Why does gets() assuming extern returning int in this code in C?

Question

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

int main() {
    int length;
    char **lines, buffer[80];

    printf("How many lines do you want to enter?.\n");
    scanf("%d", &length);
    getchar();
    lines = (char**)malloc(length * sizeof(char*));
    for (i = 0; i < length; ++i) { //Scan user's lines
        gets(buffer);
        lines[i] = (char*)malloc(strlen(buffer) * sizeof(char) + 1);
        if (!lines[i]) { //Check if allocation available
            printf("Error! Out of memory!");
            return endProgram;
        }
        strcpy(lines[i], buffer);
    }
    return 0;
}

I'd like to know why does the gets() function giving me an error about assuming extern returning int, I'm just trying to scan a string.

Answer 1

The most likely reason is that gets() function has been deprecated for some time. It looks like it has been removed from <stdio.h> on your system.

Fortunately, this situation is easy to fix: replace the call with fgets(), like this:

fgets(buffer, sizeof(buffer), stdin);

Note: fgets is not a drop-in replacement for gets, because it keeps '\n' character at the end of line inside the string that it returns. This Q&A discusses the issue, and provides a solution.

Answer 2

The gets() function has been removed from the C Standard. It was never a good function to use because there is no way to tell gets how large the argument buffer is, making it very easy to cause buffer overflow or even malicious attacks if the input file is from an untrusted source. The reason your compiler complains about assuming extern returning int is that the prototype for gets is no longer present in your <stdio.h> system header.

Modify your code this way:

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

int main(void) {
    int length;
    char **lines, buffer[81];

    printf("How many lines do you want to enter?.\n");
    if (scanf("%d", &length) != 1)
        return 1;
    getchar();  // consuming the line feed typed by the user.
    lines = malloc(length * sizeof(char*));
    if (lines == NULL) {
        printf("Error! Out of memory!\n");
        return 2;
    }
    for (i = 0; i < length; i++) { //Scan user's lines
        if (!fgets(buffer, sizeof buffer, stdin)) {
            printf("Error! Premature end of file!\n");
            return 2;
        }
        buffer[strcspn(buffer, "\n")] = '\0'; // overwrite the final \n
        lines[i] = strdup(buffer);
        if (lines[i] == NULL) {
            printf("Error! Out of memory!\n");
            return 2;
        }
    }
    // perform other work on the buffer.
    return 0;
}

Answer 3

gets() was deprecated in C99 and dropped from standard C with C11.

OP received the warning "about assuming extern returning int" because it is not prototyped in the standard include files.

To replace gets() with something else can be done approximately with fgets() @dasblinkenlight

With new code, use fgets() and its quirks.

---

To roll your own my_gets(char *dest, size_t size), with a high degree of former gets() compatibility, code should:

1. Take a size parameter that matches the size of the destination array.

2. Upon reading too many characters for the destination array, my_gets() is free to perform any reasonable functionality as the former gets() would cause undefined behavior at this point. Recommend filling the array as best as possible, consuming characters until '\n' or EOF, and returning NULL to indicate error (possible setting errno).

3. A simplistic use of fgets() is problematic as the size passed to fgets() is the space for characters, the '\n' and '\0', whereas the size passed to my_gets() really should be the space needed for the characters and '\0'. The '\n' being consumed and not saved.

A candidate function that meets these goals, using VLA, is at https://stackoverflow.com/a/34031881/2410359.