Concatenating strings in C which method is more efficient

Question

I came across these two methods to concatenate strings   Common part   char  first   First   char  second    Second   char  both   malloc strlen first    strlen second    2     Method 1   strcpy both  first   strcat both                 or space could have been part of one of the strings strcat both  second     Method 2   sprintf both    s  s   first  second     In both cases the content of both would be  First Second    I would like to know which one is more efficient  I have to perform several concatenation operations   or if you know a better way to do it

User · Answer

I don t know that in case two there s any real concatenation done  Printing them back to back doesn t constitute concatenation   Tell me though  which would be faster   1  a  copy string A to new buffer b  copy string B to buffer c  copy buffer to output buffer  or  1 copy string A to output buffer b  copy string b to output buffer

User · Answer

sprintf   is designed to handle far more than just strings  strcat   is specialist  But I suspect that you are sweating the small stuff  C strings are fundamentally inefficient in ways that make the differences between these two proposed methods insignificant  Read  Back to Basics  by Joel Spolsky for the gory details   This is an instance where C   generally performs better than C  For heavy weight string handling using std  string is likely to be more efficient and certainly safer    edit    2nd edit Corrected code  too many iterations in C string implementation   timings  and conclusion change accordingly  I was surprised at Andrew Bainbridge s comment that std  string was slower  but he did not post complete code for this test case  I modified his  automating the timing  and added a std  string test  The test was on VC   2008  native code  with default  Release  options  i e  optimised   Athlon dual core  2 6GHz  Results   C string handling   0 023000 seconds sprintf             0 313000 seconds std  string         0 500000 seconds   So here strcat   is faster by far  your milage may vary depending on compiler and options   despite the inherent inefficiency of the C string convention  and supports my original suggestion that sprintf   carries a lot of baggage not required for this purpose  It remains by far the least readable and safe however  so when performance is not critical  has little merit IMO   I also tested a std  stringstream implementation  which was far slower again  but for complex string formatting still has merit   Corrected code follows    include  lt ctime gt   include  lt cstdio gt   include  lt cstring gt   include  lt string gt   void a char  first  char  second  char  both        for  int i   0  i    1000000  i                  strcpy both  first           strcat both                strcat both  second            void b char  first  char  second  char  both        for  int i   0  i    1000000  i            sprintf both    s  s   first  second      void c char  first  char  second  char  both        std  string first s first        std  string second s second        std  string both s second         for  int i   0  i    1000000  i            both s   first s         second s      int main void        char  first   First       char  second    Second       char  both    char   malloc  strlen first    strlen second    2    sizeof char        clock t start        start   clock         a first  second  both       printf   C string handling    f seconds n    float  clock   - start  CLOCKS PER SEC         start   clock         b first  second  both       printf   sprintf              f seconds n    float  clock   - start  CLOCKS PER SEC         start   clock         c first  second  both       printf   std  string          f seconds n    float  clock   - start  CLOCKS PER SEC         return 0

User · Answer

strcpy and strcat are much simpler oprations compared to sprintf  which needs to parse the format string strcpy and strcat are small so they will generally be inlined by the compilers  saving even one more extra function call overhead  For example  in llvm strcat will be inlined using a strlen to find copy starting position  followed by a simple store instruction

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size t lf   strlen first   size t ls   strlen second    char  both    char   malloc  lf   ls   2    sizeof char     strcpy both  first    both lf         strcpy  amp both lf 1   second

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Don t worry about efficiency  make your code readable and maintainable   I doubt the difference between these methods is going to matter in your program

User · Answer

They should be pretty much the same  The difference isn t going to matter  I would go with sprintf since it requires less code

User · Answer

Here s some madness for you  I actually went and measured it  Bloody hell  imagine that  I think I got some meaningful results   I used a dual core P4  running Windows  using mingw gcc 4 4  building with  gcc foo c -o foo exe -std c99 -Wall -O2    I tested method 1 and method 2 from the original post  Initially kept the malloc outside the benchmark loop  Method 1 was 48 times faster than method 2  Bizarrely  removing -O2 from the build command made the resulting exe 30  faster  haven t investigated why yet    Then I added a malloc and free inside the loop  That slowed down method 1 by a factor of 4 4  Method 2 slowed down by a factor of 1 1   So  malloc   strlen   free DO NOT dominate the profile enough to make avoiding sprintf worth while   Here s the code I used  apart from the loops were implemented with  lt  instead of    but that broke the HTML rendering of this post    void a char  first  char  second  char  both        for  int i   0  i    1000000   48  i                  strcpy both  first           strcat both                strcat both  second            void b char  first  char  second  char  both        for  int i   0  i    1000000   1  i            sprintf both    s  s   first  second      int main void        char  first   First       char  second    Second       char  both    char   malloc  strlen first    strlen second    2    sizeof char            Takes 3 7 sec with optimisations  2 7 sec WITHOUT optimisations      a first  second  both           Takes 3 7 sec with or without optimisations       b first  second  both        return 0

User · Answer

For readability  I d go with  char   s   malloc snprintf NULL  0    s  s   first  second    1   sprintf s    s  s   first  second     If your platform supports GNU extensions   you could also use asprintf     char   s   NULL  asprintf  amp s    s  s   first  second     If you re stuck with the MS C Runtime  you have to use  scprintf   to determine the length of the resulting string   char   s   malloc  scprintf   s  s   first  second    1   sprintf s    s  s   first  second     The following will most likely be the fastest solution   size t len1   strlen first   size t len2   strlen second    char   s   malloc len1   len2   2   memcpy s  first  len1   s len1         memcpy s   len1   1  second  len2   1      includes terminating null

User · Answer

The difference is unlikely to matter    If your strings are small  the malloc will drown out the string concatenations  If your strings are large  the time spent copying the data will drown out the differences between strcat   sprintf    As other posters have mentioned  this is a premature optimization   Concentrate on algorithm design  and only come back to this if profiling shows it to be a performance problem   That said    I suspect method 1 will be faster   There is some---admittedly small---overhead to parse the sprintf format-string   And strcat is more likely  inline-able

User · Answer

Neither is terribly efficient since both methods have to calculate the string length or scan it each time   Instead  since you calculate the strlen  s of the individual strings anyway  put them in variables and then just strncpy   twice

[c] Concatenating strings in C, which method is more efficient?

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