How to read the content of a file to a string in C

Question

What is the simplest way  least error-prone  least lines of code  however you want to interpret it  to open a file in C and read its contents into a string  char   char    whatever

User · Answer

If  read its contents into a string  means that the file does not contain characters with code 0  you can also use getdelim   function  that either accepts a block of memory and reallocates it if necessary  or just allocates the entire buffer for you  and reads the file into it until it encounters a specified delimiter or end of file  Just pass   0  as the delimiter to read the entire file   This function is available in the GNU C Library  http   www gnu org software libc manual html mono libc html index-getdelim-994  The sample code might look as simple as  char  buffer   NULL  size t len  ssize t bytes read   getdelim   amp buffer   amp len    0   fp   if   bytes read    -1         Success  now the entire file is in the buffer

User · Answer

I tend to just load the entire buffer as a raw memory chunk into memory and do the parsing on my own  That way I have best control over what the standard lib does on multiple platforms   This is a stub I use for this  you may also want to check the error-codes for fseek  ftell and fread   omitted for clarity    char   buffer   0  long length  FILE   f   fopen  filename   rb     if  f      fseek  f  0  SEEK END     length   ftell  f     fseek  f  0  SEEK SET     buffer   malloc  length     if  buffer          fread  buffer  1  length  f         fclose  f      if  buffer         start to process your data   extract strings here

User · Answer

Note  This is a modification of the accepted answer above  Here s a way to do it  complete with error checking  I ve added a size checker to quit when file was bigger than 1 GiB  I did this because the program puts the whole file into a string which may use too much ram and crash a computer  However  if you don t care about that you could just remove it from the code   include  lt stdio h gt   include  lt stdlib h gt    define FILE OK 0  define FILE NOT EXIST 1  define FILE TO LARGE 2  define FILE READ ERROR 3  char   c read file const char   f name  int   err  size t   f size        char   buffer      size t length      FILE   f   fopen f name   quot rb quot        size t read length           if  f            fseek f  0  SEEK END           length   ftell f           fseek f  0  SEEK SET                       1 GiB  best not to load a whole large file in one string         if  length  gt  1073741824                 err   FILE TO LARGE                           return NULL                             buffer    char   malloc length   1                    if  length                read length   fread buffer  1  length  f                            if  length    read length                     free buffer                     err   FILE READ ERROR                    return NULL                                           fclose f                     err   FILE OK          buffer length      0            f size   length            else            err   FILE NOT EXIST                   return NULL                 return buffer     And to check for errors  int err  size t f size  char   f data   f data   c read file  quot test txt quot    amp err   amp f size    if  err           process error   else          process data     free f data

User · Answer

If you re using glib  then you can use g file get contents   gchar  contents  GError  err   NULL   g file get contents   foo txt    amp contents  NULL   amp err   g assert   contents    NULL  amp  amp  err    NULL      contents    NULL  amp  amp  err    NULL    if  err    NULL             Report error to user  and free error     g assert  contents    NULL       fprintf  stderr   Unable to read file   s n   err- gt message       g error free  err       else            Use file contents     g assert  contents    NULL

User · Answer

If you re using glib  then you can use g file get contents   gchar  contents  GError  err   NULL   g file get contents   foo txt    amp contents  NULL   amp err   g assert   contents    NULL  amp  amp  err    NULL      contents    NULL  amp  amp  err    NULL    if  err    NULL             Report error to user  and free error     g assert  contents    NULL       fprintf  stderr   Unable to read file   s n   err- gt message       g error free  err       else            Use file contents     g assert  contents    NULL

User · Answer

Another  unfortunately highly OS-dependent  solution is memory mapping the file   The benefits generally include performance of the read  and reduced memory use as the applications view and operating systems file cache can actually share the physical memory   POSIX code would look like this   int fd   open  filename   O RDONLY   int len   lseek fd  0  SEEK END   void  data   mmap 0  len  PROT READ  MAP PRIVATE  fd  0     Windows on the other hand is little more tricky  and unfortunately I don t have a compiler in front of me to test  but the functionality is provided by CreateFileMapping   and MapViewOfFile

User · Answer

I tend to just load the entire buffer as a raw memory chunk into memory and do the parsing on my own  That way I have best control over what the standard lib does on multiple platforms   This is a stub I use for this  you may also want to check the error-codes for fseek  ftell and fread   omitted for clarity    char   buffer   0  long length  FILE   f   fopen  filename   rb     if  f      fseek  f  0  SEEK END     length   ftell  f     fseek  f  0  SEEK SET     buffer   malloc  length     if  buffer          fread  buffer  1  length  f         fclose  f      if  buffer         start to process your data   extract strings here

User · Answer

I tend to just load the entire buffer as a raw memory chunk into memory and do the parsing on my own  That way I have best control over what the standard lib does on multiple platforms   This is a stub I use for this  you may also want to check the error-codes for fseek  ftell and fread   omitted for clarity    char   buffer   0  long length  FILE   f   fopen  filename   rb     if  f      fseek  f  0  SEEK END     length   ftell  f     fseek  f  0  SEEK SET     buffer   malloc  length     if  buffer          fread  buffer  1  length  f         fclose  f      if  buffer         start to process your data   extract strings here

User · Answer

Assumes the file exists and will seg  fault otherwise  const GLchar  load shader source char  filename      FILE  file   fopen filename   r                   open    fseek file  0L  SEEK END                          find the end   size t size   ftell file                          get the size in bytes   GLchar  shaderSource   calloc 1  size             allocate enough bytes   rewind file                                       go back to file beginning   fread shaderSource  size  sizeof char   file      read each char into ourblock   fclose file                                       close the stream   return shaderSource      This is a pretty crude solution because nothing is checked against null

User · Answer

If you are reading special files like stdin or a pipe  you are not going to be able to use fstat to get the file size beforehand  Also  if you are reading a binary file fgets is going to lose the string size information because of embedded   0  characters  Best way to read a file then is to use read and realloc    include  lt stdio h gt   include  lt unistd h gt   include  lt errno h gt   include  lt string h gt   int main          char buf 4096       ssize t n      char  str   NULL      size t len   0      while  n   read STDIN FILENO  buf  sizeof buf             if  n  lt  0                if  errno    EAGAIN                  continue              perror  read                break                    str   realloc str  len   n   1           memcpy str   len  buf  n           len    n          str len      0             printf     s n   len  str       return 0

User · Answer

Assumes the file exists and will seg  fault otherwise  const GLchar  load shader source char  filename      FILE  file   fopen filename   r                   open    fseek file  0L  SEEK END                          find the end   size t size   ftell file                          get the size in bytes   GLchar  shaderSource   calloc 1  size             allocate enough bytes   rewind file                                       go back to file beginning   fread shaderSource  size  sizeof char   file      read each char into ourblock   fclose file                                       close the stream   return shaderSource      This is a pretty crude solution because nothing is checked against null

User · Answer

easy and neat assuming contents in the file are less than 10000    void read whole file char fileName 1000   char buffer 10000         FILE   file   fopen fileName   r        if file    NULL                puts  File not found            exit 1             char  c      int idx 0      while  fscanf file     c    amp c     1                buffer idx    c          idx              buffer idx    0

User · Answer

Another  unfortunately highly OS-dependent  solution is memory mapping the file   The benefits generally include performance of the read  and reduced memory use as the applications view and operating systems file cache can actually share the physical memory   POSIX code would look like this   int fd   open  filename   O RDONLY   int len   lseek fd  0  SEEK END   void  data   mmap 0  len  PROT READ  MAP PRIVATE  fd  0     Windows on the other hand is little more tricky  and unfortunately I don t have a compiler in front of me to test  but the functionality is provided by CreateFileMapping   and MapViewOfFile

User · Answer

If  read its contents into a string  means that the file does not contain characters with code 0  you can also use getdelim   function  that either accepts a block of memory and reallocates it if necessary  or just allocates the entire buffer for you  and reads the file into it until it encounters a specified delimiter or end of file  Just pass   0  as the delimiter to read the entire file   This function is available in the GNU C Library  http   www gnu org software libc manual html mono libc html index-getdelim-994  The sample code might look as simple as  char  buffer   NULL  size t len  ssize t bytes read   getdelim   amp buffer   amp len    0   fp   if   bytes read    -1         Success  now the entire file is in the buffer

User · Answer

easy and neat assuming contents in the file are less than 10000    void read whole file char fileName 1000   char buffer 10000         FILE   file   fopen fileName   r        if file    NULL                puts  File not found            exit 1             char  c      int idx 0      while  fscanf file     c    amp c     1                buffer idx    c          idx              buffer idx    0

User · Answer

If the file is text  and you want to get the text line by line  the easiest way is to use fgets     char buffer 100   FILE  fp   fopen  filename    r                       do not use  rb  while  fgets buffer  sizeof buffer   fp         do something   fclose fp

User · Answer

I will add my own version  based on the answers here  just for reference  My code takes into consideration sizeof char  and adds a few comments to it      Open the file in read mode  FILE  file   fopen file name   r       Check if there was an error  if  file    NULL        fprintf stderr   Error  Can t open file   s     file name       exit EXIT FAILURE        Get the file length fseek file  0  SEEK END   long length   ftell file   fseek file  0  SEEK SET      Create the string for the file contents  char  buffer   malloc sizeof char     length   1    buffer length      0      Set the contents of the string  fread buffer  sizeof char   length  file      Close the file  fclose file      Do something with the data            Free the allocated string space  free buffer

User · Answer

If  read its contents into a string  means that the file does not contain characters with code 0  you can also use getdelim   function  that either accepts a block of memory and reallocates it if necessary  or just allocates the entire buffer for you  and reads the file into it until it encounters a specified delimiter or end of file  Just pass   0  as the delimiter to read the entire file   This function is available in the GNU C Library  http   www gnu org software libc manual html mono libc html index-getdelim-994  The sample code might look as simple as  char  buffer   NULL  size t len  ssize t bytes read   getdelim   amp buffer   amp len    0   fp   if   bytes read    -1         Success  now the entire file is in the buffer

User · Answer

Note  This is a modification of the accepted answer above  Here s a way to do it  complete with error checking  I ve added a size checker to quit when file was bigger than 1 GiB  I did this because the program puts the whole file into a string which may use too much ram and crash a computer  However  if you don t care about that you could just remove it from the code   include  lt stdio h gt   include  lt stdlib h gt    define FILE OK 0  define FILE NOT EXIST 1  define FILE TO LARGE 2  define FILE READ ERROR 3  char   c read file const char   f name  int   err  size t   f size        char   buffer      size t length      FILE   f   fopen f name   quot rb quot        size t read length           if  f            fseek f  0  SEEK END           length   ftell f           fseek f  0  SEEK SET                       1 GiB  best not to load a whole large file in one string         if  length  gt  1073741824                 err   FILE TO LARGE                           return NULL                             buffer    char   malloc length   1                    if  length                read length   fread buffer  1  length  f                            if  length    read length                     free buffer                     err   FILE READ ERROR                    return NULL                                           fclose f                     err   FILE OK          buffer length      0            f size   length            else            err   FILE NOT EXIST                   return NULL                 return buffer     And to check for errors  int err  size t f size  char   f data   f data   c read file  quot test txt quot    amp err   amp f size    if  err           process error   else          process data     free f data

User · Answer

If the file is text  and you want to get the text line by line  the easiest way is to use fgets     char buffer 100   FILE  fp   fopen  filename    r                       do not use  rb  while  fgets buffer  sizeof buffer   fp         do something   fclose fp

User · Answer

Another  unfortunately highly OS-dependent  solution is memory mapping the file   The benefits generally include performance of the read  and reduced memory use as the applications view and operating systems file cache can actually share the physical memory   POSIX code would look like this   int fd   open  filename   O RDONLY   int len   lseek fd  0  SEEK END   void  data   mmap 0  len  PROT READ  MAP PRIVATE  fd  0     Windows on the other hand is little more tricky  and unfortunately I don t have a compiler in front of me to test  but the functionality is provided by CreateFileMapping   and MapViewOfFile

User · Answer

I will add my own version  based on the answers here  just for reference  My code takes into consideration sizeof char  and adds a few comments to it      Open the file in read mode  FILE  file   fopen file name   r       Check if there was an error  if  file    NULL        fprintf stderr   Error  Can t open file   s     file name       exit EXIT FAILURE        Get the file length fseek file  0  SEEK END   long length   ftell file   fseek file  0  SEEK SET      Create the string for the file contents  char  buffer   malloc sizeof char     length   1    buffer length      0      Set the contents of the string  fread buffer  sizeof char   length  file      Close the file  fclose file      Do something with the data            Free the allocated string space  free buffer

User · Answer

If the file is text  and you want to get the text line by line  the easiest way is to use fgets     char buffer 100   FILE  fp   fopen  filename    r                       do not use  rb  while  fgets buffer  sizeof buffer   fp         do something   fclose fp

User · Answer

If the file is text  and you want to get the text line by line  the easiest way is to use fgets     char buffer 100   FILE  fp   fopen  filename    r                       do not use  rb  while  fgets buffer  sizeof buffer   fp         do something   fclose fp

User · Answer

If you are reading special files like stdin or a pipe  you are not going to be able to use fstat to get the file size beforehand  Also  if you are reading a binary file fgets is going to lose the string size information because of embedded   0  characters  Best way to read a file then is to use read and realloc    include  lt stdio h gt   include  lt unistd h gt   include  lt errno h gt   include  lt string h gt   int main          char buf 4096       ssize t n      char  str   NULL      size t len   0      while  n   read STDIN FILENO  buf  sizeof buf             if  n  lt  0                if  errno    EAGAIN                  continue              perror  read                break                    str   realloc str  len   n   1           memcpy str   len  buf  n           len    n          str len      0             printf     s n   len  str       return 0

User · Answer

Just modified from the accepted answer above    include  lt stdio h gt   include  lt stdlib h gt   include  lt assert h gt   char  readFile char  filename        FILE  f   fopen filename   rt        assert f       fseek f  0  SEEK END       long length   ftell f       fseek f  0  SEEK SET       char  buffer    char    malloc length   1       buffer length      0       fread buffer  1  length  f       fclose f       return buffer     int main         char  content   readFile     hello txt        printf   s   content

User · Answer

I tend to just load the entire buffer as a raw memory chunk into memory and do the parsing on my own  That way I have best control over what the standard lib does on multiple platforms   This is a stub I use for this  you may also want to check the error-codes for fseek  ftell and fread   omitted for clarity    char   buffer   0  long length  FILE   f   fopen  filename   rb     if  f      fseek  f  0  SEEK END     length   ftell  f     fseek  f  0  SEEK SET     buffer   malloc  length     if  buffer          fread  buffer  1  length  f         fclose  f      if  buffer         start to process your data   extract strings here

User · Answer

Another  unfortunately highly OS-dependent  solution is memory mapping the file   The benefits generally include performance of the read  and reduced memory use as the applications view and operating systems file cache can actually share the physical memory   POSIX code would look like this   int fd   open  filename   O RDONLY   int len   lseek fd  0  SEEK END   void  data   mmap 0  len  PROT READ  MAP PRIVATE  fd  0     Windows on the other hand is little more tricky  and unfortunately I don t have a compiler in front of me to test  but the functionality is provided by CreateFileMapping   and MapViewOfFile

User · Answer

If  read its contents into a string  means that the file does not contain characters with code 0  you can also use getdelim   function  that either accepts a block of memory and reallocates it if necessary  or just allocates the entire buffer for you  and reads the file into it until it encounters a specified delimiter or end of file  Just pass   0  as the delimiter to read the entire file   This function is available in the GNU C Library  http   www gnu org software libc manual html mono libc html index-getdelim-994  The sample code might look as simple as  char  buffer   NULL  size t len  ssize t bytes read   getdelim   amp buffer   amp len    0   fp   if   bytes read    -1         Success  now the entire file is in the buffer

User · Answer

Just modified from the accepted answer above    include  lt stdio h gt   include  lt stdlib h gt   include  lt assert h gt   char  readFile char  filename        FILE  f   fopen filename   rt        assert f       fseek f  0  SEEK END       long length   ftell f       fseek f  0  SEEK SET       char  buffer    char    malloc length   1       buffer length      0       fread buffer  1  length  f       fclose f       return buffer     int main         char  content   readFile     hello txt        printf   s   content

[c] How to read the content of a file to a string in C?

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