Array of c-strings and pass by pointer method does not work

Question

Assigning new values to an array of C strings using pass by pointer method does not work properly.

Inside the "LettersToCapital" method, new values are assigned to the C-string array properly, however, once the C-strings array contents are read outside the method the results are all wrong. All the function is supposed to do is to capitalize all the lowercase letters. I am definitely doing something wrong, but what could it be?

If on line 53 variable tempStr is replaced with a constant string literal e.g. "aqua" then the values remain the same outside the function. But assigning directly from a char array (tempStr) to char array pointer (*(string +i)) does not yield correct results.

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

void LettersToCapital(char **string, int size);
void ReadOutAOC(char **string, int size);

int main()
{
    char *canadianProvinces[] = {"British Columbia", "Alberta", "Saskatchewan", "Manitoba", "Ontario", "Quebec", "New Brunswick", "Nova Scotia", "Prince Edward Island", "Newfoundland", "Yukon", "Northwest Territories", "Nunavut"};

    int numOfCanProv = sizeof(canadianProvinces) / sizeof(int);

    printf("\nNumber of Canadian provinces %d\n", numOfCanProv);

    // printing all provinces before conversion
    printf("\nBefore \"all to capital conversion\"\n\n");
    ReadOutAOC(canadianProvinces, numOfCanProv);    

    LettersToCapital(canadianProvinces, numOfCanProv);
    // Temp(canadianProvinces);

    // printing all provinces after conversion
    printf("\nAfter \"all to capital conversion\"\n");
    ReadOutAOC(canadianProvinces, numOfCanProv);
}

void ReadOutAOC(char **string, int size)
{
    printf("\n");

    for(int i = 0; i < size; i++)
        printf("String outside the assignment method[%d]: %s\n", i + 1, *(string + i));
}

void LettersToCapital(char **string, int size)
{
    char tempStr[256];

    for(int i = 0; i < size; i++)
    {
        for(int j = 0; j < strlen(*(string + i)); j++)
        {
            if(*(*(string + i) + j) > 96 && *(*(string + i) + j) < 123)
                tempStr[j] = *(*(string + i) + j) - 32;
            else
                tempStr[j] = *(*(string + i) + j);
        }
        tempStr[strlen(*(string + i))] = '\0';
        *(string + i) = tempStr; // does not work
        //*(string + i) = "aqua"; // try this instead
        printf("String inside the assignment method[%d]: %s\n", i + 1, *(string + i));
    }
}

Expected output should be:

Before "all to capital conversion"

British Columbia

Alberta

Saskatchewan

...

After "all to capital conversion"

BRITISH COLUMBIA

ALBERTA

SASKATCHEWAN

...

Answer 1

The following code:

1. cleanly compiles
2. performs the desired functionality
3. note the proper method of accessing the data strings
4. note the appropriate usage of the header file: ctype.h and function: toupper()
5. unused variables, like char tempStr[256]; and all references to that variable are eliminated
6. will result in a 'seg fault' event due to trying to modify data in readonly memory

And now the faulty code:

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

void LettersToCapital(char **string, size_t size);
void ReadOutAOC(char **string, size_t size);

int main( void )
{
    char *canadianProvinces[] = 
    {
        "British Columbia", 
        "Alberta", 
        "Saskatchewan", 
        "Manitoba", 
        "Ontario", 
        "Quebec", 
        "New Brunswick", 
        "Nova Scotia", 
        "Prince Edward Island", 
        "Newfoundland", 
        "Yukon", 
        "Northwest Territories", 
        "Nunavut"
    };

    size_t numOfCanProv = sizeof(canadianProvinces) / sizeof(canadianProvinces[0]);

    printf("\nNumber of Canadian provinces %lu\n", numOfCanProv);

    // printing all provinces before conversion
    printf("\nBefore \"all to capital conversion\"\n\n");
    ReadOutAOC(canadianProvinces, numOfCanProv);    

    LettersToCapital(canadianProvinces, numOfCanProv);
    // Temp(canadianProvinces);

    // printing all provinces after conversion
    printf("\nAfter \"all to capital conversion\"\n");
    ReadOutAOC(canadianProvinces, numOfCanProv);
}

void ReadOutAOC(char **string, size_t size)
{
    printf("\n");

    for( size_t i = 0; i < size; i++)
        printf("String outside the assignment method[%lu]: %s\n", i + 1, string[i] );
}

void LettersToCapital(char **string, size_t size)
{
    for( size_t i = 0; i < size; i++)
    {
        size_t j;
        for( j = 0; j < strlen( string[i] ); j++)
        {
            string[i][j] = (char)toupper( string[i][j] );
        }

        printf("String inside the assignment method[%lu]: %s\n", i + 1, string[i] );
    }
}

Running the above code results in the following output:

Before "all to capital conversion"


String outside the assignment method[1]: British Columbia
String outside the assignment method[2]: Alberta
String outside the assignment method[3]: Saskatchewan
String outside the assignment method[4]: Manitoba
String outside the assignment method[5]: Ontario
String outside the assignment method[6]: Quebec
String outside the assignment method[7]: New Brunswick
String outside the assignment method[8]: Nova Scotia
String outside the assignment method[9]: Prince Edward Island
String outside the assignment method[10]: Newfoundland
String outside the assignment method[11]: Yukon
String outside the assignment method[12]: Northwest Territories
String outside the assignment method[13]: Nunavut
Segmentation fault (core dumped)

using gdb to step through the program shows the cause of the seg fault event is this line:

string[i][j] = (char)toupper( string[i][j] );

because it is trying to change a value/byte/char in readonly memory

Suggest modifying the definition of the data to:

#define MAX_PROV_NAME_LEN 50
char canadianProvinces[][ MAX_PROV_NAME_LEN ] = 
    {
        "British Columbia", 
        "Alberta", 
        "Saskatchewan", 
        "Manitoba", 
        "Ontario", 
        "Quebec", 
        "New Brunswick", 
        "Nova Scotia", 
        "Prince Edward Island", 
        "Newfoundland", 
        "Yukon", 
        "Northwest Territories", 
        "Nunavut"
    };

Then adjusting the rest of the code to match would correct the problem.

The corrected code:

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

#define MAX_PROV_NAME_LEN 50

void LettersToCapital(char string[][ MAX_PROV_NAME_LEN ], size_t size);
void ReadOutAOC(char string[][ MAX_PROV_NAME_LEN ], size_t size);

int main( void )
{
    char canadianProvinces[][ MAX_PROV_NAME_LEN ] = 
    {
        "British Columbia", 
        "Alberta", 
        "Saskatchewan", 
        "Manitoba", 
        "Ontario", 
        "Quebec", 
        "New Brunswick", 
        "Nova Scotia", 
        "Prince Edward Island", 
        "Newfoundland", 
        "Yukon", 
        "Northwest Territories", 
        "Nunavut"
    };

    size_t numOfCanProv = sizeof(canadianProvinces) / sizeof(canadianProvinces[0]);

    printf("\nNumber of Canadian provinces %lu\n", numOfCanProv);

    // printing all provinces before conversion
    printf("\nBefore \"all to capital conversion\"\n\n");
    ReadOutAOC(canadianProvinces, numOfCanProv);    

    LettersToCapital(canadianProvinces, numOfCanProv);
    // Temp(canadianProvinces);

    // printing all provinces after conversion
    printf("\nAfter \"all to capital conversion\"\n");
    ReadOutAOC(canadianProvinces, numOfCanProv);
}

void ReadOutAOC(char string[][ MAX_PROV_NAME_LEN ], size_t size)
{
    printf("\n");

    for( size_t i = 0; i < size; i++)
        printf("String outside the assignment method[%lu]: %s\n", i + 1, string[i] );
}

void LettersToCapital(char string[][ MAX_PROV_NAME_LEN ], size_t size)
{
    for( size_t i = 0; i < size; i++)
    {
        size_t j;
        for( j = 0; j < strlen( string[i] ); j++)
        {
            string[i][j] = (char)toupper( string[i][j] );
        }

        printf("String inside the assignment method[%lu]: %s\n", i + 1, string[i] );
    }
}

a run of the corrected code results in the following output:

Number of Canadian provinces 13

Before "all to capital conversion"


String outside the assignment method[1]: British Columbia
String outside the assignment method[2]: Alberta
String outside the assignment method[3]: Saskatchewan
String outside the assignment method[4]: Manitoba
String outside the assignment method[5]: Ontario
String outside the assignment method[6]: Quebec
String outside the assignment method[7]: New Brunswick
String outside the assignment method[8]: Nova Scotia
String outside the assignment method[9]: Prince Edward Island
String outside the assignment method[10]: Newfoundland
String outside the assignment method[11]: Yukon
String outside the assignment method[12]: Northwest Territories
String outside the assignment method[13]: Nunavut
String inside the assignment method[1]: BRITISH COLUMBIA
String inside the assignment method[2]: ALBERTA
String inside the assignment method[3]: SASKATCHEWAN
String inside the assignment method[4]: MANITOBA
String inside the assignment method[5]: ONTARIO
String inside the assignment method[6]: QUEBEC
String inside the assignment method[7]: NEW BRUNSWICK
String inside the assignment method[8]: NOVA SCOTIA
String inside the assignment method[9]: PRINCE EDWARD ISLAND
String inside the assignment method[10]: NEWFOUNDLAND
String inside the assignment method[11]: YUKON
String inside the assignment method[12]: NORTHWEST TERRITORIES
String inside the assignment method[13]: NUNAVUT

After "all to capital conversion"

String outside the assignment method[1]: BRITISH COLUMBIA
String outside the assignment method[2]: ALBERTA
String outside the assignment method[3]: SASKATCHEWAN
String outside the assignment method[4]: MANITOBA
String outside the assignment method[5]: ONTARIO
String outside the assignment method[6]: QUEBEC
String outside the assignment method[7]: NEW BRUNSWICK
String outside the assignment method[8]: NOVA SCOTIA
String outside the assignment method[9]: PRINCE EDWARD ISLAND
String outside the assignment method[10]: NEWFOUNDLAND
String outside the assignment method[11]: YUKON
String outside the assignment method[12]: NORTHWEST TERRITORIES
String outside the assignment method[13]: NUNAVUT