strdup - what does it do in C

Question

What is the purpose of the strdup   function in C

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From strdup man   The strdup   function shall return a pointer to a new string  which is a duplicate of the string pointed to by s1  The returned pointer can be passed to free    A null pointer is returned if the new string cannot be created

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No point repeating the other answers  but please note that strdup   can do anything it wants from a C perspective  since it is not part of any C standard  It is however defined by POSIX 1-2001

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From strdup man   The strdup   function shall return a pointer to a new string  which is a duplicate of the string pointed to by s1  The returned pointer can be passed to free    A null pointer is returned if the new string cannot be created

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From strdup man   The strdup   function shall return a pointer to a new string  which is a duplicate of the string pointed to by s1  The returned pointer can be passed to free    A null pointer is returned if the new string cannot be created

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The most valuable thing it does is give you another string identical to the first  without requiring you to allocate memory  location and size  yourself  But  as noted  you still need to free it  but which doesn t require a quantity calculation  either

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The strdup   function is a shorthand for string duplicate  it takes in a parameter as a string constant or a string literal and allocates just enough space for the string and writes the corresponding characters in the space allocated and finally returns the address of the allocated space to the calling routine

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It makes a duplicate copy of the string passed in by running a malloc and strcpy of the string passed in  The malloc ed buffer is returned to the caller  hence the need to run free on the return value

User · Answer

No point repeating the other answers  but please note that strdup   can do anything it wants from a C perspective  since it is not part of any C standard  It is however defined by POSIX 1-2001

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char   strdup const char   s      size t len   1 strlen s     char  p   malloc len      return p   memcpy p  s  len    NULL      Maybe the code is a bit faster than with strcpy   as the  0 char doesn t need to be searched again  It already was with strlen

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It makes a duplicate copy of the string passed in by running a malloc and strcpy of the string passed in  The malloc ed buffer is returned to the caller  hence the need to run free on the return value

User · Answer

No point repeating the other answers  but please note that strdup   can do anything it wants from a C perspective  since it is not part of any C standard  It is however defined by POSIX 1-2001

User · Answer

char   strdup const char   s      size t len   1 strlen s     char  p   malloc len      return p   memcpy p  s  len    NULL      Maybe the code is a bit faster than with strcpy   as the  0 char doesn t need to be searched again  It already was with strlen

User · Answer

No point repeating the other answers  but please note that strdup   can do anything it wants from a C perspective  since it is not part of any C standard  It is however defined by POSIX 1-2001

User · Answer

Exactly what it sounds like  assuming you re used to the abbreviated way in which C and UNIX assigns words  it duplicates strings  -   Keeping in mind it s actually not part of the ISO C standard itself a   it s a POSIX thing   it s effectively doing the same as the following code   char  strdup const char  src        char  dst   malloc strlen  src    1       Space for length plus nul     if  dst    NULL  return NULL              No memory     strcpy dst  src                           Copy the characters     return dst                                Return the new string     In other words    It tries to allocate enough memory to hold the old string  plus a   0  character to mark the end of the string   If the allocation failed  it sets errno to ENOMEM and returns NULL immediately  Setting of errno to ENOMEM is something malloc does in POSIX so we don t need to explicitly do it in our strdup  If you re not POSIX compliant  ISO C doesn t actually mandate the existence of ENOMEM so I haven t included that here b   Otherwise the allocation worked so we copy the old string to the new string c  and return the new address  which the caller is responsible for freeing at some point     Keep in mind that s the conceptual definition  Any library writer worth their salary may have provided heavily optimised code targeting the particular processor being used      a  However  functions starting with str and a lower case letter are reserved by the standard for future directions  From C11 7 1 3 Reserved identifiers      Each header declares or defines all identifiers listed in its associated sub-clause  and  optionally declares or defines identifiers listed in its associated future library directions sub-clause      The future directions for string h can be found in C11 7 31 13 String handling  lt string h gt       Function names that begin with str  mem  or wcs and a lowercase letter may be added to the declarations in the  lt string h gt  header    So you should probably call it something else if you want to be safe      b  The change would basically be replacing if  d    NULL  return NULL  with   if  d    NULL        errno   ENOMEM      return NULL         c  Note that I use strcpy for that since that clearly shows the intent  In some implementations  it may be faster  since you already know the length  to use memcpy  as they may allow for transferring the data in larger chunks  or in parallel  Or it may not  -  Optimisation mantra  1   measure  don t guess    In any case  should you decide to go that route  you would do something like   char  strdup const char  src        size t len   strlen src    1           String plus   0      char  dst   malloc len                 Allocate space     if  dst    NULL  return NULL           No memory     memcpy  dst  src  len                  Copy the block     return dst                             Return the new string

User · Answer

The strdup   function is a shorthand for string duplicate  it takes in a parameter as a string constant or a string literal and allocates just enough space for the string and writes the corresponding characters in the space allocated and finally returns the address of the allocated space to the calling routine

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It makes a duplicate copy of the string passed in by running a malloc and strcpy of the string passed in  The malloc ed buffer is returned to the caller  hence the need to run free on the return value

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strdup   does dynamic memory allocation for the character array including the end character   0  and returns the address of the heap memory   char  strdup  const char  s        char  p   malloc  strlen  s    1        allocate memory     if  p    NULL          strcpy  p s                         copy string     return p                                return the memory     So  what it does is give us another string identical to the string given by its argument  without requiring us to allocate memory  But we still need to free it  later

User · Answer

Exactly what it sounds like  assuming you re used to the abbreviated way in which C and UNIX assigns words  it duplicates strings  -   Keeping in mind it s actually not part of the ISO C standard itself a   it s a POSIX thing   it s effectively doing the same as the following code   char  strdup const char  src        char  dst   malloc strlen  src    1       Space for length plus nul     if  dst    NULL  return NULL              No memory     strcpy dst  src                           Copy the characters     return dst                                Return the new string     In other words    It tries to allocate enough memory to hold the old string  plus a   0  character to mark the end of the string   If the allocation failed  it sets errno to ENOMEM and returns NULL immediately  Setting of errno to ENOMEM is something malloc does in POSIX so we don t need to explicitly do it in our strdup  If you re not POSIX compliant  ISO C doesn t actually mandate the existence of ENOMEM so I haven t included that here b   Otherwise the allocation worked so we copy the old string to the new string c  and return the new address  which the caller is responsible for freeing at some point     Keep in mind that s the conceptual definition  Any library writer worth their salary may have provided heavily optimised code targeting the particular processor being used      a  However  functions starting with str and a lower case letter are reserved by the standard for future directions  From C11 7 1 3 Reserved identifiers      Each header declares or defines all identifiers listed in its associated sub-clause  and  optionally declares or defines identifiers listed in its associated future library directions sub-clause      The future directions for string h can be found in C11 7 31 13 String handling  lt string h gt       Function names that begin with str  mem  or wcs and a lowercase letter may be added to the declarations in the  lt string h gt  header    So you should probably call it something else if you want to be safe      b  The change would basically be replacing if  d    NULL  return NULL  with   if  d    NULL        errno   ENOMEM      return NULL         c  Note that I use strcpy for that since that clearly shows the intent  In some implementations  it may be faster  since you already know the length  to use memcpy  as they may allow for transferring the data in larger chunks  or in parallel  Or it may not  -  Optimisation mantra  1   measure  don t guess    In any case  should you decide to go that route  you would do something like   char  strdup const char  src        size t len   strlen src    1           String plus   0      char  dst   malloc len                 Allocate space     if  dst    NULL  return NULL           No memory     memcpy  dst  src  len                  Copy the block     return dst                             Return the new string

User · Answer

Exactly what it sounds like  assuming you re used to the abbreviated way in which C and UNIX assigns words  it duplicates strings  -   Keeping in mind it s actually not part of the ISO C standard itself a   it s a POSIX thing   it s effectively doing the same as the following code   char  strdup const char  src        char  dst   malloc strlen  src    1       Space for length plus nul     if  dst    NULL  return NULL              No memory     strcpy dst  src                           Copy the characters     return dst                                Return the new string     In other words    It tries to allocate enough memory to hold the old string  plus a   0  character to mark the end of the string   If the allocation failed  it sets errno to ENOMEM and returns NULL immediately  Setting of errno to ENOMEM is something malloc does in POSIX so we don t need to explicitly do it in our strdup  If you re not POSIX compliant  ISO C doesn t actually mandate the existence of ENOMEM so I haven t included that here b   Otherwise the allocation worked so we copy the old string to the new string c  and return the new address  which the caller is responsible for freeing at some point     Keep in mind that s the conceptual definition  Any library writer worth their salary may have provided heavily optimised code targeting the particular processor being used      a  However  functions starting with str and a lower case letter are reserved by the standard for future directions  From C11 7 1 3 Reserved identifiers      Each header declares or defines all identifiers listed in its associated sub-clause  and  optionally declares or defines identifiers listed in its associated future library directions sub-clause      The future directions for string h can be found in C11 7 31 13 String handling  lt string h gt       Function names that begin with str  mem  or wcs and a lowercase letter may be added to the declarations in the  lt string h gt  header    So you should probably call it something else if you want to be safe      b  The change would basically be replacing if  d    NULL  return NULL  with   if  d    NULL        errno   ENOMEM      return NULL         c  Note that I use strcpy for that since that clearly shows the intent  In some implementations  it may be faster  since you already know the length  to use memcpy  as they may allow for transferring the data in larger chunks  or in parallel  Or it may not  -  Optimisation mantra  1   measure  don t guess    In any case  should you decide to go that route  you would do something like   char  strdup const char  src        size t len   strlen src    1           String plus   0      char  dst   malloc len                 Allocate space     if  dst    NULL  return NULL           No memory     memcpy  dst  src  len                  Copy the block     return dst                             Return the new string

User · Answer

The most valuable thing it does is give you another string identical to the first  without requiring you to allocate memory  location and size  yourself  But  as noted  you still need to free it  but which doesn t require a quantity calculation  either

User · Answer

From strdup man   The strdup   function shall return a pointer to a new string  which is a duplicate of the string pointed to by s1  The returned pointer can be passed to free    A null pointer is returned if the new string cannot be created

User · Answer

The most valuable thing it does is give you another string identical to the first  without requiring you to allocate memory  location and size  yourself  But  as noted  you still need to free it  but which doesn t require a quantity calculation  either

User · Answer

strdup and strndup are defined in POSIX compliant systems as   char  strdup const char  str   char  strndup const char  str  size t len     The strdup   function allocates sufficient memory for a copy of the string str  does the copy  and returns a pointer to it   The pointer may subsequently be used as an argument to the function free   If insufficient memory is available  NULL is returned and errno is set to ENOMEM   The strndup   function copies at most len characters from the string str always null terminating the copied string

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strdup   does dynamic memory allocation for the character array including the end character   0  and returns the address of the heap memory   char  strdup  const char  s        char  p   malloc  strlen  s    1        allocate memory     if  p    NULL          strcpy  p s                         copy string     return p                                return the memory     So  what it does is give us another string identical to the string given by its argument  without requiring us to allocate memory  But we still need to free it  later

User · Answer

The most valuable thing it does is give you another string identical to the first  without requiring you to allocate memory  location and size  yourself  But  as noted  you still need to free it  but which doesn t require a quantity calculation  either

User · Answer

It makes a duplicate copy of the string passed in by running a malloc and strcpy of the string passed in  The malloc ed buffer is returned to the caller  hence the need to run free on the return value

User · Answer

Exactly what it sounds like  assuming you re used to the abbreviated way in which C and UNIX assigns words  it duplicates strings  -   Keeping in mind it s actually not part of the ISO C standard itself a   it s a POSIX thing   it s effectively doing the same as the following code   char  strdup const char  src        char  dst   malloc strlen  src    1       Space for length plus nul     if  dst    NULL  return NULL              No memory     strcpy dst  src                           Copy the characters     return dst                                Return the new string     In other words    It tries to allocate enough memory to hold the old string  plus a   0  character to mark the end of the string   If the allocation failed  it sets errno to ENOMEM and returns NULL immediately  Setting of errno to ENOMEM is something malloc does in POSIX so we don t need to explicitly do it in our strdup  If you re not POSIX compliant  ISO C doesn t actually mandate the existence of ENOMEM so I haven t included that here b   Otherwise the allocation worked so we copy the old string to the new string c  and return the new address  which the caller is responsible for freeing at some point     Keep in mind that s the conceptual definition  Any library writer worth their salary may have provided heavily optimised code targeting the particular processor being used      a  However  functions starting with str and a lower case letter are reserved by the standard for future directions  From C11 7 1 3 Reserved identifiers      Each header declares or defines all identifiers listed in its associated sub-clause  and  optionally declares or defines identifiers listed in its associated future library directions sub-clause      The future directions for string h can be found in C11 7 31 13 String handling  lt string h gt       Function names that begin with str  mem  or wcs and a lowercase letter may be added to the declarations in the  lt string h gt  header    So you should probably call it something else if you want to be safe      b  The change would basically be replacing if  d    NULL  return NULL  with   if  d    NULL        errno   ENOMEM      return NULL         c  Note that I use strcpy for that since that clearly shows the intent  In some implementations  it may be faster  since you already know the length  to use memcpy  as they may allow for transferring the data in larger chunks  or in parallel  Or it may not  -  Optimisation mantra  1   measure  don t guess    In any case  should you decide to go that route  you would do something like   char  strdup const char  src        size t len   strlen src    1           String plus   0      char  dst   malloc len                 Allocate space     if  dst    NULL  return NULL           No memory     memcpy  dst  src  len                  Copy the block     return dst                             Return the new string

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The statement   strcpy ptr2  ptr1     is equivalent to  other than the fact this changes the pointers    while  ptr2      ptr1       Whereas   ptr2   strdup ptr1     is equivalent to   ptr2   malloc strlen ptr1    1   if  ptr2    NULL  strcpy ptr2  ptr1     So  if you want the string which you have copied to be used in another function  as it is created in heap section   you can use strdup  else strcpy is enough

User · Answer

strdup and strndup are defined in POSIX compliant systems as   char  strdup const char  str   char  strndup const char  str  size t len     The strdup   function allocates sufficient memory for a copy of the string str  does the copy  and returns a pointer to it   The pointer may subsequently be used as an argument to the function free   If insufficient memory is available  NULL is returned and errno is set to ENOMEM   The strndup   function copies at most len characters from the string str always null terminating the copied string

User · Answer

The statement   strcpy ptr2  ptr1     is equivalent to  other than the fact this changes the pointers    while  ptr2      ptr1       Whereas   ptr2   strdup ptr1     is equivalent to   ptr2   malloc strlen ptr1    1   if  ptr2    NULL  strcpy ptr2  ptr1     So  if you want the string which you have copied to be used in another function  as it is created in heap section   you can use strdup  else strcpy is enough

[c] strdup() - what does it do in C?

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