There is no direct function to do that. You have to write something like this, using strchr:

char* replace_char(char* str, char find, char replace){
    char *current_pos = strchr(str,find);
    while (current_pos) {
        *current_pos = replace;
        current_pos = strchr(current_pos,find);
    }
    return str;
}

For whole strings, I refer to this answered question

Indexing in C arrays start from 0. So you have to replace c[1] = 'B' with c[0] = 'B'.

Also, see similar question from today: Smiles in output C++ - I've put a more detailed description there :)

#include <stdio.h>

int main(void) {
    char* words[3] = {"cat", "dog", "bird"};
    
    printf("The Initial List\n");
    for(int i=0; i<3; ++i)
    {
        printf("%d : %s\n", i, words[i]);
    }
    
    // Replace the middle entry
    char* replacement = "mouse";
    words[1] = replacement;
    
    printf("The Final List\n");
    for(int i=0; i<3; ++i)
    {
        printf("%d : %s\n", i, words[i]);
    }

    return 0;
}

Output:

Success #stdin #stdout 0.01s 5464KB
The Initial List
0 : cat
1 : dog
2 : bird
The Final List
0 : cat
1 : mouse
2 : bird

You don't want to modify the pointer, you want to modify what the pointer points to. Also the ret points to a character, so *ret is the first character of a sequence of characters, in this case the character 'j', so you can't just assign a string to it. You must replace each character of that sequence. The function memcpy can help you with that.

To replace the other occurrences of "john" you can use this loop:

while(ret = strstr(array, data)){
    printf("The substring is: %s\n", ret);
    memcpy(ret, "jack", strlen("jack"));
}

strstr() returns NULL if it does not find anything, and when that happens you are going to break out of the loop.

There are some problems with your code too:

• You can use argv[1] in place of data.
• It is better to copy a file to a buffer with calls to fread().

If I have correctly understood, you do not want to copy the terminating null. The standard library function to use is void *memcpy(void *dest, void *src, size_t size) that copies exactly size bytes from src to dest.

But you have to ensure that there is enough room at dest to accept the size bytes

You can do it like this:

• Compute the length of the new array by adding the number of '2's to strlen of your string plus one for null terminator
• Allocate the required number of chars using malloc
• Go through your string in a loop; if you see '2', add 'B' followed by 'a' to the output;
• Otherwise, copy the current character to the output.

Note: now that your string is allocated dynamically, you need to call free once you are done using it.

const char* str = ...
int cnt = strlen(str)+1;
for (const char *p = str ; *p ; cnt += (*p++ == '2'))
    ;
char *res = malloc(cnt);
char *out = res;
const char *in = str;
while (*in) {
    if (*in == '2') {
        *out++ = 'B';
        *out++ = 'a';
    } else {
        *out++ = *in;
    }
    in++;
}
*out++ = '\0';

Demo.

You need to replace only when you find the full substring.

There are useful standard functions :

• strstr allows to find a substring in a string
• memcpy can be used to replace the old substring by the new one

A proposal :

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

int main(int argc, char ** argv)
{
  if (argc != 4)
    printf("usage : %s <str> <old> <new>\n", *argv);
  else {
    char * old = argv[2];
    char * new = argv[3];
    size_t len = strlen(old);

    if (len != strlen(new))
      fprintf(stderr, "'%s' and '%s' do not have the same length\n", old, new);
    else {
      char * str = argv[1];
      char * p = str;

      printf("'%s' -> ", str);

      /* the loop doing the replacements */
      while ((p = strstr(p, old)) != NULL) {
        memcpy(p, new, len);
        p += len;
      }

      printf("'%s'\n", str);
    }
  }

  return 0;
}

Compilation and execution :

pi@raspberrypi:/tmp $ gcc -pedantic -Wextra -Wall r.c
pi@raspberrypi:/tmp $ ./a.out
usage : ./a.out <str> <old> <new>
pi@raspberrypi:/tmp $ ./a.out xyzdefxyzghixy xyz abc
'xyzdefxyzghixy' -> 'abcdefabcghixy'

Edit

NB: please Note that I wish to only use stdio.h

Can be :

#include <stdio.h>

int main(int argc, char ** argv)
{
  if (argc != 4)
    printf("usage : %s <str> <old> <new>\n", *argv);
  else {
    char * old = argv[2];
    char * new = argv[3];
    char * str = argv[1];

    /* check old and new have the same length */
    char * pold, * pnew;

    for (pold = old, pnew = new; *pold && *pnew; ++pold, ++pnew)
      ;

    if (*pold || *pnew)
      fprintf(stderr, "'%s' and '%s' do not have the same length\n", old, new);
    else {
      printf("'%s' -> ", str);

      char * pstr = str;

      /* the loop doing the replacements */
      while (*pstr) {
        /* check if substring */
        char * pold = old;
        char * psubstr = pstr;

        while (*pold && (*pold == *psubstr)) {
          pold += 1;
          psubstr += 1;
        }

        if (*pold == 0) {
          /* substring, replacement */
          pnew = new;

          while (*pnew)
            *pstr++ = *pnew++;
        }
        else
          pstr += 1;
      }

      printf("'%s'\n", str);
    }
  }

  return 0;
}

As you can see this is a bad choice, it is very easy to understand what the previous version does, this is not the case with that version

Replace character with character

This code will replace every occurrence of the character 'n' with the character '1'.

#include <stdio.h>

int main(void)
{
    char str[15];
    printf("Input: ");
    scanf("%14s", str); // prevent buffer overflow

    for (int i = 0; str[i]; ++i) { // iterate over str
        if (str[i] == 'n')
            str[i] = '1';
    }

    printf("Modified: %s\n", str);

    return 0;
}

Replace character with string

This code will replace every occurrence of the character 'n' with the string "1+k" using the function strcat. The modified string is saved to char mod[].

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

#include <stdio.h>

int main(void)
{
    char str[15];
    printf("Input: ");
    scanf("%14s", str); // prevent buffer overflow

    char mod[] = "";

    for (int i = 0; str[i]; ++i) { // iterate over str
        if (str[i] == 'n') {
            char *repl = "1+k";
            strcat(mod, repl); // add "1+k" to mod
        } else {
            strncat(mod, &str[i], 1); // add str[i] to mod
        }
    }

    printf("Modified: %s\n", mod);

    return 0;
}