sprintf() problem

[karlc]

I encountered the problem as follows:

#include <stdio.h>
 
char* fun(char* b)
{
  char* sTmp;
  static char d[BUFSIZ];
  bzero(d, BUFSIZ);
  sprintf(d, "%s:%s", "Test", b);
  sTmp = d;
  return  sTmp;
}
 
main()
{
  char a[100];
  char b[100];
  char tmp[100];
  bzero(tmp, 100);
  strcpy(a, "ABC123");
  strcpy(b, "CDE456");
  sprintf(tmp, "a: %s\n", fun(a));
  printf("%s", tmp);
  sprintf(tmp, "b: %s\n", fun(b));
  printf("%s", tmp);
  sprintf(tmp, "a: %s b: %s\n", fun(a), fun(b));
  printf("%s", tmp);
}
 

/* result */

a: Test:ABC123
b: Test:CDE456
a: Test:CDE456 b: Test:CDE456

The final printf returned "CDE456" twice instead of "ABC123" and "CDE456". Could you please help to resolve the problem?

Thank you

Karlc

[horace1]

your function fun() uses a static char d[] to hold the string so when you call

  sprintf(tmp, "a: %s b: %s\n", fun(a), fun(b));
 

the result of the last fun(a) or fun(b) call is printed twice, i.e. if fun(a) is called first d[] holds "ABC123" when fun(b) is called this overwrites d[] with "CDE456" and hence "CDE456" is printed twice.

note with using g++ I get
a: Test:ABC123
b: Test:CDE456
a: Test:ABC123 b: Test:ABC123

this is because the order of the evalution of the function parameters is not specified, i.e. fun(a)may be called by called before fun(b) or the other way around

[karlc]

Dear horace1

So how could I fix this problem? Please help.

Charles

your function fun() uses a static char d[] to hold the string so when you call

  sprintf(tmp, "a: %s b: %s\n", fun(a), fun(b));
 

the result of the last fun(a) or fun(b) call is printed twice, i.e. if fun(a) is called first d[] holds "ABC123" when fun(b) is called this overwrites d[] with "CDE456" and hence "CDE456" is printed twice.

note with using g++ I get
a: Test:ABC123
b: Test:CDE456
a: Test:ABC123 b: Test:ABC123

this is because the order of the evalution of the function parameters is not specified, i.e. fun(a)may be called by called before fun(b) or the other way around

[karlc]

Sorry for my mistake. There shouldn't be static keyword.

If fact, the problem still existed when I removed it.

Please help. Thank you.

[horace1]

Dear horace1

So how could I fix this problem? Please help.

Charles

you could use malloc
http://www.cplusplus.com/malloc

to create a new array each time you call the function

  char* sTmp=malloc(BUFSIZ);
 

however, remember to free() the allocated memory once you have finished with it

[horace1]

Sorry for my mistake. There shouldn't be static keyword.

If fact, the problem still existed when I removed it.

Please help. Thank you.

if you don't use static in this context the storage allocated to d[] is lost on exit from the function fun(). The progam MAY still appear to work but can cause segmentation errors when the memory is used for something else later on.

[karlc]

Dear Horace1

malloc() did solve the problem. It is what I expected. Thank you.

However, as you reommended, how could I free() the memory if I need to return sTmp in the fun()? Could I free() the memory outside the fun()?

Please help.

Thank you

Charles

[horace1]

Dear Horace1

malloc() did solve the problem. It is what I expected. Thank you.

However, as you reommended, how could I free() the memory if I need to return sTmp in the fun()? Could I free() the memory outside the fun()?

Please help.

Thank you

Charles

yes, you just need the pointer

[chella]

Sorry for my mistake. There shouldn't be static keyword.

If fact, the problem still existed when I removed it.

Please help. Thank you.

Hi karlc,
Try executing this code. I hope u'll get the desired output.

#include <stdio.h>
#include<string.h>
void fun(char* b,char *str)
{
char d[100];
sprintf(d, "%s:%s", "Test", b);
strcpy(str,d);
}

int main()
{
char a[100];
char b[100];
char tmp[100];
char *str1,*str2;
str1=(char *)malloc(sizeof(char));
str2=(char *)malloc(sizeof(char));
strcpy(a, "ABC123");
strcpy(b, "CDE456");
fun(a,tmp);
printf("%s\n", tmp);
fun(b,tmp);
printf("%s\n", tmp);
fun(a,str1);
fun(b,str2);
sprintf(tmp, "a: %s b: %s\n", str1, str2);
printf("%s", tmp);
return 0;
}

Regards,
Chella

[karlc]

Dear Horace1

Glad to receive your reply again. You example worked well. Thank for your time.

However, I would like to keep return type of fun() be char*, because the function will be called by macro like:

#define MACRO1 fun("aaa")
#define MACRO2 fun("bbb")
....

I though malloc() could help as you advised. The only problem was how to free the memory in the char* fun()

Shall I declare sTmp as global variable (it seemed not good to declare global variables ...). Do you have any other good suggestions?

BTW, Thank for your help

Karlc

Hi karlc,
Try executing this code. I hope u'll get the desired output.

#include <stdio.h>
#include<string.h>
void fun(char* b,char *str)
{
char d[100];
sprintf(d, "%s:%s", "Test", b);
strcpy(str,d);
}

int main()
{
char a[100];
char b[100];
char tmp[100];
char *str1,*str2;
str1=(char *)malloc(sizeof(char));
str2=(char *)malloc(sizeof(char));
strcpy(a, "ABC123");
strcpy(b, "CDE456");
fun(a,tmp);
printf("%s\n", tmp);
fun(b,tmp);
printf("%s\n", tmp);
fun(a,str1);
fun(b,str2);
sprintf(tmp, "a: %s b: %s\n", str1, str2);
printf("%s", tmp);
return 0;
}

Regards,
Chella

[horace1]

Dear Horace1

Glad to receive your reply again. You example worked well. Thank for your time.

However, I would like to keep return type of fun() be char*, because the function will be called by macro like:

#define MACRO1 fun("aaa")
#define MACRO2 fun("bbb")
....

I though malloc() could help as you advised. The only problem was how to free the memory in the char* fun()

Shall I declare sTmp as global variable (it seemed not good to declare global variables ...). Do you have any other good suggestions?

BTW, Thank for your help

Karlc

I was assuming you do return a char * function result as in your original code, e.g.

char* fun(char* b)
{
  char* sTmp=malloc(BUFSIZ);
  bzero(sTmp, BUFSIZ);
  sprintf(sTmp, "%s:%s", "Test", b);
  return  sTmp;
}
 
 

[karlc]

Horace1

Yes. It was what I did as you said.

Is it OK not to call free() in the fun()?

Charles

I was assuming you do return a char * function result as in your original code, e.g.

char* fun(char* b)
{
  char* sTmp=malloc(BUFSIZ);
  bzero(sTmp, BUFSIZ);
  sprintf(sTmp, "%s:%s", "Test", b);
  return  sTmp;
}