c read file from command line

#include<stdio.h>
int main(){
	FILE *in=fopen("name_of_file.txt","r");
	char c;
	while((c=fgetc(in))!=EOF)
		putchar(c);
	fclose(in);
	return 0;
}

#include <stdio.h>

int main() {
  	char contents[3000];
	FILE *fp = fopen("filename.txt", "r");
	fscanf(fp, "%s", contents);
  	printf(contents);
}

You can use as your main function:
int main(int argc, char **argv) 

So, if you entered to run your program:
C:\myprogram myfile.txt

argc will be 2
argv[0] will be myprogram
argv[1] will be myfile.txt

To read the file:
FILE *f = fopen(argv[1], "r");

#include <stdio.h>

int main(int argc, char **argv) {
    for (int i = 0; i < argc; ++i) {
        printf("argv[%d]: %s\n", i, argv[i]);
    }
}

/*
	[birryree@lilun c_code]$ ./a.out hello there
	argv[0]: ./a.out
	argv[1]: hello
	argv[2]: there
*/