Cant get first character in array of strings

Question

I want to get first char character of each string. Here a example:

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

int main() {
    int size = 2;
    char** text = (char**) malloc(sizeof(char*) * size);
    for(int i = 0; i < size; ++i) {
        char buf[80];
        fgets(buf, 80, stdin);
        text[i] = (char*)malloc(strlen(buf));
        strcpy(text[i], buf);
    }

    for(int i = 0; i < strlen(text[i]); ++i) {
        printf("%c ", text[i][0]);
    }

}

In last for loop, program falls in Segmentation fault. I dont know why.

Adrian Mole · Accepted Answer · 2020-05-09 21:34:30Z

1

The strlen function returns the number of characters in the given string not including the terminal nul character; however, the strcpy function copies all characters including that terminating nul!

So, your allocation for text[i] is not quite big enough and, by writing beyond the buffer's bounds, you are getting undefined behaviour.

Add an extra character to the malloc call:

    for(int i = 0; i < size; ++i) {
        char buf[80];
        fgets(buf, 80, stdin);
        text[i] = malloc(strlen(buf) + 1); // Need space for the terminal nul!
        strcpy(text[i], buf);
    }

Or, more simply, use the strdup function, which achieves the same result as your malloc and strcpy in one fell swoop:

    for(int i = 0; i < size; ++i) {
        char buf[80];
        fgets(buf, 80, stdin);
        text[i] = strdup(buf);
    }

Either way, don't forget to call free on all the buffers you allocate.

EDIT: You are also using the wrong 'limit' in your final output loop; this:

    for(int i = 0; i < strlen(text[i]); ++i) { // strlen() is not the # strings
        printf("%c ", text[i][0]);
    }

Should be:

    for(int i = 0; i < size; ++i) { // "size" is your number of strings!
        printf("%c ", text[i][0]);
    }

edited May 9, 2020 at 21:34

answered May 9, 2020 at 21:01

Adrian Mole

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4 Comments

Georgii Over a year ago

i add +1 at malloc and doesn't work, same as strdup. And yes, i know about free i just drop it out of snippet. onlinegdb.com/Sy12R54cL

Adrian Mole Over a year ago

But you still have for(int i = 0; i < strlen(text[i]); ++i) { in your second loop. See the "EDIT" part of my answer, and change that to for(int i = 0; i < size; ++i) {.

anastaciu Over a year ago

@Georgii, minor issue, casting malloc is unnecessary in C.

Adrian Mole Over a year ago

@anastaciu Good call! I'm normally quite pedantic about that, but somehow missed it today.

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