Determine if a file contains a string in C

Question

How can I check if a given FILE* contains a string in C running on Linux (if it matters)?

The string must consist of the whole line it's on. For example, this:

jfjfkjj
string
jfjkfjk

would be true; but this:

jffjknf
fklm...string...lflj
jfjkfnj

wouldn't. I'm essentially looking for an internal alternative to system("grep -x file")

while(fgets( ....) != NULL) and remember to remove any trailing newline. — Weather Vane
– Weather Vane, Commented Jun 27, 2018 at 18:14
@WeatherVane I just now saw your comment, that's indeed the code I came up with just now. I know this is somewhat off-topic, but what's the difference between while(fgets( ....) != NULL) and while(!fgets( ....))? — Billy
– Billy, Commented Jun 27, 2018 at 18:40

Billy · Accepted Answer · 2018-06-27 19:04:35Z

0

This reads a file line by line and checks if the line matches the string supplied in argument 1 (argv[1]) after every read. If so, it sets the bool infile (bools defined in <stdbool.h>) to true.

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

int main(int argc, char **argv[]) {
    char    *filepath = "/path/to/file";
    bool    infile = false;
    char    *line = NULL;
    size_t  len = 0;
    ssize_t read;

    FILE    *fp = fopen(filepath, "r");

    if (!fp) {
        fprintf(stderr, "Failed to open %s\n", filepath);
        return 1;
    }

    while ((read = getline(&line, &len, fp)) != -1) {
        line[strcspn(line, "\n")] = 0;
        if (!strcmp(line, argv[1])) {
            infile = true;
            break;
        }
    }
    fclose(uuidfp);

    if (line)
        free(line);

    return 0;
}

answered Jun 27, 2018 at 19:04

Billy

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9 Comments

user3185968 Over a year ago

If you're looking to improve performance, you might want to hash the target line and compare the hash to each line in the file first. You'll still need to do the strcmp() in case of hash collisions, but collisions should be rare, unless you expect adversarial input.

Steve Summit Over a year ago

@EOF I wouldn't expect that'd be worth the trouble.

user3185968 Over a year ago

@SteveSummit I'd expect it depends. An even simpler test would be to use the return value of getline(), which must either be strlen(argv[1]) or strlen(argv[1])+1 for the strings to possibly be equal.

Weather Vane Over a year ago

A reasonable solution, but has no report about whether the string from argv[1] was found in the file. And it has a flaw that you failed to check argc to ensure there is an argv[1] supplied. Moreover, the main argument is incorrect; char **argv[] should be either char *argv[] or char **argv.

chux Over a year ago

@EOF 2nd idea is a good one.

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