implicit declaration of function ‘strrev’

Question

Any ideas on why when I try to compile this code to check if a line from the file atadata I get the warning:

warning: implicit declaration of function ‘strrev’ [-Wimplicit-function-declaration]

CODE

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

int main(){
    char palindromes[100];
    char reverse[100];
    FILE *atadata = fopen("atadata","r");
    while (fgets(palindromes,sizeof(palindromes),atadata) != NULL){
        strcpy(reverse,palindromes);
        strrev(reverse);
        if( strcmp(atadata,reverse) == 0)
        fputs(atadata, stdout);
    }
    fclose(atadata);
    return 0;
}

There is no `strrev` unless you have written one. (then you need to include the header with your declaration) — David C. Rankin, Oct 28 '14 at 21:38
im new to c thank you for your help! how do you write a strrev?? — Sean O'Brien, Oct 28 '14 at 21:42
@SeanO'Brien You open a text editor, type in the necessary code, save it as a text file and compile it using a C compiler. — The Paramagnetic Croissant, Oct 28 '14 at 21:46
Start at the ends, swap chars and keep going in until you meet in the middle. — Martin James, Oct 28 '14 at 21:47

score 4 · Answer 1 · edited May 23 '17 at 11:45

4

If you are compiling in Linux, then strrev() is not part of string.h. The linked question contains an answer linking to source for an implementation you can bring into your own code directly.

edited May 23 '17 at 11:45

Community

1
1

answered Oct 28 '14 at 21:39

Alex Reynolds

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This link might be useful from Stack overflow http://stackoverflow.com/questions/8534274/is-the-strrev-function-not-available-in-linux – Manjunath N Oct 28 '14 at 21:42
1

That's the same link that is in my answer. – Alex Reynolds Oct 28 '14 at 21:43
Oh! i didn't observe that's good solution actually. – Manjunath N Oct 28 '14 at 21:48

score 3 · Answer 2 · answered Oct 28 '14 at 21:52

3

char *strrev(char *str){
    char c, *front, *back;

    if(!str || !*str)
        return str;
    for(front=str,back=str+strlen(str)-1;front < back;front++,back--){
        c=*front;*front=*back;*back=c;
    }
    return str;
}

answered Oct 28 '14 at 21:52

BLUEPIXY

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score 1 · Answer 3 · answered Oct 28 '14 at 21:58

Incase some of the other peices of code are hard to read or you do not know how to use pointers, here is a short script to reverse a string.

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

int main(void)
{
// Start here
char String1[30] = "Hello",reversed[30];
int i;
long int length = strlen(String1);

//Reverse string
for(i=0; i<length; ++i)
{
    reversed[length-i-1] = String1[i];
}

printf("First string %s\n",String1);
printf("Reversed string %s\n",reversed);

// To here if you want to copy and paste into your code without making functions 
return 0;
}

This is punk of readable coding style. I love simplicity. – pasignature Mar 04 '20 at 12:13 — pasignature, Mar 04 '20 at 12:13

David C. Rankin · Answer 4 · 2020-03-04T16:44:27.163

0

One function that will suit your needs and is efficient in its operation would take the following form:

/** strrev - reverse string, swaps src & dest each iteration.
*  Takes valid string and reverses, original is not preserved.
*  If str is valid, returns pointer to str, NULL otherwise.
*/
char *
strrev (char *str)
{
    if (!str) { 
        fprintf (stderr, "%s() Error: invalid string\n", __func__); 
        return NULL; 
    }

    char *begin = str;
    char *end = *begin ? str + strlen (str) - 1 : begin;  /* ensure non-empty */
    char tmp;

    while (end > begin)
    {
        tmp = *end;
        *end-- = *begin;
        *begin++ = tmp;
    }

    return str;
}

edited Mar 04 '20 at 16:44

answered Oct 28 '14 at 21:45

David C. Rankin

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It's a bad idea for random utility functions to spew their complaints to `stdout`/`stderr` without explicit instruction. – Deduplicator Oct 28 '14 at 21:49
I try to be over-inclusive and at least have the function make noise if it receives invalid input. I agree, with your premise regarding error handling, but providing an error handler along with an example function is guaranteed to muddy the water. – David C. Rankin Oct 28 '14 at 21:58
1

`char *end = str + strlen (str) - 1;` : if `strlen(str)==0` , `end = &str[-1]`.. UB. – BLUEPIXY Oct 28 '14 at 21:59
If you consider that invalid input (and decide to check for it), then `abort()` instead of `return 0` after complaining. – Deduplicator Oct 28 '14 at 22:00

implicit declaration of function ‘strrev’

4 Answers4