c中的strstr函数和多维数组

Question

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

void find_track(char *search_for);
char tracks[][80] = {
    "I left my heart in Harvard Med School",
    "Newark, Newark - a wonderful town",
    "Dancing with a Dork",
    "From here to maternity",
    "The girl from Iwo Jima"
};

int main() {
    char *to_search_str;
    printf("Search for: ");
    fgets(to_search_str, 80, stdin);
    find_track(to_search_str);
    return 0;
}

void find_track(char *search_for) {
    int i;
    for (i=0; i<5; i++) {
        if (strstr(tracks[i], search_for)) {
            printf("Track %d: '%s'\n", i, tracks[i]);
        }
    }
}

The program is supposed to search for a string in every string in the tracks multi dimensional array but the strstr() function in the find_track is always returning null no matter the input (even if we input the a sub string of a string from tracks multi dimensional array). I don't know why this is happening?

EDIT : After correction

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

void find_track(char *search_for);
char tracks[][80] = {
    "I left my heart in Harvard Med School",
    "Newark, Newark - a wonderful town",
    "Dancing with a Dork",
    "From here to maternity",
    "The girl from Iwo Jima"
};

int main() {
    char to_search_str[80];
    printf("Search for: ");
    fgets(to_search_str, 80, stdin);
    to_search_str[strlen(to_search_str)-1] = '\0';
    find_track(to_search_str);
    return 0;
}

void find_track(char *search_for) {
    int i;
    for (i=0; i<5; i++) {
        if (strstr(tracks[i], search_for)) {
            printf("Track %d: '%s'\n", i, tracks[i]);
        }
    }
}

Output

Answer 1

Most likely the issue with input via fgets() .

• You are reading into an uninitialized pointer to_search_str , which does not point to a valid memory location. In this case, you can simply change this to an array, like char to_search_str[80] = {0}; and get done with it.

• You need to trim the trailing newline that's stored in the input buffer.

  From the man page , ( emphasis mine )

fgets() reads in at most one less than size characters from stream and stores them into the buffer pointed to by s. Reading stops after an EOF or a newline. If a newline is read, it is stored into the buffer. A terminating null byte ( '\\0' ) is stored after the last character in the buffer.

A quick way of getting that done is to_search_str[strcspn(to_search_str, "\\n")] = 0; , but there are more robust ways mentioned in this other answer

Answer 2

You aren't allocating to_search_str pointer, the char * pointer you pass to fgets as the destination buffer. Being it actually uninitialized, this causes undefined behavior that normally ends with a program crash.

You just need to allocate it, statically or dynamically.

The simplest solution consists in just defining a static array in the stack:

#include <string.h>

#define LEN 80

int main() {
    char to_search_str[LEN];

    printf("Search for: ");
    fgets(to_search_str, LEN, stdin);

    /* Remove trailing newline from the string to search */
    to_search_str[strcspn(to_search_str, "\n")] = 0;

    find_track(to_search_str);
    return 0;
}

The size of the array is 80 because you use this number as the size parameter in fgets . Please note the use of a #define for the constant 80, making possible to change it in a easier way.

The dynamic allocation in the heap involves the use of malloc() function (and free() as soon as the array is not needed anymore):

#include <string.h>

#define LEN 80

int main() {
    char * to_search_str = malloc(LEN);

    printf("Search for: ");
    fgets(to_search_str, LEN, stdin);

    /* Remove trailing newline from the string to search */
    to_search_str[strcspn(to_search_str, "\n")] = 0;

    find_track(to_search_str);

    free(to_search_str);
    return 0;
}

Note: since fgets retains trailing newline ``\\n'` in the output buffer, we have to remove it. I used the clever oneliner solution described here .

Answer 3

char *to_search_str; is an uninitialized pointer, writing to it will result in undefined behavior. You have to allocate memory or use an array instead char to_search_str[100]; for example.

Also don't forget that fgets will also read the newline into the buffer, which you have to remove.

Answer 4

This code snippet in main

char *to_search_str;
printf("Search for: ");
fgets(to_search_str, 80, stdin);

invokes undefined behavior because the pointer to_search_str is not initialized and has indeterminate value.

It seems you at least mean

char to_search_str[80];
printf("Search for: ");
fgets(to_search_str, 80, stdin);

The function fgets can append the new line character '\\n' to the entered string.

You need to remove it for example the following way

to_search_str[ strcspn( to_search_str, "\n" ) ] = '\0';

The function find_track should be declared at least like

void find_track( const char *search_for);

Though it is a bad idea when a function definition relies on global variables.

Also the approach of finding relevant strings is not good. For example the user can enter a string that contains only one character 'a' . In this case all records will satisfy the condition. You should check that the searched string forms a word (a sequence of characters separated by spaces) in a string in the array.