How to use struct timeval to get the execution time

Question

After reading this article about elapsed time  I wrote a simple code to calculate the execution time of a loop    include  lt stdio h gt   include  lt sys time h gt   int main  int argc  char   argv        struct timeval  tvalBefore  tvalAfter       gettimeofday   amp tvalBefore  NULL       int i  0      while   i  lt  1000            i                gettimeofday   amp tvalAfter  NULL        printf  Time in microseconds   0 3f microseconds n                float  tvalAfter tv sec - tvalBefore tv sec                   return 0      The clang compiler gives me the following errors   print time c 7 16  error  expected identifier or             struct timeval   tvalBefore   tvalAfter                          print time c 13 17  error  use of undeclared identifier  tvalBefore          gettimeofday   amp tvalBefore  NULL                            print time c 19 17  error  use of undeclared identifier  tvalAfter          gettimeofday   amp tvalAfter  NULL                            print time c 22 12  error  use of undeclared identifier  tvalAfter                           float  tvalAfter tv sec - tvalBefore tv sec                                     print time c 22 31  error  use of undeclared identifier  tvalBefore                           float  tvalAfter tv sec - tvalBefore tv sec                                                        5 errors generated    I can t figure out what s wrong with my code  any idea

User · Answer

Change   struct timeval  tvalBefore  tvalAfter     Looks like an attempt to                                           delcare a variable with                                           no name       to   struct timeval tvalBefore  tvalAfter    It is less likely  IMO  to make this mistake if there is a single declaration per line   struct timeval tvalBefore  struct timeval tvalAfter    It becomes more error prone when declaring pointers to types on a single line   struct timeval  tvalBefore  tvalAfter    tvalBefore is a struct timeval  but tvalAfter is a struct timeval

User · Answer

You have two typing errors in your code    struct timeval    should be   struct timeval   and after the printf   parenthesis you need a semicolon   Also  depending on the compiler  so simple a cycle might just be optimized out  giving you a time of 0 microseconds whatever you do   Finally  the time calculation is wrong  You only take into accounts the seconds  ignoring the microseconds  You need to get the difference between seconds  multiply by one million  then add  after  tv usec and subtract  before  tv usec  You gain nothing by casting an integer number of seconds to a float   I d suggest checking out the man page for struct timeval   This is the code    include  lt stdio h gt   include  lt sys time h gt   int main  int argc  char   argv        struct timeval tvalBefore  tvalAfter      removed comma      gettimeofday   amp tvalBefore  NULL       int i  0      while   i  lt  10000            i                gettimeofday   amp tvalAfter  NULL           Changed format to long int   ld   changed time calculation      printf  Time in microseconds   ld microseconds n                 tvalAfter tv sec - tvalBefore tv sec  1000000L             tvalAfter tv usec  - tvalBefore tv usec                 Added semicolon     return 0

[c] How to use struct timeval to get the execution time?

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