08-05-2002
a problem about malloc()
1 . Thanks everyone who read the post.
2 . the programe is that :
#include <stdio.h>
#include <string.h>
void do_it(char *p)
{
p = (char *) malloc(100);
(void )strcpy(p,"1234");
}
int main(void)
{
char *p;
do_it(p);
(void )printf("p = %s \n",p);
return 0;
}
cc -g a.c -o a.out
./a.out
it will show that "memory fault(coredump)"
3 . Why ? I think that malloc() get memory from heap , and the heap can be seen at everywhere in the programe .In other words , it is "global" .
4 Then I write another programe. it works correctly .
#include <stdio.h>
#include <string.h>
char *do_it(void)
{
return ((char *) malloc(100));
}
int main(void)
{
char *p;
p = do_it();
strcpy(p, "asdfa");
(void) printf("p = %s \n",p);
}
5 . Then , the point 4 declare that point 3 is correct , but it is not we want .
6 . Then I think the OS has changed the value p when do_it() function return
10 More Discussions You Might Find Interesting
1. Programming
hello sir
since by mentioning a integer pointer and storing the integers
by incrementing the pointer value
then what is the purpose of malloc?
u can decalre it as
in t *p;
several integers can be stored by incrementing the value of p,
hence what is the diffrence between this... (2 Replies)
Discussion started by: rajashekaran
2 Replies
2. Programming
Hello
This is a simple program i carried out in my machine
i dont know how it is working
#include<alloc.h>
#include<stdio.h>
mian()
{
int *p,j;
p= (int*)malloc(1);
for(j=1;j<=580;j++)
{
*p=j;
++p;
}
p=p-580;
for(j=1;j<=580;j++)
{
printf("%d",*p);
} (7 Replies)
Discussion started by: rajashekaran
7 Replies
3. Programming
Hi All!
Does some one know I am under UNIX system can not allocate more then 1 Mb memory? It broke program down.
Any information would be greatly appreciated.
Thanks. (2 Replies)
Discussion started by: prodigal
2 Replies
4. Programming
Hi!
I hope this is the correct forum to post the question even if I'm a newbie...
I am a C-newbie (and really on the edge to be a C-addict ;) ) and have a question.
When should I use malloc?
To state it differently, when should I NOT use malloc?
For instance, if I have an array of... (5 Replies)
Discussion started by: Tonje
5 Replies
5. Programming
Some one please explain me what is Dynamic memory allocation and the use of malloc() function.How do we allocate memory dynamically and also the other way? (3 Replies)
Discussion started by: rash123
3 Replies
6. Programming
Hey Guys
I am trying to implement the malloc function for my OS class and I am having a little trouble with it. I would be really grateful if I could get some hints on this problem.
So I am using a doubly-linked list as my data structure and I have to allocate memory for it (duh...). The... (1 Reply)
Discussion started by: Gambit_b
1 Replies
7. UNIX for Dummies Questions & Answers
Do kmalloc and malloc allocate from same heap ? (3 Replies)
Discussion started by: dragonpoint
3 Replies
8. Programming
Which one is faster among malloc and new?
My understanding is that since new also has to call constructors after allocating memory it must be slower than malloc.
Am I correct? (1 Reply)
Discussion started by: rupeshkp728
1 Replies
9. UNIX for Dummies Questions & Answers
Good day! I'm a newbie in C. I'm trying to get an unlimited input from the user using malloc then printing the inputs after the user presses enter. My code works, but there's a warning that I don't know how to fix. Please help me. Thank you.
Here's my code:
#include <stdio.h>
#include... (6 Replies)
Discussion started by: eracav
6 Replies
10. Programming
Hello,
I am trying to read a text file into linked list, but always got the first and last records wrong.
1) The problem looks related to the initialization of the node temp with malloc(), but could not figure it out. No error/warning at compiling, though.
2) The output file is empty,... (10 Replies)
Discussion started by: yifangt
10 Replies
STRING(3) Library Functions Manual STRING(3)
NAME
string, strcat, strncat, strcmp, strncmp, strcpy, strncpy, strlen, index, rindex - string operations string, strcat, strncat, strcmp,
strncmp, strcpy, strncpy, strlen, strchr, strrchr, strerror, memcmp, memcpy, memmove, memchr, memset, index, rindex - string operations
SYNOPSIS
#include <sys/types.h>
#include <strings.h>
char *strcat(char *s1, const char *s2)
char *strncat(char *s1, const char *s2, size_t n)
int strcmp(const char *s1, const char *s2)
int strncmp(const char *s1, const char *s2, size_t n)
char *strcpy(char *s1, const char *s2)
char *strncpy(char *s1, const char *s2, size_t n)
size_t strlen(const char *s)
char *strchr(const char *s, int c)
char *strrchr(const char *s, int c)
char *strerror(int errnum)
int memcmp(const void *s1, const void *s2, size_t n)
void *memcpy(void *s1, const void *s2, size_t n)
void *memmove(void *s1, const void *s2, size_t n)
void *memchr(const void *s, int c, size_t n)
void *memset(void *s, int c, size_t n)
char *index(const char *s, int c)
char *rindex(const char *s, int c)
DESCRIPTION
These functions operate on null-terminated strings. They do not check for overflow of any receiving string.
Strcat appends a copy of string s2 to the end of string s1. Strncat copies at most n characters. Both return a pointer to the null-termi-
nated result.
Strcmp compares its arguments and returns an integer greater than, equal to, or less than 0, according as s1 is lexicographically greater
than, equal to, or less than s2. Strncmp makes the same comparison but looks at at most n characters.
Strcpy copies string s2 to s1, stopping after the null character has been moved. Strncpy copies exactly n characters, truncating or null-
padding s2; the target may not be null-terminated if the length of s2 is n or more. Both return s1.
Strlen returns the number of non-null characters in s.
Strchr (strrchr) returns a pointer to the first (last) occurrence of character c in string s, or null if c does not occur in the string.
Strerror returns the error string for the system call error errnum. See intro(2).
Memcmp is like strcmp except that the strings are memory blocks of length n. Null characters are treated as ordinary characters.
Memcpy copies n bytes from the location pointed to by s2 to s1. Memmove is like memcpy, except that it can handle overlap between the two
strings. Both functions return s1.
Memchr returns a pointer to the first occurrence of character c in string s, or null if c does not occur in the string.
Memset sets n bytes to c starting at location s. It returns s.
Index and rindex are obsolete versions of strchr and strrchr. New code should avoid using them.
NOTES
Characters are compared as unsigned char, whether char itself is signed or not.
4th Berkeley Distribution May 15, 1985 STRING(3)