Selecting lines which start with a string - c

Question

I want to use this function to show lines from a file that starts with a string.
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include "RangeSelection.h"
bool RangeSelection(char * buffer, int len, char * argv[])
{
int i = 0, j, c;
char word[4];
bool is_break;
while((c = getchar()) != EOF)
{
is_break = false; //a variable for checking if instruction break was used
for(j = 0; j < 3; ++j) //load three first chars
{
word[j] = c;
c = getchar(); //to load chars into c
if(c == '\0' || c == EOF) //if there's less than 3 chars in this word, don't check it
{
is_break = true;
break;
} //if that's EOF or new line, break
}
if(!is_break)
{
word[3] = '\0';
}
if(strcmp(word, argv) == 0) //they're the same
{
printf("%c", word[0]);
printf("%c", word[1]);
printf("%c", word[2]);
do //do while as it will show the output at least three times
{
printf("%c", c); //print that char
}
while((c = getchar()) != '\0' && c != EOF);
}
else
{
while((c = getchar()) != '\0' && c != EOF) //load chars until a new line
{
;
}
}
}
return false;
}
But it doesn't work as I want to. Run with parameter "xxx" it has to show only lines starting with "xxx". For this input:
xxx
dasfdad
xxx works
x
it should print only:
xxx
xxx works
while it prints:
xxx
dasfdad
xxx works
x
However, while the input is like this one:
dasfdad
xxx works
x
It prints nothing although the program ought to show
xxx works
Char * argv[] is an argument provided by the user while running the program in a console(for example xxx). Thanks in advance.

Answer 1

Use memcmp() instead of strcmp() to make life much easier for yourself. The parameter len is never used in the RangeSelection() function. This should be removed unless it is needed for some future development. Unless the string indicated by buffer will be changed in the function, it would be good to use const char *buffer in the parameter list.
Also note that char *argv[] is converted to a pointer to a pointer to char in the function parameter list; I don't think that this is what is expected in the posted code. The function strcmp() expects pointers to char (pointers to the first elements of null-terminated character arrays), so the expression strcmp(word, argv) leads to undefined behavior due to type mismatch in the function call.
Here is an example program:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdbool.h>
bool filter_line(const char * buffer, const char *tag)
{
bool is_tagged;
size_t tag_sz = strlen(tag);
if (strlen(buffer) < tag_sz) {
is_tagged = false;
} else {
is_tagged = !memcmp(buffer, tag, tag_sz);
}
return is_tagged;
}
int main(int argc, char *argv[])
{
if (argc < 2) {
fprintf(stderr, "Usage: %s line_tag\n", argv[0]);
exit(EXIT_FAILURE);
}
const char *filter_tag = argv[1];
char line[4096];
while (fgets(line, sizeof line, stdin) != NULL) {
if (filter_line(line, filter_tag)) {
printf("%s", line);
}
}
return 0;
}
Sample interactions using text files as input:
test_filter_line.txt
xxx
dasfdad
xxx works
x
abc xxx
xxx
λ> ./line_filter xxx < test_filter_line.txt
xxx
xxx works
test_filter_line2.txt:
dasfdad
xxx works
x
λ> ./line_filter xxx < test_filter_line2.txt
xxx works

Answer 2

Your program prints the whole file when it starts with given pattern and prints nothing if it doesn't because you are not matching newline character \n at any time whereas you want to read your file line by line.
I would suggest you split the job into separate functions. This is how I would do it in C99 :
int starts_with(char *buf, char *pattern)
{
char *p = pattern;
for (int i = 0; i < strlen(pattern); i++)
{
if (!buf[i] || buf[i] != pattern[i])
return 0;
}
return 1;
}
Now you can store each line inside a buffer within your RangeSelection function and check if each buffer start with your pattern