CODEWITHSUNDEEP
cpalindromeimplicit-declaration
Sean O'Brien
Sean O'Brien

Reputation: 31

implicit declaration of function ‘strrev’

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;
}

Upvotes: 3

Views: 26661

Answers (4)

David C. Rankin
David C. Rankin

Reputation: 84551

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;
}

Upvotes: 0

nsij22
nsij22

Reputation: 879

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;
}

Upvotes: 1

BLUEPIXY
BLUEPIXY

Reputation: 40145

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;
}

Upvotes: 3

Alex Reynolds
Alex Reynolds

Reputation: 96937

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.

Upvotes: 4

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