C programming: Returning string from a function - c

Question

I am tring to print compose a function can get the specific line from a file, like string=extract_line(2), then the string will be the content of the 2nd line of a file.
but when I tried to put the function in a head file, I got Segmentation fault.
Here is my code:
test.c:
#include "extract_line.h"
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char const *argv[])
{
extract_line(2);
extract_line(3);
printf("%s\n", str);
return 0;
}
extract_line.h
#include <stdio.h>
#include <stdlib.h>
#define TEMP_PATH "/home/pi/project/PCD_8544_screen/show_on_LCD/network_speed.txt"
static char str[256];
void extract_line( int Tgt_Line )
{
FILE *fp;
fp=fopen(TEMP_PATH, "r");
// char str[256];
char holder;
int line=0;
while((holder=fgetc(fp)) != EOF) {
if(holder == '\n') line++;
if(line == Tgt_Line) break; /* 1 because count start from 0,you know */
}
if(holder == EOF) {
printf("%s doesn't have the 2nd line\n", fp);
//error:there's no a 2nd
}
int i=0;
while((holder=fgetc(fp)) != EOF && (holder != '\n' )) {
// putchar(holder);
str[i] = holder;
i++;
}
// printf("%s\n",str);
fclose(fp);
// return str;
}

Answer 1

In the following line of code:
printf("%s doesn't have the 2nd line\n", fp);
You're printing a FILE * as though it's expected to point at a string (filename?). As printf expects a char *, this is likely to cause irrational behaviour. Note that your compiler is probably SCREAMING at you about this! Perhaps you meant:
printf("%s doesn't have the 2nd line\n", TEMP_PATH);
Additionally, there's a possibility for a null pointer dereference which would also cause irrational behaviour between the following two lines:
fp=fopen(TEMP_PATH, "r");
// snip! you need to check fp here!
while((holder=fgetc(fp)) != EOF) {
On that note, your loop is infinite on systems where char is an unsigned type, as EOF is negative which can't possibly compare equal to any unsigned values. As fgetc returns an int with an unsigned char within it and holder is a char, the downward conversion discards the information required for your loop to terminate. holder should be defined as an int, instead.
As there's no upper bound on how many characters are read set by your loop, it's possible that str may be accessed (and assigned) out of bounds in str[i] = holder;. This could cause irrational behaviour.
Additionally, it appears there's no explicit assignment of a '\0' string terminator. As str is defined with static storage duration (not to be confused with your static keyword, which provides internal linkage), that won't really cause irrational behaviour, in fact it's quite predictable, but your output might not be as you expect if the first call reads a line longer than the second call.
Finally, just a few nitpick notes:
I suggest using size_t rather than int for variables which are expected to count or refer to array elements (e.g. int i=0; should be size_t i=0;). This is to make it explicit that negative numbers aren't acceptable, which might also speed your code up a little.
It's a little bit strange to put code in header files. Usually definitions only go into header files, which we #include, and code goes into code files, which we link to (e.g. gcc -o x.c compiles x.c, then gcc main.c x.o compiles main.c and links it with x.o).

Answer 2

You need to have a main() function or you need to run the script from within any other function..
#include "extract_line.h"
#include <stdio.h>
#include <stdlib.h>
void main() {
extract_line(2);
extract_line(3);
printf("%s\n", str);
}