Reading line by line from a file in C - c

Question

What I am trying to do is print out the contents of a file line by line. I run the program in terminal by doing: ./test testText.txt. When I do this, random characters are printed out but not what is in the file. The text file is located in the same folder as the makefile. What's wrong?
#include <stdio.h>
FILE *fp;
int main(int argc, char *argv[])
{
char line[15];
fp = fopen(*argv, "r");
while((fgets(line, 15, fp)) != NULL)
{
printf(line);
printf("\n");
}
}

Answer 1

When I do this, random characters are printed out but not what is in the file
These characters are not random, and in fact they are coming from a file. It's not the file that you are trying to read, though - it's the executable file which you are running.
*argv represents the name of the executable; add this line to see what's in *argv:
printf("%s\n", *argv);
The actual command line arguments start at argv[1], so you need
fp = fopen(argv[1], "r");

Answer 2

The first argument passed on the command line is at argv[1], while *argv refers to argv[0]. argv[0] contains the filename of the executable - you are printing out the content of the executable.
The following code prints out the entire argv[] array, then reads your file and prints it.
#include <stdio.h>
int main( int argc, char *argv[] )
{
for( int i = 0; i < argc; i++ )
{
printf( "argv[%d] : %s\n", i, argv[i] ) ;
}
if( argc >= 2 )
{
FILE* fp = fopen( argv[1], "r" ) ;
if( fp != NULL )
{
char line[15];
while( fgets( line, sizeof(line), fp ) != NULL )
{
printf( "%s", line ) ;
}
}
}
return 0 ;
}
Note that fgets() will read an entire line including the , so there is no need to print '\n', especially because with only 15 characters, your line buffer may well not contain an entire line. Note also the tighter localisation of variables - your code needlessly made fp global.
Other refinements are the safe use of the array size rather than literal 15, and the use of a literal constant string for the format specifier. You should avoid passing a variable string for the printf() format string - if your input itself contains format specifiers, printf() will try to read data from arguments that do not exist with undefined results.

Answer 3

Q: What's wrong?
A humble critique:
#include <stdio.h>
FILE *fp; // Perhaps this should be declared inside main?
int main(int argc, char *argv[])
{
char line[15]; // Are the file lines all 14 characters or less? (seems small)
fp = fopen(*argv, "r"); // Opening the binary executable file (argv[0])? Intereting.
// Should check here to ensure that fopen() succeeded.
while((fgets(line, 15, fp)) != NULL)
OK... well, remember that this isn't a text file.. it's an executable (due to *argv). This will read some wacky (but not random) characters from the executable.
{
printf(line); // Bad practice. Should be: printf("%s", line);
Ok... now print the wacky characters?
printf("\n"); // Redundant. The '\n' characters will be supplied in 'line'.
}
// fclose() call missing.
// Integer return value for main() is missing.
}
Here is (perhaps) what was actually intended:
#include <stdio.h>
#include <errno.h>
int main(int argc, char *argv[])
{
int rCode = 0;
FILE *fp = NULL;
char line[255+1];
if(argc != 2)
{
printf("Usage: %s {filepath}\n", *argv);
goto CLEANUP;
}
errno=0;
fp = fopen(argv[1], "r");
if(NULL == fp)
{
rCode=errno;
fprintf(stderr, "fopen() failed. errno:%d\n", rCode);
goto CLEANUP;
}
while(fgets(line, sizeof(line), fp)) /* --As per 'chux' comment */
printf("%s", line);
CLEANUP:
if(fp)
fclose(fp);
return(rCode);
}
Or, if the intent is truly to print the content of the executable, perhaps this:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <errno.h>
int main(int argc, char *argv[])
{
int rCode = 0;
FILE *fp = NULL;
off_t offset = 0;
errno=0;
fp = fopen(*argv, "r");
if(NULL == fp)
{
rCode=errno;
fprintf(stderr, "fopen() failed. errno:%d\n", rCode);
goto CLEANUP;
}
for(;;)
{
char line[16];
size_t bytesRead;
int index;
char ascii[16+1];
memset(ascii, 0, sizeof(ascii));
bytesRead = fread(line, 1, sizeof(line), fp);
if(0==bytesRead)
break;
printf(" %08zX | ", offset);
for(index=0; index < bytesRead; ++index)
{
printf("%02hhX%c", line[index], 7==index ? '-' : ' ');
ascii[index] = isprint(line[index]) ? line[index] : '.';
}
printf("%*s %s\n", (16 -index) * 3, "", ascii);
offset += bytesRead;
}
if(errno)
{
rCode=errno;
fprintf(stderr, "fgets() failed. errno:%d\n", errno);
}
CLEANUP:
if(fp)
fclose(fp);
return(rCode);
}

Answer 4

your file name found at index 1 of argv.
if (argc <= 1) {
printf("no file was given\n");
exit(-1);
}
// open file from argv[1]
// ...