How to open text file and read it in C?

Text Files: Opening

You use fopen to open a file. It opens a file for a specified mode (the three most common are r, w, and a, for read, write, and append). It then returns a file pointer that you use to access the file. For example,


suppose you want to open a file and write the numbers 1 to 10 in it. You could use the following code:

#include 
#define MAX 10

int main()
{
   FILE *f;
   int x;
   f=fopen("out","w");
   if (!f)
       return 1;
   for(x=1; x<=MAX; x++)
       fprintf(f,"%d\n",x);
   fclose(f);
   return 0;
}

The fopen statement here opens a file named out with the w mode. This is a destructive write mode, which means that if out does not exist it is created, but if it does exist it is destroyed and a new file is created in its place. The fopen command returns a pointer to the file, which is stored in the variable f. This variable is used to refer to the file. If the file cannot be opened for some reason, f will contain NULL.

Main Function Return Values This program is the first program in this series that returns an error value from the main program. If the fopen command fails, f will contain a NULL value (a zero). We test for that error with the if statement. The if statement looks at the True/False value of the variable f. Remember that in C, 0 is False and anything else is true. So if there were an error opening the file, f would contain zero, which is False. The ! is the NOT operator. It inverts a Boolean value. So the if statement could have been written like this: if (f == 0) That is equivalent. However, if (!f) is more common. If there is a file error, we return a 1 from the main function. In UNIX, you can actually test for this value on the command line. See the shell documentation for details.

The fprintf statement should look very familiar: It is just like printf but uses the file pointer as its first parameter. The fclose statement closes the file when you are done.

Text Files: Reading

To read a file, open it with r mode. In general, it is not a good idea to use fscanf for reading: Unless the file is perfectly formatted, fscanf will not handle it correctly. Instead, use fgets to read in each line and then parse out the pieces you need.

The following code demonstrates the process of reading a file and dumping its contents to the screen:

#include 

int main()
{
   FILE *f;
   char s[1000];

   f=fopen("infile","r");
   if (!f)
       return 1;
   while (fgets(s,1000,f)!=NULL)
       printf("%s",s);
   fclose(f);
   return 0;
}

The fgets statement returns a NULL value at the end-of-file marker. It reads a line (up to 1,000 characters in this case) and then prints it to stdout. Notice that the printf statement does not include \n in the format string, because fgets adds \n to the end of each line it reads. Thus, you can tell if a line is not complete in the event that it overflows the maximum line length specified in the second parameter to fgets.

C Errors to Avoid Do not accidentally type close instead of fclose. The close function exists, so the compiler accepts it. It will even appear to work if the program only opens or closes a few files. However, if the program opens and closes a file in a loop, it will eventually run out of available file handles and/or memory space and crash, because close is not closing the files correctly.