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Top Forums Programming Segmentation fault when I pass a char pointer to a function in C. Post 303016197 by jose_spain on Saturday 21st of April 2018 10:16:06 AM
Old 04-21-2018
Thanks, but I want to understand pointers use and this is the reason that I do it with a pointer.

I have done new tests with this program and I have found that:

1) If tmp is declared and initialized in main so, it works right without changes in "reverse" function:

Code:
char* tmp;

  tmp = (char*) malloc ((strlen("hello") + 1) * sizeof(char));
  strcpy(tmp, "hello");
  printf("tmp=%s\n" , tmp);
  reverse(tmp);
  printf("After calling reverse: tmp=%s\n\n" , tmp);

The program prints:

Code:
tmp=hello
After calling reverse: tmp=olleh

2) If instead I do this:

Code:
char* tmp = "hello";

  printf("tmp=%s\n" , tmp);
  reverse(tmp);
  printf("After calling reverse: tmp=%s\n\n" , tmp);

Then I get the segmentation fault.

What is the difference?

In this second case, If I check the value of s in reverse, it is:
"hello" + 0 (end of string mark)


Moderator's Comments:
Mod Comment Seriously: Please use CODE tags as required by forum rules!

Last edited by RudiC; 04-21-2018 at 11:20 AM.. Reason: Added CODE tags.
 

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rmvb(9F)						   Kernel Functions for Drivers 						  rmvb(9F)

NAME
rmvb - remove a message block from a message SYNOPSIS
#include <sys/stream.h> mblk_t *rmvb(mblk_t *mp, mblk_t *bp); INTERFACE LEVEL
Architecture independent level 1 (DDI/DKI). PARAMETERS
mp Message from which a block is to be removed. mblk_t is an instance of the msgb(9S) structure. bp Message block to be removed. DESCRIPTION
rmvb() removes a message block (bp) from a message (mp), and returns a pointer to the altered message. The message block is not freed, merely removed from the message. It is the module or driver's responsibility to free the message block. RETURN VALUES
If successful, a pointer to the message (minus the removed block) is returned. The pointer is NULL if bp was the only block of the message before rmvb() was called. If the designated message block (bp) does not exist, -1 is returned. CONTEXT
rmvb() can be called from user or interrupt context. EXAMPLES
This routine removes all zero-length M_DATA message blocks from the given message. For each message block in the message, save the next message block (line 10). If the current message block is of type M_DATA and has no data in its buffer (line 11), then remove it from the message (line 12) and free it (line 13). In either case, continue with the next message block in the message (line 16). 1 void 2 xxclean(mp) 3 mblk_t *mp; 4 { 5 mblk_t *tmp; 6 mblk_t *nmp; 7 8 tmp = mp; 9 while (tmp) { 10 nmp = tmp->b_cont; 11 if ((tmp->b_datap->db_type == M_DATA) && (tmp->b_rptr == tmp->b_wptr)) { 12 (void) rmvb(mp, tmp); 13 freeb(tmp); 14 } 15 tmp = nmp; 16 } 17 } SEE ALSO
freeb(9F), msgb(9S) Writing Device Drivers STREAMS Programming Guide SunOS 5.10 11 Apr 1991 rmvb(9F)
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