I want to read lines of numbers from the text file (filename.txt) using a function in C.
How do I open this file (provided the filename is only given through a redirection on Unix)?
i.e. ./cfile < filename.txt
int main (void)
{
char filename[20];
fgets(filename, 19, stdin);
FILE *fp;
fp = fopen(filename, "r");
}
So, would this be correct; also, how do I access one line at a time from the file (all I know is EOF has to be used somewhere)?
The contents of filename.txt gets redirected to the standard input of the executable. Therefore you could simply write your code as follows:
#include <stdio.h>
#define MAXLEN 256 // Maximum number of characters in a line
int main() {
char line[MAXLEN];
while (fgets(line, MAXLEN, stdin)) {
printf("Line %s", line); // Do something with the line
}
return 0;
}
< is used to redirect the standard input from a file instead of the keyboard, in this case you don't need fopen:
int main(void)
{
char buffer[100];
/* Don't use magic numbers with fgets, sizeof buffer is the correct size */
while (fgets(buffer, sizeof buffer, stdin)) {
printf("%s", buffer);
}
return 0;
}
You can fopen a file using an argument passed to main:
int main(int argc, char *argv[])
{
char buffer[100];
FILE *fp;
if (argc == 2) {
fp = fopen(argv[1], "r");
if (fp == NULL) {
perror("fopen");
exit(EXIT_FAILURE);
}
while (fgets(buffer, sizeof buffer, fp)) {
printf("%s", buffer);
}
fclose(fp);
}
return 0;
}
Launch it using ./cfile filename.txt (without the redirection).
On Linux, when you run a program, 3 files are opened for the standard input (fd 0), output (fd 1) and error (fd 2). By default, these files are your terminal :
% cat &
% ls -l /proc/`pidof cat`/fd
total 0
lrwx------ 1 max max 64 mars 21 10:34 0 -> /dev/pts/0
lrwx------ 1 max max 64 mars 21 10:34 1 -> /dev/pts/0
lrwx------ 1 max max 64 mars 21 10:34 2 -> /dev/pts/0
But you can also specify the file that should be used with <, > and 2> :
% cat > /dev/null &
% ls -l /proc/`pidof cat`/fd
total 0
lrwx------ 1 max max 64 mars 21 10:36 0 -> /dev/pts/0
l-wx------ 1 max max 64 mars 21 10:36 1 -> /dev/null
lrwx------ 1 max max 64 mars 21 10:36 2 -> /dev/pts/0
So, the file is already opened and is respectively in the FILE* stdin, stdout and stderr. For this last example it's in stdout but in your case (with <) it will be in the FILE* stdin.
So basically, you don't need to do the fopen, and you can use stdin.
