Need help with opening and reading a txt file in C

[technmom]

I'm learning C and will shortly be going on to C++. I need to open a file and read the line then check for a character. For now I just want to print the line out. I get a segmentation error when running. I'm brand new at C. I have really old old programming experience ( a lot of it but very long ago).

Here is my code (don't laugh I'm learning):


#include <stdio.h>
#include <string.h>


int main()

{

int i;
int length;
char line [128];
char fname [128];

// Ask for filename of text file
printf("Enter c program filename: ");
FILE *fr = 0;
fgets (fname, 80, stdin);


//Open file

fr = fopen(fname, "r");
printf(" %s ", fname);

while ( fgets(line, sizeof(line), fr) != NULL)
printf("%s\n", line);


return 0;


}

[soltanis]

You don't check to see if you opened the file successfully after the fopen call:



if (!(fr = fopen(fname, "r"))) {
perror("fopen");
exit(1);
}


Trying to read from a file pointer that is NULL will probably cause a segfault.

[technmom]

Okay so I do get an error no such file or directory. Did I say I'm also new to Linux and the file paths.
I am trying to open the same file that I am compiling or another one in the same directory as the executable.
I suppose I need to enter the entire file path.

This brings me to another question. I have these c programs in my home directory because I don't really know the commands to navigate around and to tell terminal what to look for. I would like to organize them in another directory.

Bye the way exit gave me a compile warning:
mp1.c:26: warning: incompatible implicit declaration of built-in function ‘exit’

[dwhitney67]

soltanis is correct in that you should check to see if you successfully opened the file before attempting to read from it.

When passing a file name, you need to pass the full path leading to the file; otherwise fopen() will assume the file is in the current directory.

If you do not already have the man-pages on your system, you need to install them; the reason is so that you do not scratch your head when GCC report something trivial like exit() has not been defined. The man-pages will provide you with the information necessary to know that the header file stdlib.h needs to be included.


sudo apt-get install manpages-dev

man 3 exit # the 3 is used to indicate which section of the man-pages


The man-pages will also provide information on fgets(). It would tell you that it will stop reading the stdin input either when the number of characters specified have been reached OR a newline character is read. The last part concerning the newline is important.

You have fname declared as an array of 128 characters; from your code, you are only attempting to read 80 characters (odd, but whatever). If you enter "mp1.c", then your buffer is going to have the following in it: "mp1.c\n"

The call to fopen() will fail each and everytime because it will never find a file called "mp1.c\n". You, as the programmer, need to remove the newline from fname. An easy way to do this is as follows:


...

printf("Enter C program filename: ");
fgets(fname, sizeof(fname), stdin);

/* remove the newline if it exists */
char* nl = strchr(fname, '\n');

if (nl != NULL)
{
*nl = '\0';
}

...

/* open the file */
FILE* fr = fopen(fname, "r");

if (fr != NULL)
{
/* proceed to read from the file. */
}
else
{
/* error */
}

[technmom]

Thanks I'll try that......I do have man pages just did not think to use it. I just threw the 80 in there don't ask why???

[technmom]

Thanks that worked and I learned a little more C today

[trent.josephsen]

I'm learning C
Okay.

and will shortly be going on to C++.
Why? If you're still wrestling with these basic principles in C (no offense), you don't need to be jumping to another language any time soon. Unless it's just because you decided C was too hard and wanted to go with something simpler (which is perfectly understandable, but C++ would not be your best option in that case).

C and C++ don't share much besides lineage. If you want to program in C, learn C; if you want to program in C++, learn C++. But don't learn half of C and go on to supplement it with half of C++; you'll just end up writing badly designed C programs in C++, or vice versa. Learn to write in a language idiomatically before using it in Real Life.

$0.02 :)

[technmom]

Thanks for you input, but I am learning with some other folks. Our main objective is to learn C++ but the instructor is having us do some C. It is not a class for credit but to learn for using in the near future. So I'm not driving the path but have plenty of holes.

[dwhitney67]

Thanks for you input, but I am learning with some other folks. Our main objective is to learn C++ but the instructor is having us do some C. It is not a class for credit but to learn for using in the near future. So I'm not driving the path but have plenty of holes.

Unfortunately, your instructor is worth <snip>. If his/hers goal is too teach C++, then the STL (o)fsream, along with the STL string, would have veen of beter use.



std::string fname
std::cout << "Enter filename: ";
std::getline(fname, std::cin);

std::fstream file(fname.c_str(), std::ios::in);
...

[technmom]

Maybe you are right about a better way. He didn't suggest anything so what you see is my stumbling effort to do the task nothing from him. No need for criticism.