Almost. Strings are NUL terminated, so you probably want to allocate an extra byte to store the NUL byte. That is, even though sides is 1 character long, it really is 2 bytes: {5,'\0'}.
So it would be:
tempSides = (char *)malloc((strlen(sides)+1)*sizeof(char));
and if you wanna copy it in:
strcpy(tempSides, sides);
Answer from Claudiu on Stack OverflowAlmost. Strings are NUL terminated, so you probably want to allocate an extra byte to store the NUL byte. That is, even though sides is 1 character long, it really is 2 bytes: {5,'\0'}.
So it would be:
tempSides = (char *)malloc((strlen(sides)+1)*sizeof(char));
and if you wanna copy it in:
strcpy(tempSides, sides);
Note that:
- Strings are zero-terminated (\0), and strlen() doesn't count it;
- By definition, sizeof(char) is 1 (byte), so it's not required;
- If you use a C (not C++) compiler, there's no need to cast it to
char *;
So that would be:
char *tempSides = malloc(strlen(inSides) + 1);
Still, if you want to duplicate the contents of inSides, you can use strdup, e.g.:
char *tempSides = strdup(inSides);
if (tempSides != NULL) {
// do whatever you want...
free(tempSides);
}
c - Memory allocation for char array - Stack Overflow
sizeof Char / Array length Memory allocation in C, - Stack Overflow
c - Char memory allocation - Stack Overflow
c - Dynamic memory allocation with char - Stack Overflow
Videos
Hey friends,
I'm sure somebody on the internet jas alread answered this, but I honestly haven't found a suitable answer for me.
Let's say we have a struct like this:
typedef struct {
char *name;
int age;
} Human;And a kind of "constructor" function like this:
Human *human_new(char *name, int age)
{
Human *this = (Human *) malloc(sizeof(Human));
this->name = name;
this->age = age;
return this;
}
My question is, do I need to allocate memory for the char pointer name of the struct or is the assignment of this->name enough?
Declaring Static Character Arrays (strings)
When you know (or have a reasonable idea how large your array needs to be, you can simply declare an array of sufficient size to handle your input (i.e. if you names are no longer than 25 characters, then you could safely declare name[26]. For strings, you always need at minimum the number of chars to store + 1 (for the null-terminating character).
If there may be a few characters more than 25, there's nothing wrong with declaring your array few bytes longer than needed to protect against accidental writing beyond the end of the array. Say name[32].
Let's declare an array of 5-characters below and look at how the information is stored in memory.
char name[5] = {0}; /* always initialize your arrays */
The above declaration creates an array of 5-contiguous bytes on the stack for your use. e.g., you can visualize the 5-bytes of memory initialized to zero as follows:
+---+---+---+---+---+
name | 0 | 0 | 0 | 0 | 0 |
+---+---+---+---+---+
\
'name' holds the starting address for
the block of memory that is the array
(i.e. the address of the first element
like: 0x7fff050bf3d0)
Note: when using name to hold a 'character string', the actual string can be no longer than 4 chars because you must end a sting with a null-terminating character, which is the null-character '\0' (or simply numeric 0, both are equivalent)
To store information in name, you can either do it by assigning characters one-at-a-time:
name[0] = 'J';
name[1] = 'o';
name[2] = 'h';
name[3] = 'n';
name[4] = 0; /* null-terminate. note: this was already done by initialization */
In memory you now have:
+---+---+---+---+---+
name | J | o | h | n | 0 | /* you actually have the ASCII value for */
+---+---+---+---+---+ /* each letter stored in the elements */
Of course, nobody assigns one character at a time in this manner. You options are many, using one of the functions provided by C, e.g. strcpy, strncpy, memcpy, or by reading information from a file stream, or file descriptor with fgets, getline, or by using a simple loop and index variable to do the assignment, or by using one of the string formatting functions, e.g. sprintf, etc... For example you can accomplish the same thing with any of the following:
/* direct copy */
strcpy (name, "John");
strncpy (name, "John", 5);
memcpy (name, "John", sizeof "John"); /* include copy of the terminating char */
/* read from stdin into name */
printf ("Please enter a name (4 char max): ");
scanf ("%[^\n]%*c", name);
Note: above with strncpy, if you had NOT initialized all element to 0 (the last of which will serve as your null-terminating character, and then used strncpy (name, "John", 4); you would need to manually terminate the string with name[4] = 0;, otherwise you would not have a valid string (you would have an unterminated array of chars which would lead to undefined behavior if you used name where a string was expected.)
If you do not explicitly understand this STOP, go read and understand what a null-terminated string is and how it differs from an array of characters. Seriously, stop now and go learn, it is that fundamental to C. (if it doesn't end with a null-terminating character - it isn't a c-string.
What if I don't know how many characters I need to store?
Dynamic Allocations of Character Strings
When you do not know how many characters you need to store (or generally how many of whatever data type), the normal approach is to declare a pointer to type, and then allocate a reasonably anticipated amount of memory (just based on your best understanding of what you are dealing with), and then reallocate to add additional memory as required. There is no magic to it, it is just a different way of telling the compiler how to manage the memory. Just remember, when you allocate the memory, you own it. You are responsible for (1) preserving a pointer to the beginning address of the memory block (so it can be freed later); and (2) freeing the memory when you are done with it.
A simple example will help. Most of the memory allocation/free functions are declared in stdlib.h.
char *name = NULL; /* declare a pointer, and initialize to NULL */
name = malloc (5 * sizeof *name); /* allocate a 5-byte block of memory for name */
if (!name) { /* validate memory was allocated -- every time */
fputs ("error: name allocation failed, exiting.", stderr);
exit (EXIT_FAILURE);
}
/* Now use name, just as you would the statically declared name above */
strncpy (name, "John", 5);
printf (" name contains: %s\n", name);
free (name); /* free memory when no longer needed.
(if reusing name, set 'name = NULL;')
*/
Note: malloc does NOT initialize the contents of the memory it allocates. If you want to initialize your new block of memory with zero (as we did with the static array), then use calloc instead of malloc. You can also use malloc and then call memset as well.
What happens if I allocate memory, then need More?
As mentioned above discussing dynamic memory, the general scheme is to allocate a reasonable anticipated amount, then realloc as required. You use realloc to reallocate the original block of memory created by malloc. realloc essentially creates a new block of memory, copies the memory from your old block to the new, and then frees the old block of memory. Since the old block of memory is freed, you want to use a temporary pointer for reallocation. If reallocation fails, you still have your original block of memory available to you.
You are free to add as little or as much memory as you like at any call to realloc. The standard scheme usually seen is to start with some initial allocation, then reallocate twice that amount each time you run out. (the means you need to keep track of how much memory is currently allocated).
To sew this up, let's end with a simple example that simply reads a string of any length as the first argument to the program (use "quotes" if your string contains whitespace). It will then allocates space to hold the string, then reallocate to append more text to the end of the original string. Finally it will free all memory in use before exit:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main (int argc, char **argv) {
if (argc < 2) { /* validate input */
fprintf (stderr, "error: insufficient input. usage: %s \"name\"\n",
argv[0]);
return 1;
}
size_t len = strlen (argv[1]); /* length of input */
size_t sz_mem = len + 1; /* memory required */
char *name = malloc (sz_mem * sizeof *name); /* allocate memory for name */
if (!name) { /* validate memory created successfully or throw error */
fputs ("error: name allocation failed, exiting.", stderr);
return 1;
}
printf ("\n allocated %zu bytes of memory for 'name'\n", sz_mem);
memset (name, 0, sz_mem); /* initialize memory to zero (optional) */
strncpy (name, argv[1], sz_mem); /* copy the null-terminator as well */
printf (" name: '%s' (begins at address: %p)\n", name, name);
/* realloc - make name twice as big */
void *tmp = realloc (name, 2 * sz_mem); /* use a temporary pointer */
if (!tmp) { /* check realloc succeeded */
fprintf (stderr, "error: virtual memory exhausted, realloc 'name'\n");
return 1;
}
memset (tmp + sz_mem, 0, sz_mem * sizeof *name); /* zero new memory */
name = tmp; /* assign new block to name */
sz_mem += sz_mem; /* update current allocation size */
printf (" reallocated 'name' to %zu bytes\n", sz_mem);
strncat (name, " reallocated", sizeof " reallocated");
printf ("\n final name : '%s'\n\n", name);
free (name);
return 0;
}
Use/Output
$ ./bin/arraybasics "John Q. Public"
allocated 15 bytes of memory for 'name'
name: 'John Q. Public' (begins at address: 0xf17010)
reallocated 'name' to 30 bytes
final name : 'John Q. Public reallocated'
Memory Check
When you dynamically allocate memory, it is up to you to validate you are using the memory correctly and that you track and free all the memory you allocate. Use a memory error checker like valgrind to veryify your memory use is correct. (there is no excuse not to, it is dead-bang-simple to do) Just type valgrind yourprogramexe
$ valgrind ./bin/arraybasics "John Q. Public"
==19613== Memcheck, a memory error detector
==19613== Copyright (C) 2002-2012, and GNU GPL'd, by Julian Seward et al.
==19613== Using Valgrind-3.8.1 and LibVEX; rerun with -h for copyright info
==19613== Command: ./bin/arraybasics John\ Q.\ Public
==19613==
allocated 15 bytes of memory for 'name'
name: 'John Q. Public' (begins at address: 0x51e0040)
reallocated 'name' to 30 bytes
final name : 'John Q. Public reallocated'
==19613==
==19613== HEAP SUMMARY:
==19613== in use at exit: 0 bytes in 0 blocks
==19613== total heap usage: 2 allocs, 2 frees, 45 bytes allocated
==19613==
==19613== All heap blocks were freed -- no leaks are possible
==19613==
==19613== For counts of detected and suppressed errors, rerun with: -v
==19613== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 2 from 2)
In the output, the following lines are of particular significance:
==19613== HEAP SUMMARY:
==19613== in use at exit: 0 bytes in 0 blocks
==19613== total heap usage: 2 allocs, 2 frees, 45 bytes allocated
This tells you that all memory allocated during your program has been properly freed. (make sure you close all open file streams, they are dynamically allocated as well).
Of equal importance is the ERROR SUMMARY:
==19613== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 2 from 2)
There are no errors in the memory use. If you attempt to read, write or free memory from a location outside your block, or from an unitialized location or that would leave other memory unreachable, that information will show as an error.
(the suppressed: 2 from 2 just relate to additional debug libraries not present on my system)
This ended up longer than intended, but if it helps, it was worth it. Good luck.
How can I announce the array and then define it's size?
Don't; 'announce' it when you know what size it needs to be. You can't use it before then anyway.
In C99 and later, you can define variables when needed โ anywhere in a statement block. You can also use VLAs (variable-length arrays) where the size is not known until runtime. Don't create enormous arrays as VLAs (e.g. 1 MiB or more โ but tune the limit to suit your machine and prejudices); use dynamic memory allocation after all.
If you're stuck with the archaic C89/C90 standard, then you can only define variables at the start of a block, and arrays have sizes known at compile time, so you have to use dynamic memory allocation โ malloc(), free() etc.
You are not printing addresses correctly: an address is not an int, so you cannot use %d to print it. Use %p (for "pointer") instead, and cast the address to void* for printing:
printf ("This is the Address of val1 %p\n", (void*)&foo[1]);
Now your program produces this or similar output:
This is the Address of val1 0xffbd2fcf
This is the Address of val2 0xffbd2fd0
The size of each array member is 2
Two pointers are off by 1, which is the size of a single char. The size of the entire array is 2. If you want to print the size of a single element, use foo[0].
Memory Allocation is a bit tricky, but it's easier than you think. It is, as the name implies, the usage and selection of data to optimize program function and speed. Basically, Memory Allocation is a "perk" of languages such as C/C++ that allow programmers to only use EXACTLY as much data as needed, freeing up memory for other computer functions.
Some good info to know about memory...
- memory is known in "bytes", these are 8-bit "groups" of data.
- A "bit" is a value that is either 0 or 1.
Variable sizes:
- Char: 1 byte
- Int: 4 bytes
- double: 8 bytes
When neighboring elements are of the same type (for example, an array of chars), they will have a difference in address that increments/decrements by the value of the memory size. Since a char variable has a size of 1 byte, neighboring elements will have addresses that differ by 1.
Ex: char addresses: 1204, 1205, 1206, 1207... (1 byte)
int addresses: 1204, 1208, 1212, 1216... (4 bytes)
Do like this:
char** list;
list = malloc(sizeof(char*)*number_of_row);
for(i=0;i<number_of_row; i++)
list[i] = malloc(sizeof(char)*number_of_col);
Additionally, if you are allocating memory dynamically. you are to free it as work done:
for(i=0;i<number_of_row; i++)
free(list[i] );
free(list);
EDIT
In you revised question:
int wordLine = 0, counter = 0, i;
wordLine and counter are 0
before this code:
list = malloc(sizeof(char*)*wordLine+1);
for(i = 0;i < wordLine ; i++)
list[i] = malloc(sizeof(char)*counter);
you have to assign value to wordLine and counter variable
Also memory allocation should be before the following loop(outside):
while((c = fgetc(myFile)) != EOF){
:
:
}
EDIT:
New your third version of question. You are reading file two times. So you need to fseek(), rewind() to first char before second loop starts.
try with:
fseek(fp, 0, SEEK_SET); // same as rewind()
rewind(fp); // same as fseek(fp, 0, SEEK_SET)
also I have doubt in your logic to calculate numberOfLines and maxNumberOfChars. please check that also
EDIT
I think your calculation for maxNumberOfChars = 0, numberOfLines = 0 is wrong try like this:
maxNumberOfChars = 0, numberOfLines = 0, numberOfChars = 0;
while((c = fgetc(myFile)) !=EOF){
if(c == '\n'){
numberOfLines++;
if(maxNumberOfChars < numberOfChars)
maxNumberOfChars = numberOfChars;
numberOfChars=0
}
numberOfChars++;
}
maxNumberOfChars is max number of chars in a line.
Also change code:
malloc(sizeof(char)*(maxNumberOfChars + 1));
If I were you, I'd map the file to private memory, using mmap, and then iterate over the file, storing starts of words in an array of char** that you can increase as you go with realloc, and replacing line breaks with 0.
That way, you have your words in memory as a continuous block, you don't have to care about file I/O, because you have the entire text file in memory as char*, and you don't have to malloc an array of arrays.
For information on the functions, see the respective man pages, or drop me a comment :)
EDIT: If you don't know mmap yet, have a look at this: http://www.jimscode.ca/index.php/component/content/article/13-c/45-c-simple-mmap-example
Most C programmers today still try to read files into memory using fopen and friends, but that's completely unnecessary and introduces additionel levels of complexity. (buffering, growing arrays, ...) and mmap is a nice alternative that moves all the nasty work down to the OS
Having e.g.
char *p;
p = malloc(SOME_SIZE);
p = "some string";
is equivalent to
int i;
i = 5;
i = 10;
When you reassign a pointer to point somewhere else, you loose the original value of the pointer, where it originally pointed. That will lead to a memory leak.
The simple answer is no. Once you have overwritten your copy of the pointer, then you can't get it back again.
In reality, the memory manager has knowledge of the pointer, but there is no standard way to get it.
It is printing everything but it terminating ...
Consider your memory allocation statements:
char *a;
a=(char *)malloc(sizeof(char));
By allocating only sizeof(char) bytes to the buffer a, then attempting to write anything more than the null terminator to it, you are invoking undefined behavior. (Note: sizeof(char) in C is by definition equal to 1, always)
C strings are defined as a null terminated character array. You have allocated only one byte. The only legal C string possible is one containing only the null termination byte. But your code attempts to write much more, and in so doing encroaches on memory locations not owned by your process. So in general, when creating strings, follow two simple rules:
- Determine
max lengthof string you need - allocate memory to
max length + 1bytes to accommodate termination byte.
Example if max string is x characters long, create the memory for x + 1 characters:
char inputStr[] = {"This string is x char long"};
char string = malloc(strlen(inputStr) +1); //+1 for null byte
strcpy(string, inputStr);
Note, in C it is not recommended to cast the return of malloc() and family.
You've two problems with your code. Firstly, you only allocate enough space for 1 character and since strings have to be NUL terminated, the longest string you could have is 0 characters long. Since you don't know how long the text you're going to read in, you could start with an arbitrary size (say 1024).
a=malloc(1024);
Secondly, scanf will only read up to the next space when you use "%s". It also isn't constrained by the available space in a. A better way to to read in an entire line of text, is to use fgets like this
fgets(a,1024,stdin);
This will read up to 1023 characters or up to and including the next newline character. It will NUL terminate the string for you as well.
You can then print it as a string.
printf("%s",a);