Line by line read in C

Question

I have such task: I have to find some piece of text in file, but I don't know exactly a name of the file or smth, my teacher will check my program that way: ./program 12345 < file.txt, where 12345 - piece of text the program should find, file.txt - file where the program should find it. And I have a problem with reading the file. As far as I understand it should be smth like line = fgets(text, 100, stdin). and then I'll check if the line has the needed piece of text in this line with argv[]. But I don't know how to use fgets correctly to check EACH line. I tried to make a while-loop but it reads only the first line. Can somebody explain me how can I do that? It's forbidden to work with dynamic memory (malloc, free, etc). Thanks in advance.

Answer 1

The function you are looking for is strstr, found in <string.h>.

Defined as:

char *strstr(const char *str, const char *substr);

Finds the first occurrence of the null-terminated byte string pointed to by substr in the null-terminated byte string pointed to by str. The terminating null characters are not compared.

Combine this with fgets in a loop.

A cursory example of its use:

#include <stdio.h>
#include <string.h>

int main(int argc, char **argv)
{
    if (argc < 2) {
        fprintf(stderr, "usage: %s STRING\n"
                "\t-- Searches standard input for STRING.\n",
                argv[0]);
        return 1;
    }

    char line[512];
    unsigned line_no = 1;

    while (fgets(line, sizeof line, stdin)) {
        if (strstr(line, argv[1])) {
            printf("%u: %s", line_no, line);
        }

        line_no++;
    }
}

Running this program on its own source code, ./a.out line < find.c:

13:     char line[512];
14:     unsigned line_no = 1;
16:     while (fgets(line, sizeof line, stdin)) {
17:         if (strstr(line, argv[1])) {
18:             printf("%u: %s", line_no, line);
21:         line_no++;