I've been trying to work out exactly what this function's purpose is I've come across..
The code intentionally has bad code practices, so I am trying to figure out if this is one of them.
Here is the function:
void clear_mem(char *memblock, int siz) {
register int i;
for (i=0; i<=siz;i++)
*(memblock+i) = 0;
}
The function is called within the following function:
char *get_argument(char line[], int argno){
char *argument = malloc(512);
char clone[512];
strncpy(clone, line, strlen(line)+1);
int current_arg = 0;
char *splitted = strtok(clone, " ");
while (splitted != NULL){
if (splitted[0] != ':'){
current_arg++;
}
if (current_arg == argno+1){
clear_mem(argument, 512); //Here
strncpy(argument, splitted, strlen(splitted)+1);
return argument;
free(argument);
}
splitted = strtok(NULL, " ");
}
if (current_arg != argno){
argument[0] = '\0';
}
free(argument);
return argument;
}
Thanks in advance!
In this code:
for (i=0; i<=siz;i++)
*(memblock+i) = 0;
memblock+i adds the integer i to the pointer memblock. The result points i elements beyond where memblock points. Since memblock is a pointer to char, the result points i characters beyond where memblock points.
Then *(memblock+i) refers to the character at that address. *(memblock+i) is equivalent to memblock[i]. *(memblock+i) = 0 sets the character to zero.
So the effect of this code is to set all characters indexed by i during the loop to zero. It clears a block of memory.
The for (i=0; i<=siz;i++) causes the loop to iterate with i taking all values from zero up to and including siz. Thus, siz+1 characters will be set to zero.
We can see this is an error because get_argument allocates 512 bytes for argument and then later calls clear_mem(argument, 512), which clears 513 bytes. The resulting behavior is not defined by the C code.
Related
I'm using an array of strings in C to hold arguments given to a custom shell. I initialize the array of buffers using:
char *args[MAX_CHAR];
Once I parse the arguments, I send them to the following function to determine the type of IO redirection if there are any (this is just the first of 3 functions to check for redirection and it only checks for STDIN redirection).
int parseInputFile(char **args, char *inputFilePath) {
char *inputSymbol = "<";
int isFound = 0;
for (int i = 0; i < MAX_ARG; i++) {
if (strlen(args[i]) == 0) {
isFound = 0;
break;
}
if ((strcmp(args[i], inputSymbol)) == 0) {
strcpy(inputFilePath, args[i+1]);
isFound = 1;
break;
}
}
return isFound;
}
Once I compile and run the shell, it crashes with a SIGSEGV. Using GDB I determined that the shell is crashing on the following line:
if (strlen(args[i]) == 0) {
This is because the address of arg[i] (the first empty string after the parsed commands) is inaccessible. Here is the error from GDB and all relevant variables:
(gdb) next
359 if (strlen(args[i]) == 0) {
(gdb) p args[0]
$1 = 0x7fffffffe570 "echo"
(gdb) p args[1]
$2 = 0x7fffffffe575 "test"
(gdb) p args[2]
$3 = 0x0
(gdb) p i
$4 = 2
(gdb) next
Program received signal SIGSEGV, Segmentation fault.
parseInputFile (args=0x7fffffffd570, inputFilePath=0x7fffffffd240 "") at shell.c:359
359 if (strlen(args[i]) == 0) {
I believe that the p args[2] returning $3 = 0x0 means that because the index has yet to be written to, it is mapped to address 0x0 which is out of the bounds of execution. Although I can't figure out why this is because it was declared as a buffer. Any suggestions on how to solve this problem?
EDIT: Per Kaylum's comment, here is a minimal reproducible example
#include<stdio.h>
#include<string.h>
#include<stdlib.h>
#include<unistd.h>
#include<sys/types.h>
#include<sys/wait.h>
#include <sys/stat.h>
#include<readline/readline.h>
#include<readline/history.h>
#include <fcntl.h>
// Defined values
#define MAX_CHAR 256
#define MAX_ARG 64
#define clear() printf("\033[H\033[J") // Clear window
#define DEFAULT_PROMPT_SUFFIX "> "
char PROMPT[MAX_CHAR], SPATH[1024];
int parseInputFile(char **args, char *inputFilePath) {
char *inputSymbol = "<";
int isFound = 0;
for (int i = 0; i < MAX_ARG; i++) {
if (strlen(args[i]) == 0) {
isFound = 0;
break;
}
if ((strcmp(args[i], inputSymbol)) == 0) {
strcpy(inputFilePath, args[i+1]);
isFound = 1;
break;
}
}
return isFound;
}
int ioRedirectHandler(char **args) {
char inputFilePath[MAX_CHAR] = "";
// Check if any redirects exist
if (parseInputFile(args, inputFilePath)) {
return 1;
} else {
return 0;
}
}
void parseArgs(char *cmd, char **cmdArgs) {
int na;
// Separate each argument of a command to a separate string
for (na = 0; na < MAX_ARG; na++) {
cmdArgs[na] = strsep(&cmd, " ");
if (cmdArgs[na] == NULL) {
break;
}
if (strlen(cmdArgs[na]) == 0) {
na--;
}
}
}
int processInput(char* input, char **args, char **pipedArgs) {
// Parse the single command and args
parseArgs(input, args);
return 0;
}
int getInput(char *input) {
char *buf, loc_prompt[MAX_CHAR] = "\n";
strcat(loc_prompt, PROMPT);
buf = readline(loc_prompt);
if (strlen(buf) != 0) {
add_history(buf);
strcpy(input, buf);
return 0;
} else {
return 1;
}
}
void init() {
char *uname;
clear();
uname = getenv("USER");
printf("\n\n \t\tWelcome to Student Shell, %s! \n\n", uname);
// Initialize the prompt
snprintf(PROMPT, MAX_CHAR, "%s%s", uname, DEFAULT_PROMPT_SUFFIX);
}
int main() {
char input[MAX_CHAR];
char *args[MAX_CHAR], *pipedArgs[MAX_CHAR];
int isPiped = 0, isIORedir = 0;
init();
while(1) {
// Get the user input
if (getInput(input)) {
continue;
}
isPiped = processInput(input, args, pipedArgs);
isIORedir = ioRedirectHandler(args);
}
return 0;
}
Note: If I forgot to include any important information, please let me know and I can get it updated.
When you write
char *args[MAX_CHAR];
you allocate room for MAX_CHAR pointers to char. You do not initialise the array. If it is a global variable, you will have initialised all the pointers to NULL, but you do it in a function, so the elements in the array can point anywhere. You should not dereference them before you have set the pointers to point at something you are allowed to access.
You also do this, though, in parseArgs(), where you do this:
cmdArgs[na] = strsep(&cmd, " ");
There are two potential issues here, but let's deal with the one you hit first. When strsep() is through the tokens you are splitting, it returns NULL. You test for that to get out of parseArgs() so you already know this. However, where your program crashes you seem to have forgotten this again. You call strlen() on a NULL pointer, and that is a no-no.
There is a difference between NULL and the empty string. An empty string is a pointer to a buffer that has the zero char first; the string "" is a pointer to a location that holds the character '\0'. The NULL pointer is a special value for pointers, often address zero, that means that the pointer doesn't point anywhere. Obviously, the NULL pointer cannot point to an empty string. You need to check if an argument is NULL, not if it is the empty string.
If you want to check both for NULL and the empty string, you could do something like
if (!args[i] || strlen(args[i]) == 0) {
If args[i] is NULL then !args[i] is true, so you will enter the if body if you have NULL or if you have a pointer to an empty string.
(You could also check the empty string with !(*args[i]); *args[i] is the first character that args[i] points at. So *args[i] is zero if you have the empty string; zero is interpreted as false, so !(*args[i]) is true if and only if args[i] is the empty string. Not that this is more readable, but it shows again the difference between empty strings and NULL).
I mentioned another issue with the parsed arguments. Whether it is a problem or not depends on the application. But when you parse a string with strsep(), you get pointers into the parsed string. You have to be careful not to free that string (it is input in your main() function) or to modify it after you have parsed the string. If you change the string, you have changed what all the parsed strings look at. You do not do this in your program, so it isn't a problem here, but it is worth keeping in mind. If you want your parsed arguments to survive longer than they do now, after the next command is passed, you need to copy them. The next command that is passed will change them as it is now.
In main
char input[MAX_CHAR];
char *args[MAX_CHAR], *pipedArgs[MAX_CHAR];
are all uninitialized. They contain indeterminate values. This could be a potential source of bugs, but is not the reason here, as
getInput modifies the contents of input to be a valid string before any reads occur.
pipedArgs is unused, so raises no issues (yet).
args is modified by parseArgs to (possibly!) contain a NULL sentinel value, without any indeterminate pointers being read first.
Firstly, in parseArgs it is possible to completely fill args without setting the NULL sentinel value that other parts of the program should rely on.
Looking deeper, in parseInputFile the following
if (strlen(args[i]) == 0)
contradicts the limits imposed by parseArgs that disallows empty strings in the array. More importantly, args[i] may be the sentinel NULL value, and strlen expects a non-NULL pointer to a valid string.
This termination condition should simply check if args[i] is NULL.
With
strcpy(inputFilePath, args[i+1]);
args[i+1] might also be the NULL sentinel value, and strcpy also expects non-NULL pointers to valid strings. You can see this in action when inputSymbol is a match for the final token in the array.
args[i+1] may also evaluate as args[MAX_ARGS], which would be out of bounds.
Additionally, inputFilePath has a string length limit of MAX_CHAR - 1, and args[i+1] is (possibly!) a dynamically allocated string whose length might exceed this.
Some edge cases found in getInput:
Both arguments to
strcat(loc_prompt, PROMPT);
are of the size MAX_CHAR. Since loc_prompt has a length of 1. If PROMPT has the length MAX_CHAR - 1, the resulting string will have the length MAX_CHAR. This would leave no room for the NUL terminating byte.
readline can return NULL in some situations, so
buf = readline(loc_prompt);
if (strlen(buf) != 0) {
can again pass the NULL pointer to strlen.
A similar issue as before, on success readline returns a string of dynamic length, and
strcpy(input, buf);
can cause a buffer overflow by attempting to copy a string greater in length than MAX_CHAR - 1.
buf is a pointer to data allocated by malloc. It's unclear what add_history does, but this pointer must eventually be passed to free.
Some considerations.
Firstly, it is a good habit to initialize your data, even if it might not matter.
Secondly, using constants (#define MAX_CHAR 256) might help to reduce magic numbers, but they can lead you to design your program too rigidly if used in the same way.
Consider building your functions to accept a limit as an argument, and return a length. This allows you to more strictly track the sizes of your data, and prevents you from always designing around the maximum potential case.
A slightly contrived example of designing like this. We can see that find does not have to concern itself with possibly checking MAX_ARGS elements, as it is told precisely how long the list of valid elements is.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAX_ARGS 100
char *get_input(char *dest, size_t sz, const char *display) {
char *res;
if (display)
printf("%s", display);
if ((res = fgets(dest, sz, stdin)))
dest[strcspn(dest, "\n")] = '\0';
return res;
}
size_t find(char **list, size_t length, const char *str) {
for (size_t i = 0; i < length; i++)
if (strcmp(list[i], str) == 0)
return i;
return length;
}
size_t split(char **list, size_t limit, char *source, const char *delim) {
size_t length = 0;
char *token;
while (length < limit && (token = strsep(&source, delim)))
if (*token)
list[length++] = token;
return length;
}
int main(void) {
char input[512] = { 0 };
char *args[MAX_ARGS] = { 0 };
puts("Welcome to the shell.");
while (1) {
if (get_input(input, sizeof input, "$ ")) {
size_t argl = split(args, MAX_ARGS, input, " ");
size_t redirection = find(args, argl, "<");
puts("Command parts:");
for (size_t i = 0; i < redirection; i++)
printf("%zu: %s\n", i, args[i]);
puts("Input files:");
if (redirection == argl)
puts("[[NONE]]");
else for (size_t i = redirection + 1; i < argl; i++)
printf("%zu: %s\n", i, args[i]);
}
}
}
I am trying to write a program to count letters in every word of a given string and append the count to the word.
Example - I/p: The cake O/p: The3 cake4
In my logic, I want to reset a counter variable named icnt of type integer(which is done) & and an array named temp of type character(which is not working).
I did some research on StackOverflow and implemented memset to reset but looks like it didn't work for me!
Kindly correct my program if there are mistakes.
#include<stdio.h>
#include<string.h>
void fun(char*,char*);
char ch[50];
char temp[10]={0};
int main()
{
printf("Enter string :");
scanf("%[^'\n']s",&ch);
fun(ch,temp);
}
void fun(char *ptr,char *tptr)
{
int icnt = 0;
while(*ptr!='\0')
{
if(*ptr!=' ')
{
*tptr = *ptr;
tptr++;
icnt++;
}
else if(*ptr==' ')
{
printf("In IF bcoz of space\n");
printf("%s %d\n",temp,icnt);
icnt = 0;
//temp[10] = {0}; //*
memset(temp,0,10); //*
//Tried reseting in both the above ways *
}
ptr++;
}
if(*ptr=='\0')
{
printf("%s%d\n",temp,icnt);
}
}
Output of the above program: The34
Looks like nothing is getting stored the 2nd time in the temp variable
Excepted output: The3 cake4
printf("%s %d\n",temp,icnt);
temp[10] = {0};
Your function takes temp as argument and accesses that global variable directly. This is not a bug up to here but it makes no sense: The function will only work if the tptr argument has a certain value. So why using an argument at all?
temp[10] = {0};
memset(temp,0,10);
Because your function overwrites the content of that array with the line *tptr = *ptr it is not neccessary to initialize the temp array at all!
Instead you only have to ensure that the last value in the array is a zero. You do this the following way:
*tptr = 0;
printf(...,temp,...);
Theoretically you could also use memset but this is not neccessary and will take more computation time.
Output of the above program: The34
Your program increments tptr (tptr++) but it never re-sets it!
Therefore the temp array has the following content:
/* Before the first printf: */ "The\0\0\0\0\0\0\0"
/* After memset: */ "\0\0\0\0\0\0\0\0\0\0"
/* Before the second printf: */ "\0\0\0cake\0\0\0"
If your input is longer than 10 characters your program may even crash because it writes to memory it is not allowed to write to.
printf will write an empty string the second time because the first character of the temp array is NUL...
Kindly correct my program if there are mistakes.
I would do the following changes:
I would use two different variables for tptr:
void fun(char *ptr,char *output)
{
char *tptr = output; /* here */
Instead of referencing temp in the function I would reference the argument which I would never modify. Before printf I would write the NUL byte to the end of the array and after printf I would reset the write pointer (tptr):
*tptr = *ptr; /* This remains tptr; does not become output */
*tptr++; /* This also */
...
*tptr = 0; /* This is new: Ensure the last byte is NUL */
printf(...,output,...); /* Not test, but output */
tptr = output; /* This is new: Reset the write pointer */
/* memset(...) - No longer needed */
#include<stdio.h>
#include<string.h>
void fun(char*,char*);
char ch[50];
char temp[10]={0};
int main()
{
printf("Enter string :");
scanf("%[^'\n']s",ch);
fun(ch,temp);
}
void fun(char *ptr,char *tptr)
{
int icnt = 0;
while(*ptr!='\0')
{
if(*ptr!=' ')
{
*tptr = *ptr;
tptr++;
icnt++;
}
else if(*ptr==' ')
{
printf("In IF bcoz of space\n");
printf("%s %d\n",temp,icnt);
icnt = 0;
//temp[10] = {0}; //*
memset(temp,0,10);
tptr = temp; /* The tptr was not reinitialised to point to base address */
//Tried reinitialization in both the above ways *
}
ptr++;
}
if(*ptr=='\0')
{
printf("%s%d\n",temp,icnt); /* Here you are printing from the base address.*/
}
}
The print was done from the base address of the array and the after reinitialisation the tptr was not set to point to the base address of the array. Code attached should work fine.
You have to add after "inct = 0;" the instruction : " tptr = temp;" because if you don't add it you will go out from the memory space of the table "temp". And you have also to add "*tptr='\0' " just before "printf("%s %d\n",temp,icnt);".
So I have the following question:
I have this struct ListAut
struct ListAut{
char* biggestn;
int sizeof_biggestn;
int total_len;
struct node* avl;
};
Its typedef is as it follows:
typedef struct ListAut *IndexOfAuts;
IndexOfAuts *newIndexOfAuts()
{
int i;
IndexOfAuts *ioa = malloc(27 * sizeof(struct ListAut));
for (i = 0; i < 27; i++)
{
ioa[i]->biggestn = "";
ioa[i]->sizeof_biggestn = 0;
ioa[i]->total_len = 0;
ioa[i]->avl = NULL;
}
return ioa;
}
void insertName(IndexOfAuts * ioa, char *nome)
{
char *aux = malloc(sizeof(nome));
aux = trim(nome);
int index = getIndexOfLetter(aux);
if (nameLen(aux) > getSizeOfLongName(ioa[index]))
{
strcpy(ioa[index]->biggestn, aux);
ioa[index]->sizeof_biggestn = nameLen(aux);
}
ioa[index]->total_len += nameLen(aux);
insert(ioa[index]->avl, aux);
}
This is an important part of a module I need for a project, and on its main it's Seg Faulting. I suspect it's on the creation of an "object" newIndexOfAuts(),
The idea of this module is to have an array of 27 pointers to those structures, one to each letter and another to the special characters;
Now I'm just confused because it might be from the problem above or from a module loader I made:
void loadModules(char *filename, IndexOfAuts * ioa, StatTable st)
{
char *nameofile = malloc(20);
strcpy(nameofile, filename);
FILE *file = fopen(nameofile, "r");
if (file != NULL)
{
int counter, ano;
char *buff, *field, *auxil;
buff = malloc(1024);
field = malloc(200);
auxil = malloc(200);
while (fgets(buff, 1024, file))
{
counter = 0;
field = strtok(buff, ",");
printf("inserting 1st name\n");
insertName(ioa, field);
counter++;
while (!atoi(field))
{
if ((auxil = strtok(NULL, ",")) != NULL)
{
counter++;
field = auxil;
insertName(ioa, field);
}
}
ano = atoi(field);
incPub(st, ano, counter - 1);
}
fclose(file);
}
}
When i run this in main that has the following lines:
printf("Creating Stat Table");
StatTable st=newStatTable();\\This Line is correct, i checked it,i hope
printf("Creating index");
IndexOfAuts* ioa=newIndexOfAuts();
printf("Loading Modules");
loadModules(filename,ioa,st);
Those prints were for me to see where was the cause of the seg fault, but the last line printed was the "Creating Index".
There are several cases of undefined behavior and one memory leak (and a possible case of undefined behavior too):
You have this initialization ioa[i]->biggestn=""; It make the biggestn member point to a constant array of one character (the '\0' character). You then do strcpy(ioa[index]->biggestn,aux); which will write over a constant character, and then go outside of the array writing into unknown memory.
You have this: char* aux=malloc(sizeof(nome)); That allocates only 4 or 8 bytes, which the size of the pointer and not what the pointer points to. Use strlen to get the length of a string.
For the above allocation you also need to allocate a byte extra, as strlen only returns the length of the string without the terminator.
You have aux=trim(nome); This overwrites the pointer you just allocated, leading to a memory leak.
The above call might also lead to undefined behavior if you return a pointer to a local variable or array.
There are probably other problematic lines, these were just the ones I found on a quick glance.
And a general tip: Learn to use a debugger! The debugger is a programmers best tool next to the compiler. If you run your program in a debugger, the debugger will stop at the location of the crash, and let you examine (and also walk up) the function call stack, as well as let you examine values of variables.
I create an array (char *charheap;) of length 32 bytes in the heap, and initialize all the elements to be \0. Here is my main function:
int main(void) {
char *str1 = alloc_and_print(5, "hello");
char *str2 = alloc_and_print(5, "brian");
}
char *alloc_and_print(int s, const char *cpy) {
char *ncb = char_alloc(s);// allocate the next contiguous block
if (ret == NULL) {
printf("Failed\n");
} else {
strcpy(ncb, cpy);
arr_print();// print the array
}
return ncb;
}
Here is what I implement:
/char_alloc(s): find the FIRST contiguous block of s+1 NULL ('\0')
characters in charheap that does not contain the NULL terminator
of some previously allocated string./
char *char_alloc(int s) {
int len = strlen(charheap);
for (int i = 0; i < len; i++) {
if (charheap[0] == '\0') {
char a = charheap[0];
return &a;
} else if (charheap[i] == '\0') {
char b = charheap[i+1];
return &b;
}
}
return NULL;
}
Expected Output: (\ means \0)
hello\\\\\\\\\\\\\\\\\\\\\\\\\\\
hello\brian\\\\\\\\\\\\\\\\\\\\\
This solution is completely wrong and I just print out two failed. :(
Actually, the char_alloc should return a pointer to the start of contiguous block but I don't know how to implement it properly. Can someone give me a hint or clue ?
Your function is returning a pointer to a local variable, therefore the caller receives a pointer to invalid memory. Just return the pointer into the charheap, which is what you want.
return &charheap[0]; /* was return &a; which is wrong */
return &charheap[i+1]; /* was return &b; which is wrong */
Your for loop uses i < len for the terminating condition, but, since charheap is \0 filled, strlen() will return a size of 0. You want to iterate through the whole charheap, so just use the size of that array (32 in this case).
int len = 32; /* or sizeof(charheap) if it is declared as an array */
The above two fixes should be enough to get your program to behave as you expect (see demonstration).
However, you do not place a check to make sure there is enough room in your heap to accept the allocation check. Your allocation should fail if the distance between the start of the available memory and the end of the charheap is less than or equal to the desired size. You can enforce this easily enough by setting the len to be the last point you are willing to check before you know there will not be enough space.
int len = 32 - s;
Finally, when you try to allocate a third string, your loop will skip over the first allocated string, but will overwrite the second allocated string. Your loop logic needs to change to skip over each allocated string. You first check if the current location in your charheap is free or not. If it is not, you advance your position by the length of the string, plus one more to skip over the '\0' terminator for the string. If the current location is free, you return it. If you are not able to find a free location, you return NULL.
char *char_alloc(int s) {
int i = 0;
int len = 32 - s;
while (i < len) {
if (charheap[i] == '\0') return &charheap[i];
i += strlen(charheap+i) + 1;
}
return NULL;
}
I need to create program that takes input from stdin in this format:
abcde //number of characters in word = number of words => square shape
fghij
klmno
pqrst
uvwxy
// \n separates first half from second
word1word //any amount of characters, any amount of words
word
word2
sdf
// \n to end input
My code works, but only about 50% of the time. I have couple of example inputs, that I use for testing, but for some of them my readwords function fails.
Here is my function, that reads words. Since I have no idea how many words or how long they are going to be, I use dynamic arrays and getchar() function.
void readWords(char **p,int *n,int w) /* before calling: n = 50; w = 20; p = 50x20 char array */
{
int i = 0,j = 0,x;
char tmp,prevtmp;
while (1)
{
prevtmp = tmp;
tmp = getchar();
if ((prevtmp == '\n' && tmp == '\n') || feof(stdin))
break; /* no more words to read */
if (tmp == '\n') /* end of word */
{
p[i][j] = '\0'; /* add \0 to create string format */
i++;
j = 0;
if (i == *n) /* if there is more words than there is space for them, double the size */
if (realloc(p,*n*2) != NULL)
*n*=2;
continue;
}
p[i][j] = tmp;
j++;
if (j == w) /* if width of word is larger than allocated space, double it */
{
for (x = 0; x < *n;x++);
if(realloc (p[x],w*2) != NULL);
w=w*2;
}
}
*n = i;
}
This is example of input for which this works (note:this function only reads second half after line with only \n):
dsjellivhsanxrr
riemjudhgdffcfz
<skipping>
atnaltapsllcelo
ryedunuhyxhedfy
atlanta
saltlakecity
<skipping 15 words>
hartford
jeffersoncity
And this is input that my function doesn't read properly:
<skipping>
...oywdz.ykasm.pkfwb.zazqy...
....ynu...ftk...zlb...akn....
missouri
delaware
<skipping>
minnesota
southdakota
What my function reads from this input:
e
yoming
xas
florida
lvania
ana
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There is no difference between those two inputs (except different words and different amount and length of words), the first half gets read properly no matter what, but only the second half bugs out. How do I fix this?
P.S. sorry for long post, in case you want to see full input without skipped bytes, here is pastebin: http://pastebin.com/hBGn2tej
realloc() returns the address of the newly allocated memory, it does not update the argument passed into it. So this (and the other use of realloc()) is incorrect:
if (realloc(p,*n*2) != NULL)
and will results in the code accessing memory incorrectly, causing undefined behaviour. Store the result of realloc() to a temporary variable and check for non-NULL before updating p. The argument to realloc() also indicates the number of bytes, not the number of elements so the size argument calculation is incorrect as p is an array of char* so it should be realloc(p, sizeof(char*) * (*n * 2));. However, the change to p would not be visible to the caller. Also note that the only legal arguments to realloc() are pointers obtained from a previous call to malloc(), realloc() or calloc(). The comment p = 50x20 char array in the code suggests this is not the case.
Here is a small example that allocates an array of char* which should be helpful:
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
void f(char*** p)
{
/* Allocate space for two 'char*' elements.
Add a NULL pointer element as sentinel value
so caller knows where to find end of list. */
*p = malloc(sizeof(**p) * 3);
/* Allocate space for the two strings
and populate. */
(*p)[0] = malloc(10);
(*p)[1] = malloc(10);
strcpy((*p)[0], "hello");
strcpy((*p)[1], "world");
(*p)[2] = NULL;
/* Add a third string. */
char** tmp = realloc(*p, sizeof(**p) * 4);
if (tmp)
{
*p = tmp;
(*p)[2] = malloc(10);
strcpy((*p)[2], "again");
(*p)[3] = NULL;
}
}
int main()
{
char** word_list = 0;
f(&word_list);
if (word_list)
{
for (int i = 0; word_list[i]; i++)
{
printf("%s\n", word_list[i]);
free(word_list[i]);
}
}
free(word_list);
return 0;
}
Additionally:
prevtmp has an unknown value upon its first use.
getchar() actually returns an int and not a char.