Unclear pointer and array Post: 302884282

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Top Forums Programming Unclear pointer and array Post 302884282 by Corona688 on Sunday 19th of January 2014 06:22:32 PM

01-19-2014

Registered User

Quote:

The most difficult parts with C to me are: 1) dynamic memory allocation of pointers

I think you're overthinking this quite a whole lot. You've divided this into a lot of separate categories like "the correct way to use pointers when malloc is involved", "the correct way to use pointers when printf is involved", "the correct way to use pointers when sprintf is involved", "the correct way to use pointers when strtok is involved", etc -- as if these things had nothing to do with each other.

If you look at them though, their arguments help give you an idea how they work.

Code:

$ man malloc

void *malloc(size_t size);

$ man strtok

char *strtok(char *str, const char *delim);

$ man strcpy

char *strcpy(char *dest, const char *src);

$ man sprintf

int sprintf(char *str, const char *format, ...);

$

malloc returns a 'void *'. It's not const, so the memory it returns is memory you're allowed to write to. 'void' means its of no particular type, that you're expected to cast it into the type you need. So malloc returns memory which you're supposed to cast into some other type, memory which you're allowed to write to.

strtok takes a 'char *', which means you're giving it memory that strtok's allowed to alter, as well as a 'const char *', memory it's not allowed to change -- the list of tokens. It returns a 'char *', a pointer to memory you're allowed to write to. (As it turns out, a pointer to the same memory, even.)

strcpy's arguments show you which one is src and which one is dest, even without their names -- the 'const char *' has to be the source because that's memory strcpy can't write to.

and sprintf takes a pointer to memory it can write to(the output string), memory it can't write to(the command string), and a variable number of arguments just like printf.

As for the way printf and sscanf demand pointers and non-pointers, there's nothing really deep to learn about C there, it's just the way the printf and scanf calls were built. They could have made printf demand everything be pointers, just like scanf does, but didn't.

Corona688

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LEARN ABOUT SUSE

string

STRING(3)						     Linux Programmer's Manual							 STRING(3)

NAME

       stpcpy, strcasecmp, strcat, strchr, strcmp, strcoll, strcpy, strcspn, strdup, strfry, strlen, strncat, strncmp, strncpy, strncasecmp, strp-
       brk, strrchr, strsep, strspn, strstr, strtok, strxfrm, index, rindex - string operations

SYNOPSIS

       #include <strings.h>

       int strcasecmp(const char *s1, const char *s2);

       int strncasecmp(const char *s1, const char *s2, size_t n);

       char *index(const char *s, int c);

       char *rindex(const char *s, int c);

       #include <string.h>

       char *stpcpy(char *dest, const char *src);

       char *strcat(char *dest, const char *src);

       char *strchr(const char *s, int c);

       int strcmp(const char *s1, const char *s2);

       int strcoll(const char *s1, const char *s2);

       char *strcpy(char *dest, const char *src);

       size_t strcspn(const char *s, const char *reject);

       char *strdup(const char *s);

       char *strfry(char *string);

       size_t strlen(const char *s);

       char *strncat(char *dest, const char *src, size_t n);

       int strncmp(const char *s1, const char *s2, size_t n);

       char *strncpy(char *dest, const char *src, size_t n);

       char *strpbrk(const char *s, const char *accept);

       char *strrchr(const char *s, int c);

       char *strsep(char **stringp, const char *delim);

       size_t strspn(const char *s, const char *accept);

       char *strstr(const char *haystack, const char *needle);

       char *strtok(char *s, const char *delim);

       size_t strxfrm(char *dest, const char *src, size_t n);

DESCRIPTION

       The string functions perform string operations on null-terminated strings.  See the individual man pages for descriptions of each function.

SEE ALSO

       index(3), rindex(3), strcasecmp(3), stpcpy(3), strcat(3), strchr(3), strcmp(3), strcoll(3), strcpy(3),  strcspn(3),  strdup(3),	strfry(3),
       strlen(3),  strncasecmp(3),  strncat(3),  strncmp(3),  strncpy(3),  strpbrk(3),	strrchr(3),  strsep(3),  strspn(3),  strstr(3), strtok(3),
       strxfrm(3)

COLOPHON

       This page is part of release 3.25 of the Linux man-pages project.  A description of the project, and information about reporting bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

								    2010-02-25								 STRING(3)