How correct that Access Violation error in c?-CodePudding

I´m getting a "Access violation writing location" error when I run that code. I know that probably is because I´m trying change a const value, but I dont know how correct the code.

OBS: I want create n arrays to storage strings with 25 characters.

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

int main() {

    //variables
    int i = 0, n = 0;

    //code
    scanf("%d\n", &n); //define the number of arrays
    char* exp;
    exp = (char*)malloc(n * 25); //allocate memory for the string size
    

    for (i = 0; i < n; i  )
    {
 
        //takes strings from keyboard
        fgets(exp[i], n*25, stdin); //the below error takes place here.
        
    }
    
    //prints the first string taked from stdin
    printf("%s", exp[0]);

}

ERROR: Exception thrown at 0x00007FFBC6EC916F (ucrtbased.dll) in BEE-URI 1022.exe: 0xC0000005: Access violation writing location 0xFFFFFFFFFFFFFFCD.

CodePudding user response：

The first argument expression of this call shall have the type char *, So instead of

fgets(exp[i], n*25, stdin);

you need at least to write

fgets(exp, n*25, stdin);

And the used conversion specifier

printf("%s", exp[0]);

is also incorrect. Either use

printf("%c", exp[0]);

printf("%s", exp);

Pay attention to that it is unclear what you are trying to achieve using this for loop

for (i = 0; i < n; i  )
{

    fgets(exp, n*25, stdin); //the below error takes place here.
    
}

Maybe you mean something like the following

char ( *exp )[25];
exp = malloc( n * sizeof( char[25] ) );

for (i = 0; i < n; i  )
{

    fgets(exp[i], sizeof( exp[i] ), stdin);
}

// either
// printf( "%s", exp[0] );
// or the loop below
for (i = 0; i < n; i  )
{
    printf( "%s", exp[i] );
}

free( exp );

CodePudding user response：

You say you want n arrays of 25 characters each, but what you malloc is one array of n * 25 characters. I think you want something like this:

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

int main(void) {
    int n;
    char **exp;

    scanf("%d", &n);                // get the number of arrays
    exp = malloc(n * sizeof *exp);  // allocate memory for n string pointers
    for (int i = 0; i < n; i  )
        exp[i] = malloc(25 * sizeof **exp);
    for (int i = 0; i < n; i  )
        fgets(exp[i], 25, stdin);   // takes strings from keyboard
    printf("%s", exp[0]);           // print 1st string taken from stdin
}

Note that you should also check the return values from your malloc calls so you know if they succeeded or failed. Same with fgets and scanf.

BTW, remember that a string of 25 characters can only hold 24 characters because you must have room for the terminating null character.