What s the best way to check if a file exists in C

Question

Is there a better way than simply trying to open the file   int exists const char  fname        FILE  file      if   file   fopen fname   r                   fclose file           return 1            return 0

User · Answer

Yes  Use stat    See the man page forstat 2    stat   will fail if the file doesn t exist  otherwise most likely succeed  If it does exist  but you have no read access to the directory where it exists  it will also fail  but in that case any method will fail  how can you inspect the content of a directory you may not see according to access rights  Simply  you can t    Oh  as someone else mentioned  you can also use access    However I prefer stat    as if the file exists it will immediately get me lots of useful information  when was it last updated  how big is it  owner and or group that owns the file  access permissions  and so on

User · Answer

Yes  Use stat    See the man page forstat 2    stat   will fail if the file doesn t exist  otherwise most likely succeed  If it does exist  but you have no read access to the directory where it exists  it will also fail  but in that case any method will fail  how can you inspect the content of a directory you may not see according to access rights  Simply  you can t    Oh  as someone else mentioned  you can also use access    However I prefer stat    as if the file exists it will immediately get me lots of useful information  when was it last updated  how big is it  owner and or group that owns the file  access permissions  and so on

User · Answer

Yes  Use stat    See the man page forstat 2    stat   will fail if the file doesn t exist  otherwise most likely succeed  If it does exist  but you have no read access to the directory where it exists  it will also fail  but in that case any method will fail  how can you inspect the content of a directory you may not see according to access rights  Simply  you can t    Oh  as someone else mentioned  you can also use access    However I prefer stat    as if the file exists it will immediately get me lots of useful information  when was it last updated  how big is it  owner and or group that owns the file  access permissions  and so on

User · Answer

FILE  file      if  file   fopen  sample txt   r     NULL                           file exists             fclose file                 else                         File not found  no memory leak since  file     NULL               fclose file  would cause an error

User · Answer

Usually when you want to check if a file exists  it s because you want to create that file if it doesn t   Graeme Perrow s answer is good if you don t want to create that file  but it s vulnerable to a race condition if you do  another process could create the file in between you checking if it exists  and you actually opening it to write to it    Don t laugh    this could have bad security implications if the file created was a symlink    If you want to check for existence and create the file if it doesn t exist  atomically so that there are no race conditions  then use this    include  lt fcntl h gt   include  lt errno h gt   fd   open pathname  O CREAT   O WRONLY   O EXCL  S IRUSR   S IWUSR   if  fd  lt  0         failure      if  errno    EEXIST           the file already existed                  else        now you can use the file

User · Answer

Use stat like this    include  lt sys stat h gt       stat  include  lt stdbool h gt        bool type  bool file exists  char  filename      struct stat   buffer       return  stat  filename   amp buffer     0       and call it like this    include  lt stdio h gt          printf  int main int ac  char   av        if  ac    2          return 1       if  file exists av 1            printf   s exists n   av 1        else         printf   s does not exist n   av 1         return 0

User · Answer

Usually when you want to check if a file exists  it s because you want to create that file if it doesn t   Graeme Perrow s answer is good if you don t want to create that file  but it s vulnerable to a race condition if you do  another process could create the file in between you checking if it exists  and you actually opening it to write to it    Don t laugh    this could have bad security implications if the file created was a symlink    If you want to check for existence and create the file if it doesn t exist  atomically so that there are no race conditions  then use this    include  lt fcntl h gt   include  lt errno h gt   fd   open pathname  O CREAT   O WRONLY   O EXCL  S IRUSR   S IWUSR   if  fd  lt  0         failure      if  errno    EEXIST           the file already existed                  else        now you can use the file

User · Answer

I think that access   function  which is found in unistd h is a good choice for Linux  you can use stat too    You can Use it like this    include  lt stdio h gt   include  lt stdlib h gt   include lt unistd h gt   void fileCheck const char  fileName    int main  void        char  fileName     etc sudoers        fileCheck fileName       return 0     void fileCheck const char  fileName        if  access fileName  F OK             printf  The File  s t was Found n  fileName        else          printf  The File  s t not Found n  fileName              if  access fileName  R OK             printf  The File  s t can be read n  fileName        else          printf  The File  s t cannot be read n  fileName              if  access  fileName  W OK             printf  The File  s t it can be Edited n  fileName        else          printf  The File  s t it cannot be Edited n  fileName              if  access  fileName  X OK             printf  The File  s t is an Executable n  fileName        else          printf  The File  s t is not an Executable n  fileName             And you get the following Output   The File  etc sudoers    was Found The File  etc sudoers    cannot be read The File  etc sudoers    it cannot be Edited The File  etc sudoers    is not an Executable

User · Answer

From the Visual C   help  I d tend to go with     ACCESS C  This example uses  access to check the    file named  ACCESS C  to see if it exists and if    writing is allowed        include   lt io h gt   include   lt stdio h gt   include   lt stdlib h gt   void main  void           Check for existence       if    access   ACCESS C   0       -1              printf   File ACCESS C exists n              Check for write permission          if    access   ACCESS C   2       -1            printf   File ACCESS C has write permission n              Also worth noting mode values of  access const char  path int mode     00  Existence only 02  Write permission  04  Read permission 06  Read and write permission    As your fopen could fail in situations where the file existed but could not be opened as requested   Edit  Just read Mecki s post   stat   does look like a neater way to go  Ho hum

User · Answer

Usually when you want to check if a file exists  it s because you want to create that file if it doesn t   Graeme Perrow s answer is good if you don t want to create that file  but it s vulnerable to a race condition if you do  another process could create the file in between you checking if it exists  and you actually opening it to write to it    Don t laugh    this could have bad security implications if the file created was a symlink    If you want to check for existence and create the file if it doesn t exist  atomically so that there are no race conditions  then use this    include  lt fcntl h gt   include  lt errno h gt   fd   open pathname  O CREAT   O WRONLY   O EXCL  S IRUSR   S IWUSR   if  fd  lt  0         failure      if  errno    EEXIST           the file already existed                  else        now you can use the file

User · Answer

You can use realpath   function   resolved file   realpath file path  NULL   if   resolved keyfile         File dosn t exists      perror keyfile      return -1

User · Answer

Use stat like this    include  lt sys stat h gt       stat  include  lt stdbool h gt        bool type  bool file exists  char  filename      struct stat   buffer       return  stat  filename   amp buffer     0       and call it like this    include  lt stdio h gt          printf  int main int ac  char   av        if  ac    2          return 1       if  file exists av 1            printf   s exists n   av 1        else         printf   s does not exist n   av 1         return 0

User · Answer

From the Visual C   help  I d tend to go with     ACCESS C  This example uses  access to check the    file named  ACCESS C  to see if it exists and if    writing is allowed        include   lt io h gt   include   lt stdio h gt   include   lt stdlib h gt   void main  void           Check for existence       if    access   ACCESS C   0       -1              printf   File ACCESS C exists n              Check for write permission          if    access   ACCESS C   2       -1            printf   File ACCESS C has write permission n              Also worth noting mode values of  access const char  path int mode     00  Existence only 02  Write permission  04  Read permission 06  Read and write permission    As your fopen could fail in situations where the file existed but could not be opened as requested   Edit  Just read Mecki s post   stat   does look like a neater way to go  Ho hum

User · Answer

I can t remember where I got this from that I used in my assignment  I usually copy paste the URL where I found it as an easy form of citation  and I cannot decipher what the trailing  file    0 is although my best guess is that it is redundancy to set any references to file to 0   define fclose file     file    fclose file    0   file    0   file if a pointer declared as  FILE  file NULL

User · Answer

Yes  Use stat    See the man page forstat 2    stat   will fail if the file doesn t exist  otherwise most likely succeed  If it does exist  but you have no read access to the directory where it exists  it will also fail  but in that case any method will fail  how can you inspect the content of a directory you may not see according to access rights  Simply  you can t    Oh  as someone else mentioned  you can also use access    However I prefer stat    as if the file exists it will immediately get me lots of useful information  when was it last updated  how big is it  owner and or group that owns the file  access permissions  and so on

User · Answer

From the Visual C   help  I d tend to go with     ACCESS C  This example uses  access to check the    file named  ACCESS C  to see if it exists and if    writing is allowed        include   lt io h gt   include   lt stdio h gt   include   lt stdlib h gt   void main  void           Check for existence       if    access   ACCESS C   0       -1              printf   File ACCESS C exists n              Check for write permission          if    access   ACCESS C   2       -1            printf   File ACCESS C has write permission n              Also worth noting mode values of  access const char  path int mode     00  Existence only 02  Write permission  04  Read permission 06  Read and write permission    As your fopen could fail in situations where the file existed but could not be opened as requested   Edit  Just read Mecki s post   stat   does look like a neater way to go  Ho hum

User · Answer

Use stat like this    include  lt sys stat h gt       stat  include  lt stdbool h gt        bool type  bool file exists  char  filename      struct stat   buffer       return  stat  filename   amp buffer     0       and call it like this    include  lt stdio h gt          printf  int main int ac  char   av        if  ac    2          return 1       if  file exists av 1            printf   s exists n   av 1        else         printf   s does not exist n   av 1         return 0

User · Answer

From the Visual C   help  I d tend to go with     ACCESS C  This example uses  access to check the    file named  ACCESS C  to see if it exists and if    writing is allowed        include   lt io h gt   include   lt stdio h gt   include   lt stdlib h gt   void main  void           Check for existence       if    access   ACCESS C   0       -1              printf   File ACCESS C exists n              Check for write permission          if    access   ACCESS C   2       -1            printf   File ACCESS C has write permission n              Also worth noting mode values of  access const char  path int mode     00  Existence only 02  Write permission  04  Read permission 06  Read and write permission    As your fopen could fail in situations where the file existed but could not be opened as requested   Edit  Just read Mecki s post   stat   does look like a neater way to go  Ho hum

User · Answer

Look up the access   function  found in unistd h  You can replace your function with if  access  fname  F OK      0            file exists   else          file doesn t exist    You can also use R OK  W OK  and X OK in place of F OK to check for read permission  write permission  and execute permission  respectively  rather than existence  and you can OR any of them together  i e  check for both read and write permission using R OK W OK  Update  Note that on Windows  you can t use W OK to reliably test for write permission  since the access function does not take DACLs into account  access  fname  W OK   may return 0  success  because the file does not have the read-only attribute set  but you still may not have permission to write to the file

User · Answer

You can use realpath   function   resolved file   realpath file path  NULL   if   resolved keyfile         File dosn t exists      perror keyfile      return -1

User · Answer

I can t remember where I got this from that I used in my assignment  I usually copy paste the URL where I found it as an easy form of citation  and I cannot decipher what the trailing  file    0 is although my best guess is that it is redundancy to set any references to file to 0   define fclose file     file    fclose file    0   file    0   file if a pointer declared as  FILE  file NULL

User · Answer

Look up the access   function  found in unistd h  You can replace your function with if  access  fname  F OK      0            file exists   else          file doesn t exist    You can also use R OK  W OK  and X OK in place of F OK to check for read permission  write permission  and execute permission  respectively  rather than existence  and you can OR any of them together  i e  check for both read and write permission using R OK W OK  Update  Note that on Windows  you can t use W OK to reliably test for write permission  since the access function does not take DACLs into account  access  fname  W OK   may return 0  success  because the file does not have the read-only attribute set  but you still may not have permission to write to the file

User · Answer

Usually when you want to check if a file exists  it s because you want to create that file if it doesn t   Graeme Perrow s answer is good if you don t want to create that file  but it s vulnerable to a race condition if you do  another process could create the file in between you checking if it exists  and you actually opening it to write to it    Don t laugh    this could have bad security implications if the file created was a symlink    If you want to check for existence and create the file if it doesn t exist  atomically so that there are no race conditions  then use this    include  lt fcntl h gt   include  lt errno h gt   fd   open pathname  O CREAT   O WRONLY   O EXCL  S IRUSR   S IWUSR   if  fd  lt  0         failure      if  errno    EEXIST           the file already existed                  else        now you can use the file

User · Answer

Use stat like this    include  lt sys stat h gt       stat  include  lt stdbool h gt        bool type  bool file exists  char  filename      struct stat   buffer       return  stat  filename   amp buffer     0       and call it like this    include  lt stdio h gt          printf  int main int ac  char   av        if  ac    2          return 1       if  file exists av 1            printf   s exists n   av 1        else         printf   s does not exist n   av 1         return 0

User · Answer

I think that access   function  which is found in unistd h is a good choice for Linux  you can use stat too    You can Use it like this    include  lt stdio h gt   include  lt stdlib h gt   include lt unistd h gt   void fileCheck const char  fileName    int main  void        char  fileName     etc sudoers        fileCheck fileName       return 0     void fileCheck const char  fileName        if  access fileName  F OK             printf  The File  s t was Found n  fileName        else          printf  The File  s t not Found n  fileName              if  access fileName  R OK             printf  The File  s t can be read n  fileName        else          printf  The File  s t cannot be read n  fileName              if  access  fileName  W OK             printf  The File  s t it can be Edited n  fileName        else          printf  The File  s t it cannot be Edited n  fileName              if  access  fileName  X OK             printf  The File  s t is an Executable n  fileName        else          printf  The File  s t is not an Executable n  fileName             And you get the following Output   The File  etc sudoers    was Found The File  etc sudoers    cannot be read The File  etc sudoers    it cannot be Edited The File  etc sudoers    is not an Executable

User · Answer

FILE  file      if  file   fopen  sample txt   r     NULL                           file exists             fclose file                 else                         File not found  no memory leak since  file     NULL               fclose file  would cause an error

User · Answer

Look up the access   function  found in unistd h  You can replace your function with if  access  fname  F OK      0            file exists   else          file doesn t exist    You can also use R OK  W OK  and X OK in place of F OK to check for read permission  write permission  and execute permission  respectively  rather than existence  and you can OR any of them together  i e  check for both read and write permission using R OK W OK  Update  Note that on Windows  you can t use W OK to reliably test for write permission  since the access function does not take DACLs into account  access  fname  W OK   may return 0  success  because the file does not have the read-only attribute set  but you still may not have permission to write to the file

User · Answer

Look up the access   function  found in unistd h  You can replace your function with if  access  fname  F OK      0            file exists   else          file doesn t exist    You can also use R OK  W OK  and X OK in place of F OK to check for read permission  write permission  and execute permission  respectively  rather than existence  and you can OR any of them together  i e  check for both read and write permission using R OK W OK  Update  Note that on Windows  you can t use W OK to reliably test for write permission  since the access function does not take DACLs into account  access  fname  W OK   may return 0  success  because the file does not have the read-only attribute set  but you still may not have permission to write to the file

[c] What's the best way to check if a file exists in C?

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