "fgets is deprecated" is only a gcc propaganda in favor to their specific getline().
A possible flaw in fgets() is that it doesn't tell you anything if there are null bytes being read, something you can get away with getline().
But then again if you don't like gcc, or use something different, use fgets(). If you are stuck with gcc, then use getline().
I'm trying to read a txt file containing strings of 1s and 0s and print it out in the manner below. I tried my code a couple of months ago and it worked fine in reading the text file. Now when I tried it, it outputs something really strange. Also I tried changing the directory of the file to a non-existant file but it still outputs the same thing when it should've quit the program immediately. Please help!
The content of txt file:-
10000001
01110111
01111111
01111010
01111010
01110111
Expected output:-
data_in<=24'b10000001;
#10000;
Real output:-
data_in<=24'b(some weird symbol that changes everytime I recompile);
#10000;
My code:-
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main (int argc, char *argv[])
{
int i, j;
j = 0;
char words[50];
FILE *fp;
fp = fopen (argv[1], "r");
if (fp == NULL) {
printf ("Can't open file\n");
}
while (feof (fp) == 0) {
fscanf (fp, "%s", words);
printf ("data_in<=24'b%s\n", words);
printf ("#10000\n");
}
fclose (fp);
system ("PAUSE");
return 0;
}
The input argument is the following:-
"C:\Users\Beanz\Documents\MATLAB\football frame\frame1.txt"
Read each line one by one with getline(3) -if available- or with fgets (you'll then need a large enough line buffer, at least 256 bytes), then parse each line buffer appropriately, using sscanf (the %n might be useful, and you should test the scanned item count result of sscanf) or other functions (e.g. strtok, strtol, etc...)
Remember that 'feof()' is only set AFTER trying to read PAST the end of the file, not when at the end of the file.
So the final iteration through the loop will try to read/process data that contains trash or prior contents.
Always check the returned value from 'fscanf()' before trying to use the associated data.
strongly suggest
eliminate the call to feof() and use the fscanf() to control the loop
So im working on learning how to do file I/O, but the book I'm using is terrible at teaching how to receive input from a file. Below is is their example of how to receive input from a file, but it doesn't work. I have copied it word for word, and it should loop through a list of names until it reaches the end of the file( or so they say in the book), but it doesn't. In fact if I leave the while loop in there, it doesn't print anything.
#include <stdio.h>
#include <conio.h>
#define MAX 250
int main()
{
char name[MAX];
FILE*pRead;
pRead=fopen("test.txt", "r");
if (pRead==NULL)
{
printf("file cannot be opened");
}else
printf("contents of test.txt");
while(fgets(name,sizeof(name),pRead)!=NULL){
{
printf("%s\n",name);
fscanf(pRead, "%s", name);
}
getch();
}
Even online, every beginners tutorial I see does some variation of this, but I can't seem to get it to work even a little bit.
I believe your array is too small and therefore when you are reading fscanf overwrites memory causing bizarre behavior
If you just want to read the file - presuming now there is one name per line followed by newline in the input file - just read the file using fgets() instead.
#define MAXLINE 256
char name[MAXLINE];
while (fgets(name,sizeof(name),pRead)!=NULL)
{
// do whatever
}
I know this is a dumb question, but how would I load data from a multiline text file?
while (!feof(in)) {
fscanf(in,"%s %s %s \n",string1,string2,string3);
}
^^This is how I load data from a single line, and it works fine. I just have no clue how to load the same data from the second and third lines.
Again, I realize this is probably a dumb question.
Edit: Problem not solved. I have no idea how to read text from a file that's not on the first line. How would I do this? Sorry for the stupid question.
Try something like:
/edited/
char line[512]; // or however large you think these lines will be
in = fopen ("multilinefile.txt", "rt"); /* open the file for reading */
/* "rt" means open the file for reading text */
int cur_line = 0;
while(fgets(line, 512, in) != NULL) {
if (cur_line == 2) { // 3rd line
/* get a line, up to 512 chars from in. done if NULL */
sscanf (line, "%s %s %s \n",string1,string2,string3);
// now you should store or manipulate those strings
break;
}
cur_line++;
}
fclose(in); /* close the file */
or maybe even...
char line[512];
in = fopen ("multilinefile.txt", "rt"); /* open the file for reading */
fgets(line, 512, in); // throw out line one
fgets(line, 512, in); // on line 2
sscanf (line, "%s %s %s \n",string1,string2,string3); // line 2 is loaded into 'line'
// do stuff with line 2
fgets(line, 512, in); // on line 3
sscanf (line, "%s %s %s \n",string1,string2,string3); // line 3 is loaded into 'line'
// do stuff with line 3
fclose(in); // close file
Putting \n in a scanf format string has no different effect from a space. You should use fgets to get the line, then sscanf on the string itself.
This also allows for easier error recovery. If it were just a matter of matching the newline, you could use "%*[ \t]%*1[\n]" instead of " \n" at the end of the string. You should probably use %*[ \t] in place of all your spaces in that case, and check the return value from fscanf. Using fscanf directly on input is very difficult to get right (what happens if there are four words on a line? what happens if there are only two?) and I would recommend the fgets/sscanf solution.
Also, as Delan Azabani mentioned... it's not clear from this fragment whether you're not already doing so, but you have to either define space [e.g. in a large array or some dynamic structure with malloc] to store the entire dataset, or do all your processing inside the loop.
You should also be specifying how much space is available for each string in the format specifier. %s by itself in scanf is always a bug and may be a security vulnerability.
First off, you don't use feof() like that...it shows a probable Pascal background, either in your past or in your teacher's past.
For reading lines, you are best off using either POSIX 2008 (Linux) getline() or standard C fgets(). Either way, you try reading the line with the function, and stop when it indicates EOF:
while (fgets(buffer, sizeof(buffer), fp) != 0)
{
...use the line of data in buffer...
}
char *bufptr = 0;
size_t buflen = 0;
while (getline(&bufptr, &buflen, fp) != -1)
{
...use the line of data in bufptr...
}
free(bufptr);
To read multiple lines, you need to decide whether you need previous lines available as well. If not, a single string (character array) will do. If you need the previous lines, then you need to read into an array, possibly an array of dynamically allocated pointers.
Every time you call fscanf, it reads more values. The problem you have right now is that you're re-reading each line into the same variables, so in the end, the three variables have the last line's values. Try creating an array or other structure that can hold all the values you need.
The best way to do this is to use a two dimensional array and and just write each line into each element of the array. Here is an example reading from a .txt file of the poem Ozymandias:
int main() {
char line[15][255];
FILE * fpointer = fopen("ozymandias.txt", "rt");
for (int a = 0; a < 15; a++) {
fgets(line[a], 255, fpointer);
}
for (int b = 0; b < 15; b++) {
printf("%s", line[b]);
}
return 0;
This produces the poem output. Notice that the poem is 14 lines long, it is more difficult to print out a file whose length you do not know because reading a blank line will produce the output "x�oA". Another issue is if you check if the next line is null by writing
while (fgets(....) != NULL)) {
each line will be skipped. You could try going back a line each time to solve this but i think this solution is fine for all intents.
I have an even EASIER solution with no confusing snippets of puzzling methods (no offense to the above stated) here it is:
#include <iostream>
#include <fstream>
#include <string>
using namespace std;
int main()
{
string line;//read the line
ifstream myfile ("MainMenu.txt"); // make sure to put this inside the project folder with all your .h and .cpp files
if (myfile.is_open())
{
while ( myfile.good() )
{
getline (myfile,line);
cout << line << endl;
}
myfile.close();
}
else cout << "Unable to open file";
return 0;
}
Happy coding